WO1998032314A2 - Solder joints for surface mount chips - Google Patents

Solder joints for surface mount chips Download PDF

Info

Publication number
WO1998032314A2
WO1998032314A2 PCT/IB1997/001602 IB9701602W WO9832314A2 WO 1998032314 A2 WO1998032314 A2 WO 1998032314A2 IB 9701602 W IB9701602 W IB 9701602W WO 9832314 A2 WO9832314 A2 WO 9832314A2
Authority
WO
WIPO (PCT)
Prior art keywords
solder
pad
lifter
printed circuit
solder joint
Prior art date
Application number
PCT/IB1997/001602
Other languages
French (fr)
Other versions
WO1998032314A3 (en
Inventor
Richard Keith Ii Mcmillan
Vivek Amir Jairazbhoy
Yi-Hsin Pao
Original Assignee
Ford Global Technologies, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ford Global Technologies, Inc. filed Critical Ford Global Technologies, Inc.
Priority to CA002278019A priority Critical patent/CA2278019A1/en
Priority to JP53403498A priority patent/JP2001508949A/en
Priority to BR9714289-1A priority patent/BR9714289A/en
Priority to EP97947200A priority patent/EP0953277B1/en
Priority to DE69709172T priority patent/DE69709172T2/en
Publication of WO1998032314A2 publication Critical patent/WO1998032314A2/en
Publication of WO1998032314A3 publication Critical patent/WO1998032314A3/en

Links

Classifications

    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00Apparatus or processes for manufacturing printed circuits
    • H05K3/30Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/34Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
    • H05K3/341Surface mounted components
    • H05K3/3431Leadless components
    • H05K3/3442Leadless components having edge contacts, e.g. leadless chip capacitors, chip carriers
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2201/00Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/09Shape and layout
    • H05K2201/09209Shape and layout details of conductors
    • H05K2201/09372Pads and lands
    • H05K2201/09427Special relation between the location or dimension of a pad or land and the location or dimension of a terminal
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2201/00Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/09Shape and layout
    • H05K2201/09209Shape and layout details of conductors
    • H05K2201/09654Shape and layout details of conductors covering at least two types of conductors provided for in H05K2201/09218 - H05K2201/095
    • H05K2201/09781Dummy conductors, i.e. not used for normal transport of current; Dummy electrodes of components
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2201/00Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/10Details of components or other objects attached to or integrated in a printed circuit board
    • H05K2201/10613Details of electrical connections of non-printed components, e.g. special leads
    • H05K2201/10621Components characterised by their electrical contacts
    • H05K2201/10636Leadless chip, e.g. chip capacitor or resistor
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/04Soldering or other types of metallurgic bonding
    • H05K2203/0465Shape of solder, e.g. differing from spherical shape, different shapes due to different solder pads
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05KPRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/30Details of processes not otherwise provided for in H05K2203/01 - H05K2203/17
    • H05K2203/306Lifting the component during or after mounting; Increasing the gap between component and PCB
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates generally to printed circuit boards. More particularly, the present invention relates to printed circuit boards having surface mount devices with improved solder joints.
  • FIG. 1A shows a typical surface mount PCB having an SMD 14 joined to a mounting pad 12 on a PCB substrate 10 by a conventional solder joint 24.
  • the solder joint 24 has an inner fillet 26 adjacent the device's bottom surface 18 and the mounting pad 12, and an outer fillet 28 extending between the device's perimeter wall 20 and the mounting pad 12.
  • conventional solder joint inner and outer fillets are concave in shape.
  • crack initiation time is the time required for a crack to first form in the solder joint. As FIG. IB illustrates, crack initiation generally begins on the inner fillet surface 27. Crack propagation time, on the other hand, is the time from crack initiation until the solder joint fails electrically.
  • the propagation phase consists of two stages: propagation of the crack under the device end termination 22, as shown in FIG. IC, and propagation in the outer fillet 28, as shown in FIG. ID. Crack propagation in the outer fillet typically occurs along a line that makes a 45 degree angle with the horizontal extending from the bottom edge 21 of the device up to the outer surface 29 of the fillet.
  • a solder joint fails electrically when the crack propagates to the outer fillet outer surface 29 or substantially thereto, as illustrated in FIG. IE, such that electrical continuity is functionally broken between the termination 22 and its associated mounting pad 12.
  • solder joint life is affected by three aspects of solder joint geometry: (1) solder joint height h 0 , defined as the vertical distance between the device's bottom terminations and the vertically adjacent mounting pads, as shown in FIG. 1A; (2) solder joint inner fillet shape; and (3) solder joint outer fillet shape.
  • Crack initiation time tends to increase with increased solder joint height h 0 and with appropriately designed inner fillet shape (i.e., with the inner fillet angle being greater than a certain minimum number of degrees).
  • Crack propagation time under the terminations i.e., stage I
  • stage I tends to increase with increased solder joint height.
  • One known method for increasing solder joint height is to include "lifter pads" 30 beneath the non-solderable bottom surface of the SMD, as shown in FIG. 2. According to the prior art, these lifter pads are round in shape. When a solder mass 32 deposited on a lifter pad melts during reflow soldering, pressure within the molten solder mass provides an upward force F L which tends to lift the component, ideally maintaining it at or above a minimum solder joint height h 0 until the solder mass returns to a solid state. While this method does tend to increase solder joint height, it makes no provision for the effect of the lifter pad on solder joint geometry.
  • solder quantity at both the inner and outer fillet areas is decreased as solder flows under the device termination area to fill in the increased solder height, thereby detrimentally altering solder joint geometry.
  • surface tension and pressure forces within the solder joint fillets detrimentally affect the shape and lifting effectiveness of the molten solder masses atop the lifter pads.
  • inner and outer solder joint fillets tend to change shape and decrease in size, while lifter pad effectiveness in increasing solder joint height is reduced due to the effect of surface tension and pressure forces acting at the solder joints.
  • overall solder joint height may be increased, decreased overall crack propagation length and unfavorable interior fillet angles ⁇ may result, such that little or no overall improvement in crack initiation and propagation time is achieved.
  • FIG. 4 shows a free body diagram of a conventionally soldered (i.e., concave solder joint) SMD and the forces acting thereon during and after reflow.
  • a solder joint height h 0 is achieved when the net downward force on the SMD (i.e., the weight W of the device, the ambient pressure p a , and the vertical components of the outer and inner fillet surface tension forces, Fj . and F 2 , respectively) reaches equilibrium with the net upward force (i.e., the buoyancy and contact force p D provided by the solder joint) . Compare this with FIG.
  • FIG. 5 shows a free body diagram of an SMD and the forces acting thereon during and after reflow, utilizing convex solder joints.
  • the vertical components of the surface tension forces Fi and F 2 exert less downward force on the SMD than do the vertical components of Fi and F 2 in FIG. 4.
  • the convex shape in FIG. 5 guarantees that p 0 > p a (i.e., a net upward pressure force), while in FIG. 4 the concave shape suggests that p a > p o (i.e., a net downward pressure force), particularly for devices having relatively wide solder joints.
  • a greater solder height h 0 can be achieved than can be realized by using the conventional concave solder joint geometry.
  • the prior art lifter pad method produces concave solder joints having a decreased distance between the inner fillet surface and the outer fillet surface (i.e., decreased overall crack propagation length) .
  • the prior art convex geometry method provides a solder joint height which would increase stage II crack propagation time somewhat
  • neither this method nor the prior art lifter pad method addresses how to appropriately design the inner fillet geometry, nor how to optimize the mounting pad design to maximize the outer fillet crack propagation distance. It is desirable, therefore, to provide a method for improving solder joint durability by achieving increased solder joint height, optimized solder joint inner fillet shape, and optimized solder joint outer fillet size and shape .
  • a printed circuit board comprising: a printed circuit board substrate; at least one surface mount device having a bottom surface and a bottom edge about said surface, said device having terminations on said bottom surface adjacent said bottom edge; at least two mounting pads for each of said surface mount devices, said mounting pads being disposed on a top surface of said substrate in matched relation with said terminations of said surface mount device; a solder joint connecting each of said terminations with its respective mounting pad, said solder joint having inner and outer fillets and consisting of a predetermined amount of solder; at least one lifter pad for each surface mount device, each lifter pad being disposed on said top surface of said substrate amid said mounting pads; and a solder mass for each lifter pad, wherein each solder mass is disposed between and in contact with its respective lifter pad and said bottom surface of said surface mount device, each solder mass consisting of a predetermined amount of solder; wherein said predetermined amounts of solder on said mounting pads and each lifter pad in a molten state
  • the embodiments of the present invention overcome the disadvantages of the prior art by providing a surface mount PCB having a substrate, at least one surface mount device, at least two mounting pads per device, solder joints connecting the terminations of the device to their respective mounting pads, at least one rectangular lifter pad on the substrate amid the mounting pads, and a solder mass on each lifter pad in contact with the bottom surface of the device.
  • the inner and outer extensions of the mounting pads, the size, number, and shape of the lifter pads, and the amounts of solder deposited on the mounting and lifter pads are optimized such that the solder joint has preferably convex outer fillets, the device is maintained at a predetermined height above the mounting pads, the inner fillet angle is maintained above a predetermined minimum angle, and the overall solder joint crack propagation length is increased.
  • An alternative embodiment also includes plugged vias under the lifter pads and/or mounting pads, with gas pockets trapped between the solder masses/solder joints and the plugged vias. This trapped gas pocket provides additional buoyant force upon the SMD during reflow.
  • Another advantage is that the rectangular-shaped lifter pads provide a more effective means for providing buoyant force upon the SMD than conventional round lifter pads.
  • FIGS. 1A-1E are longitudinal cross-section views of a portion of a surface mount device soldered to a printed circuit board, showing the successive phases of crack initiation, crack propagation, and solder joint failure;
  • FIG. 2 is a longitudinal cross-section view of a surface mount device soldered to a printed circuit board according to the prior art, showing conventional solder joints with lifter pads;
  • FIG. 3 shows a longitudinal cross-section view of a solder joint according to the prior art having convex fillets
  • FIG. 4 shows a free body diagram of a solder joint according to the prior art having concave fillets
  • FIG. 5 shows a free body diagram of a solder joint according to the prior art having convex fillets
  • FIGS. 6A and 6B are longitudinal cross-section views of a surface mount device soldered to a printed circuit board according to the present invention, showing improved solder fillet shapes in conjunction with either optimized lifter pads or a plugged via/optimized lifter pad combination;
  • FIG. 6C is an enlarged cross-section view of a solder joint according to the present invention.
  • FIG. 7 shows a representative plot of l c versus 1 0 within the range 0.5H ⁇ 1 0 ⁇ 2H;
  • FIG. 8 shows an elevational view of a lifter pad and solder mass according to the present invention.
  • FIG. 6A shows a printed circuit board with an SMD mounted thereon with optimized solder joints according to a first embodiment of the present invention.
  • This embodiment includes a PCB substrate 10 having a generally planar top surface, on which at least two mounting pads 12 are disposed. These mounting pads 12 are arranged on the top surface of the substrate 10 in matched relation with terminations 22 of an SMD 14. Also on this top surface are at least one lifter pad 30 for each SMD 14. The lifter pads 30 for each SMD 14 are arranged amid the corresponding mounting pads 12 for each respective SMD 14.
  • each PCB has a substrate 10 with one group of mounting pads 12 and lifter pads 30 on its top surface for each individual SMD 14 mounted thereon.
  • the SMD 14 is a leadless surface mount device, such as a leadless ceramic chip resistor (LCCR) .
  • the device 14 has a generally rectangular top surface 16, a similarly shaped bottom surface 18 oriented substantially parallel to the top surface 16, and a perimeter wall 20 abutting the top and bottom surfaces 16/18 and running around the entire perimeter of the device 14.
  • a bottom edge 21 is defined by the intersection of the bottom surface 18 with the perimeter wall 20.
  • the SMD 14 also has terminations 22 on the bottom surface 18 adjacent the device's bottom edge 21, arranged in matched relation with the mounting pads 12 on the substrate 10. These terminations 22 may also extend up some or all of the perimeter wall 20, and also onto some portion of the top surface 16.
  • the terminations 22 are electrically and mechanically connected to their respective mounting pads 12 by means of solder joints 24, each joint 24 consisting of a predetermined amount of solder. Between each lifter pad 30 and the bottom surface 18 of the SMD 14 is a solder mass 32, consisting of a predetermined amount of solder. Each solder mass 32 is in contact with both its respective lifter pad 30 and the bottom surface 18 of the device 14.
  • the PCB according to the present invention is produced by first providing a PCB substrate 10 having at least two mounting pads 12 arranged on the top surface of the substrate 10 in matched relation with the terminations 22 of an SMD 14, and having at least one rectangular lifter pad 30 arranged on the surface amid the mounting pads 12. A predetermined amount of solder is applied to the mounting pads 12 and each lifter pad 30.
  • the SMD 14 is placed onto the mounting pads 12 with the terminations 22 in matched relation thereto.
  • the solder on the mounting pads 12 and lifter pads 30 is heated, such as by placing the entire PCB in a solder reflow oven. The heat will then melt the solder and the SMD 14 will float thereon above the mounting pads 12 due to the buoyant force provided by the molten solder.
  • the molten solder is cooled, forming a solidified solder joint 24 atop each mounting pad 12 and a solidified solder mass 32 atop each lifter pad 30, such that the terminations 22 are situated at the desired solder joint height h 0 above the top surface of the mounting pads 12, the interior angle ⁇ is greater than a predetermined minimum angle, the outer fillet 28 is in contact with substantially the entire height of the perimeter wall 20, and the solder joint outer fillet 28 is preferably convex in shape. This creates a solder joint 24 which provides the desired solder joint height h ⁇ while also providing optimized solder joint structure which increases crack initiation and crack propagation times .
  • solder joints refers to joints 24 having interior angles ⁇ greater than a predetermined minimum angle, outer fillets 28 contacting substantially the entire height of the device's perimeter wall 20, a maximum stage II crack propagation length l c , and a solder volume V M sufficient to provide the desired solder joint height h 0 , as illustrated in FIG. 6C.
  • greater than a predetermined minimum angle
  • outer fillets 28 contacting substantially the entire height of the device's perimeter wall 20
  • l c maximum stage II crack propagation length
  • V M solder volume
  • optimal lifter pads refers to lifter pads 30 being rectangular (or, preferably, square) in shape, and having sufficient number, size, and solder mass volume V LP so as to provide the buoyant force necessary to maintain the desired solder joint height h 0 .
  • solder volume V LP calculated to promote this is referred to as being “optimized”.
  • the procedure for designing the first embodiment of the present invention involves two general processes: (1) optimizing the solder joints 24 and mounting pads 12, and then (2) optimizing the solder masses 32 and lifter pads 30.
  • these two processes are: the dimensions and weight of the SMD 14, the solder paste material properties (e.g., percent solder), the desired solder joint height h 0 (e.g., 7 mils), the printing/overprinting strategy for solder deposition onto the mounting pads 12 and lifter pads 30 (e.g., 10 mil overprinting on three sides of each mounting pad) , and the number and dimensions of lifter pads 30 (e.g., six per SMD).
  • the desired outputs from the two optimization processes will be: the optimum mounting pad inner extension l (i.e., the horizontal distance from the termination inner edge 23 to the mounting pad inner edge 13) which optimizes crack initiation time at the inner fillet 26, the optimum mounting pad outer extension 1 0 (i.e., the horizontal distance from the SMD bottom edge 21 to the mounting pad outer edge 15) that optimizes crack propagation length l c in the outer fillet 28, and the volume of solder paste on each mounting pad V M and each lifter pad V LP .
  • the optimum mounting pad inner extension l i.e., the horizontal distance from the termination inner edge 23 to the mounting pad inner edge 13
  • the optimum mounting pad outer extension 1 0 i.e., the horizontal distance from the SMD bottom edge 21 to the mounting pad outer edge 15
  • the first process determines the geometric dimensions of the mounting pad 12 necessary to promote optimized crack initiation and propagation times, while the second process determines the solder mass volume necessary to float the device 14 (in conjunction with the buoyant force exerted by the solder joint 24) to the desired solder joint height h 0 and maintain the desired solder joint geometry .
  • h 0 desired solder joint height
  • t thickness of termination 22 on the bottom surface 18 of SMD 14
  • h height of solder joint at inner fillet 26 between top of mounting pad 12 and bottom surface 18
  • H height of solder on perimeter wall 20 measured from the bottom edge 21 (usually the same as the height of the SMD 14)
  • w width of SMD 14 per solder joint 24
  • W weight of SMD 14 per pair of solder joints
  • V Ca i c calculated volume of solder on each mounting pad 12;
  • G p gap between mounting pads 12 on respective opposite edges of the device 14;
  • G c gap between termination inner edges 23 on respective opposite edges of the device 14;
  • G c , m a ⁇ maximum value of any G c measurements for a given device 14;
  • a ⁇ , ma ⁇ cross-sectional area of solder joint 24;
  • RP radius of curvature of solder mass outer surface 33.
  • Rchip the distance along the bottom surface 18 from the centerline of the solder mass 32 to the solder mass outer surface 33;
  • J point-of-origin located vertically halfway between bottom edge 21 and the top surface of mounting pad 12;
  • the first step in optimizing the solder joints 24 is to design the inner fillet 26 such that the angle ⁇ between the fillet's free surface 27 and the bottom surface 18 of the device 14 is no smaller than about 30 degrees. This is accomplished by dimensioning the inner extension li such that
  • the remaining steps of the solder joint optimization process focus on determining the outer extension 1 0 , given the volume of solder V M provided by the selected printing strategy, so as to form a preferably convex joint which achieves the desired solder joint height h 0 while optimizing the crack propagation length l c as' measured along line JK.
  • the second step is to select a starting value for the mounting pad outer extension 1 0 .
  • a starting value is needed because finding the optimum 1 0 value is an iterative process.
  • the volume V ca ic of the solder joint 24 is determined by calculating the amount of solder which can be deposited given the selected 1 Q , the calculated l l r the l m associated with the given SMD, the width of the solder joint w, and the particular printing or overprinting strategy to be used (e.g., 10 mil overprinting using a 10 mil thickness of solder paste having 50% solder by weight) . This represents the amount of solder available during reflow to form the solder joint 24.
  • V M w(l 0 /2) (H + ho) + w(l 1 /2)h + l m h 0 +
  • ⁇ 0 3C/2, -- (12) wherein C can be r determined using the latter part of Eqn . 7 above .
  • a point M(x c , y c ) is defined as the center of a circle including arc AKB and having radius R; that is, point (x c , y c ) is the center of curvature of the outer fillet free surface 29.
  • the ninth and final step of the solder joint optimization process is to repeat the second through eighth steps above in order to plot l c versus 1 0 within the range suggested in the first step (i.e., 0.5H ⁇ 1 0 ⁇ 2H) .
  • FIG. 7 shows a representative plot of l c versus 1 0 for the suggested range. Then, the value of 1 0 is picked which corresponds to l C/max , the maximum value of l c . For example, in FIG. 7 the 1 0 value of 0.83H corresponds to the maximum value of l c .
  • a critical aspect of lifter pad design is the ability to predict solder joint height h 0 given (1) the geometry of the device 14 and the mounting pads 12, (2) the number, shape, and size of the lifter pads 30, and (3) the solder volume V LP on each lifter pad 30.
  • the geometry of the device 14 i.e., G c , ma ⁇ and l m
  • the mounting pad geometry i.e., 1 0 and l x
  • the number and size of the lifter pads 30 is somewhat affected by the method of solder deposition.
  • the best consistency i.e., minimum percent variation in solder volume
  • stencil apertures between 12 and 26 mils. Since it is desirable to have many rather than fewer lifter pads (for improved standard deviation of deposited solder volume) , a stencil aperture corresponding to the lower end of this range is recommended. Also, smaller pads are more effective for overprinting, and have less tendency to form solder balls than do larger pads.
  • the size and number of lifter pads 30 will depend on the available space underneath the component (i.e., G p ) , the stencil aperture, the minimum recommended spacing between the stencil apertures, and the desired overprinting scheme. It has been found that a generally recommended number of lifter pads is three per solder joint, while the size R pad of the pads depends much more closely on the particular G p , stencil/deposition characteristics, and overprinting strategy used.
  • the shape of the lifter pads 30 conventionally a circular shape is used. However, according to the present invention it is recommended that rectangular-shaped lifter pads be used. Rectangular, and preferably square, pads are more effective than circular ones for overprinting (i.e., deposition of excess solder) since the corners of the pad assist during solder "pullback" and wetting, but are not covered by the bulk of the solder mass 32 after reflow.
  • a square lifter pad 30 can be overprinted with solder 10 mils beyond each of the square's four edges, thus forming a square footprint of solder paste larger than and covering the square lifter pad.
  • solder paste will melt and the solder paste footprint will shrink to a roughly circular footprint as surface tension forces cause the solder mass 32 (and the SMD placed thereon) to rise, transforming the heretofore flat, square solder deposition into a roughly columnar mass atop the lifter pad 30 whose diameter straddles and is essentially equivalent to the width of the square pad.
  • solder mass volume V LP an optimum solder volume is desirable since inadequate solder volume will result in low, perhaps concave fillets, while excessive solder quantities will raise the fillet heights inordinately and solder will be drawn under the component termination 22 from the bulk of the outer fillet 28, resulting in a shorter crack propagation length in the outer fillet 28.
  • the steps given below provide the approximate design criteria to estimate the solder volume V LP required to achieve a desired solder joint height h 0 given the necessary geometric information.
  • the first step toward determining V LP is to calculate the relative pressure in the solder joint 24 using:
  • the fourth step is to select a starting value for ⁇ LP , the angle between hypotenuse C LP and the solder mass outer surface 33, as shown in FIG. 8.
  • ⁇ c is an interface angle determined by the physical characteristics of the solder and the bottom surface 18 in contact with each other, as can be determined by one having skill in the art.
  • the inner extension l can be used with the given input information to construct the foregoing first embodiment .
  • FIG. 6B A second embodiment of the present invention is shown in FIG. 6B .
  • This embodiment is similar to the first, with the addition of plugged vias 34 and gas pockets 40 filled with gas 42 which assist the lifter pads 30 and solder masses 32 in providing additional upward force F L against the SMD 14.
  • the plugged via 34 consists of a via hole 36 formed through the substrate 10, with the hole 36 being at least partially filled with a plug material 38.
  • the lifter pads 30 have a hole 44 formed therethrough in this embodiment, the hole 44 thus defining an interior wall of the lifter pad 30.
  • a pocket 40 is formed at the top of the plugged via 34, generally bounded on its top by the solder mass 32, on its sides by the solder mass 32 and/or interior wall of the lifter pad 30 and/or via hole 36, and on its bottom by the plug material 38. It is also possible to fill the via hole 36 entirely with plug material 38, so long as some amount of pocket 40 is formed; however, preferably the via 36 is only partially filled with plug material 38, so that a larger pocket 40 is formed and thereby permitting greater lift from the subsequent expansion of gas 42 trapped in the pocket 40.
  • the plugged vias 34 are designed to trap gas 42 during reflow, and provide gas expansion that tends to increase the solder mass height h beneficially while reducing the solder deposition quantity required for the given increase in solder joint height h ⁇ .
  • the gas 42 may consist of air or other ambient gas trapped in the pocket 40 when the solder mass 32 is deposited atop the lifter pads 30, as well as volatile gases released internally by the solder mass 32 during the high heat of reflow.
  • solder joints 24 and solder masses 32 may be composed of the same solder or solder paste.

Abstract

There is disclosed herein a surface mount printed circuit board having a substrate (10), at least one surface mount device (14), at least two mounting pads (12) per device (14), solder joints (24) connecting the terminations (22) of the device (14) to their respective mounting pads (12), at least one rectangular lifter pad (30) on the substrate (10) amid the mounting pads (12), and a solder mass (32) on each lifter pad (30) in contact with the bottom surface (18) of the device (14). The solder joint (24) has preferably convex outer fillets (28), the device (14) is maintained at a predetermined height (ho) above the mounting pads (12), the inner fillet angle (α) is maintained above a predetermined minimum angle to increase solder joint crack initiation time, and the overall solder joint crack propagation length is increased (li and lo). An alternative embodiment also includes plugged vias (34) under the lifter pads (30) and/or mounting pads (12), with gas pockets (40) trapped between the solder masses (32)/solder joints (24) and the plugged vias (34). This trapped gas pocket (40) provides additional buoyant force upon the SMD (14) during reflow.

Description

OPTIMIZED SOLDER JOINTS FOR SURFACE MOUNT CHIPS
The present invention relates generally to printed circuit boards. More particularly, the present invention relates to printed circuit boards having surface mount devices with improved solder joints.
In the field of surface mount printed circuit boards (PCBs), an important indicator of solder joint durability is the time reguired for a solder joint of a surface mount device (SMD) to fail under given conditions of cyclic temperature variation. FIG. 1A shows a typical surface mount PCB having an SMD 14 joined to a mounting pad 12 on a PCB substrate 10 by a conventional solder joint 24. The solder joint 24 has an inner fillet 26 adjacent the device's bottom surface 18 and the mounting pad 12, and an outer fillet 28 extending between the device's perimeter wall 20 and the mounting pad 12. As FIG. 1A illustrates, conventional solder joint inner and outer fillets are concave in shape.
When a solder joint fails, two successive phases of joint failure occur: crack initiation and crack propagation. Crack initiation time is the time required for a crack to first form in the solder joint. As FIG. IB illustrates, crack initiation generally begins on the inner fillet surface 27. Crack propagation time, on the other hand, is the time from crack initiation until the solder joint fails electrically. The propagation phase consists of two stages: propagation of the crack under the device end termination 22, as shown in FIG. IC, and propagation in the outer fillet 28, as shown in FIG. ID. Crack propagation in the outer fillet typically occurs along a line that makes a 45 degree angle with the horizontal extending from the bottom edge 21 of the device up to the outer surface 29 of the fillet. A solder joint fails electrically when the crack propagates to the outer fillet outer surface 29 or substantially thereto, as illustrated in FIG. IE, such that electrical continuity is functionally broken between the termination 22 and its associated mounting pad 12.
It has been demonstrated that solder joint life is affected by three aspects of solder joint geometry: (1) solder joint height h0, defined as the vertical distance between the device's bottom terminations and the vertically adjacent mounting pads, as shown in FIG. 1A; (2) solder joint inner fillet shape; and (3) solder joint outer fillet shape. Crack initiation time tends to increase with increased solder joint height h0 and with appropriately designed inner fillet shape (i.e., with the inner fillet angle being greater than a certain minimum number of degrees). Crack propagation time under the terminations (i.e., stage I) tends to increase with increased solder joint height. It also tends to increase with increased distance between the inner fillet surface 27 and the bottom edge 21 of the terminations; however, this distance is determined by the geometry of the device ' s terminations and is fixed for a given device. As for crack propagation time in the outer fillet (i.e., stage II), this tends to increase with appropriate outer fillet shape, particularly where the shape requires that the crack propagate a longer distance. Thus, overall solder joint life can be improved generally by increasing the solder joint height and by appropriately designing the shape of the inner and outer fillets .
One known method for increasing solder joint height is to include "lifter pads" 30 beneath the non-solderable bottom surface of the SMD, as shown in FIG. 2. According to the prior art, these lifter pads are round in shape. When a solder mass 32 deposited on a lifter pad melts during reflow soldering, pressure within the molten solder mass provides an upward force FL which tends to lift the component, ideally maintaining it at or above a minimum solder joint height h0 until the solder mass returns to a solid state. While this method does tend to increase solder joint height, it makes no provision for the effect of the lifter pad on solder joint geometry. As the component is lifted by the lifter pads, solder quantity at both the inner and outer fillet areas is decreased as solder flows under the device termination area to fill in the increased solder height, thereby detrimentally altering solder joint geometry. Similarly, surface tension and pressure forces within the solder joint fillets detrimentally affect the shape and lifting effectiveness of the molten solder masses atop the lifter pads. Thus, using conventional lifter pads methods, inner and outer solder joint fillets tend to change shape and decrease in size, while lifter pad effectiveness in increasing solder joint height is reduced due to the effect of surface tension and pressure forces acting at the solder joints. Thus, while overall solder joint height may be increased, decreased overall crack propagation length and unfavorable interior fillet angles α may result, such that little or no overall improvement in crack initiation and propagation time is achieved.
Another approach for increasing solder joint height is disclosed in "Prediction of Equilibrium Shapes and Pedestal Heights of Solder Joints for Leadless Chip Components" (IEEE Transactions on Components, Packaging, and Mfg. Tech.), illustrated in FIG 3. Rather than using lifter pads, this approach utilizes mounting pads whose outer edge 15 is spaced closer to the SMD than is the case for conventional mounting pads, so as to encourage the formation of convex inner and outer fillets, rather than the typical concave fillets. According to this approach, an amount of solder is deposited on the mounting pads sufficient to "float" the device on the solder to a desired solder joint height when subsequently reflowed.
This approach can be further illustrated by referring to FIGS. 4 and 5. FIG. 4 shows a free body diagram of a conventionally soldered (i.e., concave solder joint) SMD and the forces acting thereon during and after reflow. A solder joint height h0 is achieved when the net downward force on the SMD (i.e., the weight W of the device, the ambient pressure pa, and the vertical components of the outer and inner fillet surface tension forces, Fj. and F2, respectively) reaches equilibrium with the net upward force (i.e., the buoyancy and contact force pD provided by the solder joint) . Compare this with FIG. 5, which shows a free body diagram of an SMD and the forces acting thereon during and after reflow, utilizing convex solder joints. Note that the vertical components of the surface tension forces Fi and F2 exert less downward force on the SMD than do the vertical components of Fi and F2 in FIG. 4. In addition, the convex shape in FIG. 5 guarantees that p0 > pa (i.e., a net upward pressure force), while in FIG. 4 the concave shape suggests that pa > po (i.e., a net downward pressure force), particularly for devices having relatively wide solder joints. Thus, by utilizing the convex joint geometry illustrated in FIG. 5, a greater solder height h0 can be achieved than can be realized by using the conventional concave solder joint geometry.
However, in practice both of the above prior art methods are limited in their applicability by limits between design of the mounting or lifter pads and the quantity of solder which can be deposited thereon using the standard solder paste deposition process (i.e., screen printing). While other methods exist which can deposit additional solder paste (e.g., dispensing), these are generally much slower than screen printing and so are most practically employed at additional cost only when needed.
Furthermore, while both of these approaches may be effective in achieving a desired solder joint height, they unfortunately may have detrimental effects on solder joint life. For example, the prior art lifter pad method produces concave solder joints having a decreased distance between the inner fillet surface and the outer fillet surface (i.e., decreased overall crack propagation length) . Also, while the prior art convex geometry method provides a solder joint height which would increase stage II crack propagation time somewhat, neither this method nor the prior art lifter pad method addresses how to appropriately design the inner fillet geometry, nor how to optimize the mounting pad design to maximize the outer fillet crack propagation distance. It is desirable, therefore, to provide a method for improving solder joint durability by achieving increased solder joint height, optimized solder joint inner fillet shape, and optimized solder joint outer fillet size and shape .
According to the present invention, there is provided a printed circuit board, comprising: a printed circuit board substrate; at least one surface mount device having a bottom surface and a bottom edge about said surface, said device having terminations on said bottom surface adjacent said bottom edge; at least two mounting pads for each of said surface mount devices, said mounting pads being disposed on a top surface of said substrate in matched relation with said terminations of said surface mount device; a solder joint connecting each of said terminations with its respective mounting pad, said solder joint having inner and outer fillets and consisting of a predetermined amount of solder; at least one lifter pad for each surface mount device, each lifter pad being disposed on said top surface of said substrate amid said mounting pads; and a solder mass for each lifter pad, wherein each solder mass is disposed between and in contact with its respective lifter pad and said bottom surface of said surface mount device, each solder mass consisting of a predetermined amount of solder; wherein said predetermined amounts of solder on said mounting pads and each lifter pad in a molten state provide a net buoyant force on said surface mount device such that said device rises to a predetermined height above said mounting pads, and wherein said solder in a solidified state maintains said device at substantially said predetermined height above said mounting pads .
The embodiments of the present invention overcome the disadvantages of the prior art by providing a surface mount PCB having a substrate, at least one surface mount device, at least two mounting pads per device, solder joints connecting the terminations of the device to their respective mounting pads, at least one rectangular lifter pad on the substrate amid the mounting pads, and a solder mass on each lifter pad in contact with the bottom surface of the device. The inner and outer extensions of the mounting pads, the size, number, and shape of the lifter pads, and the amounts of solder deposited on the mounting and lifter pads are optimized such that the solder joint has preferably convex outer fillets, the device is maintained at a predetermined height above the mounting pads, the inner fillet angle is maintained above a predetermined minimum angle, and the overall solder joint crack propagation length is increased. An alternative embodiment also includes plugged vias under the lifter pads and/or mounting pads, with gas pockets trapped between the solder masses/solder joints and the plugged vias. This trapped gas pocket provides additional buoyant force upon the SMD during reflow.
It is an advantage of the embodiments that the use of a minimum inner fillet angle, in conjunction with an increased solder joint height, promotes increased crack initiation time . It is a further advantage that the optimized outer fillet shape, in conjunction with an increased solder joint height, promotes increased crack propagation time.
Another advantage is that the rectangular-shaped lifter pads provide a more effective means for providing buoyant force upon the SMD than conventional round lifter pads.
Yet another advantage is that the rectangular lifter pads and optimized solder joints, along with the plugged vias in the alternate embodiment, act together to float the surface mount device to a desired solder joint height above the PCB mounting pads without detrimentally affecting fillet shape and overall crack propagation length.
The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :
FIGS. 1A-1E are longitudinal cross-section views of a portion of a surface mount device soldered to a printed circuit board, showing the successive phases of crack initiation, crack propagation, and solder joint failure;
FIG. 2 is a longitudinal cross-section view of a surface mount device soldered to a printed circuit board according to the prior art, showing conventional solder joints with lifter pads;
FIG. 3 shows a longitudinal cross-section view of a solder joint according to the prior art having convex fillets; FIG. 4 shows a free body diagram of a solder joint according to the prior art having concave fillets;
FIG. 5 shows a free body diagram of a solder joint according to the prior art having convex fillets;
FIGS. 6A and 6B are longitudinal cross-section views of a surface mount device soldered to a printed circuit board according to the present invention, showing improved solder fillet shapes in conjunction with either optimized lifter pads or a plugged via/optimized lifter pad combination;
FIG. 6C is an enlarged cross-section view of a solder joint according to the present invention;
FIG. 7 shows a representative plot of lc versus 10 within the range 0.5H < 10 < 2H; and
FIG. 8 shows an elevational view of a lifter pad and solder mass according to the present invention.
Referring now to the drawings, FIG. 6A shows a printed circuit board with an SMD mounted thereon with optimized solder joints according to a first embodiment of the present invention. This embodiment includes a PCB substrate 10 having a generally planar top surface, on which at least two mounting pads 12 are disposed. These mounting pads 12 are arranged on the top surface of the substrate 10 in matched relation with terminations 22 of an SMD 14. Also on this top surface are at least one lifter pad 30 for each SMD 14. The lifter pads 30 for each SMD 14 are arranged amid the corresponding mounting pads 12 for each respective SMD 14. Thus, each PCB has a substrate 10 with one group of mounting pads 12 and lifter pads 30 on its top surface for each individual SMD 14 mounted thereon.
The SMD 14 is a leadless surface mount device, such as a leadless ceramic chip resistor (LCCR) . The device 14 has a generally rectangular top surface 16, a similarly shaped bottom surface 18 oriented substantially parallel to the top surface 16, and a perimeter wall 20 abutting the top and bottom surfaces 16/18 and running around the entire perimeter of the device 14. A bottom edge 21 is defined by the intersection of the bottom surface 18 with the perimeter wall 20. The SMD 14 also has terminations 22 on the bottom surface 18 adjacent the device's bottom edge 21, arranged in matched relation with the mounting pads 12 on the substrate 10. These terminations 22 may also extend up some or all of the perimeter wall 20, and also onto some portion of the top surface 16. The terminations 22 are electrically and mechanically connected to their respective mounting pads 12 by means of solder joints 24, each joint 24 consisting of a predetermined amount of solder. Between each lifter pad 30 and the bottom surface 18 of the SMD 14 is a solder mass 32, consisting of a predetermined amount of solder. Each solder mass 32 is in contact with both its respective lifter pad 30 and the bottom surface 18 of the device 14. The PCB according to the present invention is produced by first providing a PCB substrate 10 having at least two mounting pads 12 arranged on the top surface of the substrate 10 in matched relation with the terminations 22 of an SMD 14, and having at least one rectangular lifter pad 30 arranged on the surface amid the mounting pads 12. A predetermined amount of solder is applied to the mounting pads 12 and each lifter pad 30. This can be accomplished by screen printing or other conventional deposition means, usually according to a given overprinting strategy. After this step the SMD 14 is placed onto the mounting pads 12 with the terminations 22 in matched relation thereto. Next, the solder on the mounting pads 12 and lifter pads 30 is heated, such as by placing the entire PCB in a solder reflow oven. The heat will then melt the solder and the SMD 14 will float thereon above the mounting pads 12 due to the buoyant force provided by the molten solder. Finally, the molten solder is cooled, forming a solidified solder joint 24 atop each mounting pad 12 and a solidified solder mass 32 atop each lifter pad 30, such that the terminations 22 are situated at the desired solder joint height h0 above the top surface of the mounting pads 12, the interior angle α is greater than a predetermined minimum angle, the outer fillet 28 is in contact with substantially the entire height of the perimeter wall 20, and the solder joint outer fillet 28 is preferably convex in shape. This creates a solder joint 24 which provides the desired solder joint height hσ while also providing optimized solder joint structure which increases crack initiation and crack propagation times .
It should be noted that "optimized" solder joints, as used herein, refers to joints 24 having interior angles α greater than a predetermined minimum angle, outer fillets 28 contacting substantially the entire height of the device's perimeter wall 20, a maximum stage II crack propagation length lc, and a solder volume VM sufficient to provide the desired solder joint height h0, as illustrated in FIG. 6C. Thus, it is often (but not always) the case that the outer solder fillet 28 produced will be convex in shape. (It should be noted that when the outer fillet 28 is convex, so is the inner fillet 26, generally; likewise, when the outer fillet 28 is concave, so is the inner fillet 26.) However, there may be cases, such as when a very large solder joint height h0 is sought or a particular solder formulation is used, that the desired lc crack propagation length and other characteristics may be optimized while nonetheless producing concave outer fillets. The inner and outer extensions li/lo and solder volume VM which promote these solder joint characteristics are thus also referred to as being "optimized". Also, "optimized" lifter pads, as used herein, refers to lifter pads 30 being rectangular (or, preferably, square) in shape, and having sufficient number, size, and solder mass volume VLP so as to provide the buoyant force necessary to maintain the desired solder joint height h0. Similarly, the solder volume VLP calculated to promote this is referred to as being "optimized". Furthermore,
"optimized" crack initiation and propagation times refer to the maximization of these times, as produced by the combination of the foregoing optimized features.
The procedure for designing the first embodiment of the present invention, as illustrated m FIGS. 6A and 6C, involves two general processes: (1) optimizing the solder joints 24 and mounting pads 12, and then (2) optimizing the solder masses 32 and lifter pads 30. Given as inputs into these two processes are: the dimensions and weight of the SMD 14, the solder paste material properties (e.g., percent solder), the desired solder joint height h0 (e.g., 7 mils), the printing/overprinting strategy for solder deposition onto the mounting pads 12 and lifter pads 30 (e.g., 10 mil overprinting on three sides of each mounting pad) , and the number and dimensions of lifter pads 30 (e.g., six per SMD). Using these inputs, the desired outputs from the two optimization processes will be: the optimum mounting pad inner extension l (i.e., the horizontal distance from the termination inner edge 23 to the mounting pad inner edge 13) which optimizes crack initiation time at the inner fillet 26, the optimum mounting pad outer extension 10 (i.e., the horizontal distance from the SMD bottom edge 21 to the mounting pad outer edge 15) that optimizes crack propagation length lc in the outer fillet 28, and the volume of solder paste on each mounting pad VM and each lifter pad VLP . Thus, given the provided inputs, the first process determines the geometric dimensions of the mounting pad 12 necessary to promote optimized crack initiation and propagation times, while the second process determines the solder mass volume necessary to float the device 14 (in conjunction with the buoyant force exerted by the solder joint 24) to the desired solder joint height h0 and maintain the desired solder joint geometry .
Each of these two optimizing processes will now be discussed in greater detail, using quantities from the following list of variables and constants: h0 = desired solder joint height; t = thickness of termination 22 on the bottom surface 18 of SMD 14; h = height of solder joint at inner fillet 26 between top of mounting pad 12 and bottom surface 18; H = height of solder on perimeter wall 20 measured from the bottom edge 21 (usually the same as the height of the SMD 14) ; w = width of SMD 14 per solder joint 24; W = weight of SMD 14 per pair of solder joints; VCaic = calculated volume of solder on each mounting pad 12;
VM = volume of solder on each mounting pad 12; VLP = volume of solder in each solder mass 32; L = length of SMD 14; lc = crack propagation length in outer fillet 28, measured along line JK;
10 = outward extension of mounting pad 12; (the horizontal distance from the bottom edge 21 to the mounting pad outer edge 15); lm = length of the bottom surface of termination
22; 1± = inner extension of mounting pad 12 (the horizontal distance from the termination inner edge 23 to the mounting pad inner edge 13) ;
Gp = gap between mounting pads 12 on respective opposite edges of the device 14;
Gc = gap between termination inner edges 23 on respective opposite edges of the device 14;
Gc,m= maximum value of any Gc measurements for a given device 14; Aτ,maχ = cross-sectional area of solder joint 24;
Acs = cross-sectional area of solder mass circular segment 35, as shown in FIG. 8;
C0 = hypotenuse of triangle ABC in FIG. 6C; d = hypotenuse of triangle A'B'C in FIG. 6C; CLP = hypotenuse of triangle A''B''C' in FIG. 8; p0 = pressure in the solder joint 24; pa = atmospheric (reference) pressure; σ = surface tension;
F = downward force acting on a single lifter pad 30;
F0 = upward force acting on SMD 14 due to the solder joint 24;
N = number of lifter pads 30 for every outer pad; θ0 = angle between hypotenuse CQ and the outer fillet's free surface 29; θx = angle between hypotenuse C and the inner fillet's free surface 27;
ΘLP = angle between hypotenuse CLP and the solder mass outer surface 33; θ0' = angle between hypotenuse C0 and the vertical; θi' = angle between hypotenuse C and the vertical; ΘLP' = angle between hypotenuse CLP and the vertical; θc = angle between the solder mass free surface 33 and the bottom surface 18 of the SMD 14; = angle between the inner fillet surface 27 and the bottom surface 18 of the SMD 14; R radius of curvature of outer fillet surface
27;
RP = radius of curvature of solder mass outer surface 33. Rchip = the distance along the bottom surface 18 from the centerline of the solder mass 32 to the solder mass outer surface 33;
Rpad = the distance along the top of the lifter pad 30 from the centerline of the solder mass 32 to the solder mass outer surface 33; rCG = radial distance from centerline of solder mass 32 to centroid CG of solder mass circular segment 35;
J = point-of-origin located vertically halfway between bottom edge 21 and the top surface of mounting pad 12;
K = point of intersection between outer fillet surface 27, and a crack propagation line of length lc starting at point J and making a 45 degree angle with the perimeter wall 20; xC Yc = x- and y-coordinates of point M, the origin of a circle which describes arc AKB; and
C = temporary variable (defined below) .
Optimizing the Solder Joints
The first step in optimizing the solder joints 24 is to design the inner fillet 26 such that the angle α between the fillet's free surface 27 and the bottom surface 18 of the device 14 is no smaller than about 30 degrees. This is accomplished by dimensioning the inner extension li such that
Gp = Gc,max + 3h, (1) giving li = (L - 21m - Gp)/2. — (2)
Once the inner extension li has been determined, the remaining steps of the solder joint optimization process focus on determining the outer extension 10, given the volume of solder VM provided by the selected printing strategy, so as to form a preferably convex joint which achieves the desired solder joint height h0 while optimizing the crack propagation length lc as' measured along line JK.
The second step is to select a starting value for the mounting pad outer extension 10. A starting value is needed because finding the optimum 10 value is an iterative process. A suggested range is 0.5H < 10 < 2H, so a starting value of 10 = 0.75H is recommended.
Third, the volume Vcaic of the solder joint 24 is determined by calculating the amount of solder which can be deposited given the selected 1Q, the calculated ll r the lm associated with the given SMD, the width of the solder joint w, and the particular printing or overprinting strategy to be used (e.g., 10 mil overprinting using a 10 mil thickness of solder paste having 50% solder by weight) . This represents the amount of solder available during reflow to form the solder joint 24.
Using the anticipated geometric features of reflowed solder joint 24 as depicted in FIG. 6C, the volume VM of solder in the solder joint 24 can be expressed as: VM = w(l0/2) (H + ho) + w(l1/2)h + lmh0 +
( w/ 4 ) { Co 2 [ θocsc2θ0 - cotθ0] +
Figure imgf000016_0001
- cotθi ] } ( 3 ) with
Aτ, max = VM/w, ( 4 ) where C0 = V ( 10 2 + ( H + h0 ) 2 ) --- ( 5 ) and d = V ( l,2 + h2 ) . — ( 6 )
In Eqn . 3 , all the quantitie s are known except for VM, θ0 , and ± . The angle θ0 can be determined ( and the angle θ eliminated ) by setting VM equal to Vcalc proceeding as follows . Fourth, a temporary variable C is defined by ignoring the portion of Eqn. 3 corresponding to the miniscule volume of the inner fillet circular segment 25 (i.e., the Ci2[θicsc2θi - cotθi] term), solving the equation in terms of the [θ0csc2θ0 - cotθ0] binomial term, and setting the result equal to C:
C = [θ0csc2θ0 - cotθ0] = (4/C0 2) [Vcalc/w - (lo/2) (H + hQ) -
Figure imgf000017_0001
Fifth, the csc2θ0 term of Eqn. 7 is converted into (cot2θ0 + 1), yielding:
C = θ0csc2θ0 - cotθo = θ0cot2θ0 + θ0 - cotθ0. (8)
The cotangent terms of Eqn. 8 are then series expanded to include only the first three terms in the series, giving: C = θ0(l/θo - θ0/3 - θ0 3/45)2 + θ0 -
(1/θo - θ0/3 - θ0 3/45) . — (9)
Expanding the squared trinomial, ignoring the resulting higher-order terms, and combining like terms gives: C = 2θ0/3 + 4θ0 3/45, — (10) or further,
0 = θ0 3 + 15θQ/2 - 45C/4. — (11)
Ignoring the third-order term in Eqn. 11, a first-order approximation of θ0 can be stated as: θ0 = 3C/2, -- (12) wherein C can ber determined using the latter part of Eqn . 7 above .
Once θ0 is determined, the sixth step is to calculate θ0' using: θ0' = tan_1[l0/(H + h0) ] . -- (13)
Seventh, a point M(xc, yc) is defined as the center of a circle including arc AKB and having radius R; that is, point (xc, yc) is the center of curvature of the outer fillet free surface 29. xc ι Ya and R are calculated usirtg:
Figure imgf000018_0001
xc = -l0/2 + Rcos (θo/2) sιnθ0' , -- (15) and yc = H/2 - Rcos (θ0/2) cosθ0' . — (16)
Eighth, the length lc of the outer extension is determined using:
lc = abs val [ (xc - yc) + V(2R2 - (xc + yc)2 ) ] — (17) 2
The ninth and final step of the solder joint optimization process is to repeat the second through eighth steps above in order to plot lc versus 10 within the range suggested in the first step (i.e., 0.5H < 10 < 2H) . FIG. 7 shows a representative plot of lc versus 10 for the suggested range. Then, the value of 10 is picked which corresponds to lC/max, the maximum value of lc. For example, in FIG. 7 the 10 value of 0.83H corresponds to the maximum value of lc. Once the optimum inner extension ll r optimum outer extension 10, and solder joint volume VM are determined based on the given inputs, the lifter pads 30 and solder masses 32 can then be designed.
Optimizing the Lifter Pads
A critical aspect of lifter pad design is the ability to predict solder joint height h0 given (1) the geometry of the device 14 and the mounting pads 12, (2) the number, shape, and size of the lifter pads 30, and (3) the solder volume VLP on each lifter pad 30. The geometry of the device 14 (i.e., Gc,maχ and lm) is determined by the particular device used in each case, while the mounting pad geometry (i.e., 10 and lx) is calculated m the above solder joint optimization process. The number and size of the lifter pads 30 is somewhat affected by the method of solder deposition. For instance, in the conventional screen printing solder deposition method, the best consistency (i.e., minimum percent variation in solder volume) is usually obtained with stencil apertures between 12 and 26 mils. Since it is desirable to have many rather than fewer lifter pads (for improved standard deviation of deposited solder volume) , a stencil aperture corresponding to the lower end of this range is recommended. Also, smaller pads are more effective for overprinting, and have less tendency to form solder balls than do larger pads. (That is, in depositing as much solder paste as possible without forming solder balls, a higher solder volume-to-lifter pad area ratio is possible with smaller pads than is with larger pads, generally.) Thus, the size and number of lifter pads 30 will depend on the available space underneath the component (i.e., Gp) , the stencil aperture, the minimum recommended spacing between the stencil apertures, and the desired overprinting scheme. It has been found that a generally recommended number of lifter pads is three per solder joint, while the size Rpad of the pads depends much more closely on the particular Gp, stencil/deposition characteristics, and overprinting strategy used.
As for the shape of the lifter pads 30, conventionally a circular shape is used. However, according to the present invention it is recommended that rectangular-shaped lifter pads be used. Rectangular, and preferably square, pads are more effective than circular ones for overprinting (i.e., deposition of excess solder) since the corners of the pad assist during solder "pullback" and wetting, but are not covered by the bulk of the solder mass 32 after reflow. For example, a square lifter pad 30 can be overprinted with solder 10 mils beyond each of the square's four edges, thus forming a square footprint of solder paste larger than and covering the square lifter pad. Then, during reflow, the solder paste will melt and the solder paste footprint will shrink to a roughly circular footprint as surface tension forces cause the solder mass 32 (and the SMD placed thereon) to rise, transforming the heretofore flat, square solder deposition into a roughly columnar mass atop the lifter pad 30 whose diameter straddles and is essentially equivalent to the width of the square pad.
As for the solder mass volume VLP, an optimum solder volume is desirable since inadequate solder volume will result in low, perhaps concave fillets, while excessive solder quantities will raise the fillet heights inordinately and solder will be drawn under the component termination 22 from the bulk of the outer fillet 28, resulting in a shorter crack propagation length in the outer fillet 28. The steps given below provide the approximate design criteria to estimate the solder volume VLP required to achieve a desired solder joint height h0 given the necessary geometric information. (It should be noted that while the following equations are for the case in which no metallizations corresponding to each lifter pad exist on the underside of the SMD, equations describing this case (e.g., for flip- chips) may be found in the literature.)
The first step toward determining VLP is to calculate the relative pressure in the solder joint 24 using:
(Po - Pa) = (2σsinθ0) /C0. — (18)
Second, the net upward force Fc acting on the SMD 14 due to the solder joint 24 is calculated using:
F0 = w{lm[p0 - pa - W/(21mw) - σ(cosθx + cosθ3] } —(19) where
and
Figure imgf000020_0001
Third, the downward force F acting on a single lifter pad 30 is calculated using:
F = -F0/N, -- (22) where N is the number of lifter pads 30 for every mounting pad 12. The fourth step is to select a starting value for ΘLP, the angle between hypotenuse CLP and the solder mass outer surface 33, as shown in FIG. 8. A suggested range is 10° < ΘLP < 40°, so a starting value of ΘLP = 25° is recommended.
Fifth, RChιpr CLP, and RLP are calculated using: Rchip = Rpad " h cot(θLP + θc) , -- (23)
CLP = hV(l + cot2LP + θc) ) = h/[sin(θLP + θc)],— (24) and
RLP = -CLP/(2sinθLP) = -h/[2sinθLPsin(θLP + θc) ] , -- (25)
where θc is an interface angle determined by the physical characteristics of the solder and the bottom surface 18 in contact with each other, as can be determined by one having skill in the art.
Sixth, the force on an individual lifter pad 30 is calculated using: F' = πσRchιp 2/R0 - 2πRchιpσsin (θLP + θc) — (26) where Ro = -RLP- — (27)
Seventh, the fourth through sixth steps are iterated until a value for ΘLP is arrived at such that the value of F' equals the value of F calculated in the third step (Eqn. 22) above .
Eighth, ACs and rCG are calculated using: cs = ^(Ro2) (2θLP - sin 2θLP) -- (28) and rCG = (Rchip + Rpad) /2 + R0[ (4sin3θLP) / ( 6θLP - 3sin2θLP) - cosθLP]sin(θLP + θc) - — (29)
The ninth and final step is to calculate the volume VLP of each lifter pad solder mass 32 using: VLP = π [ hRchιP 2 + h ( Rpad - Rchip ) ( 2 Rchιp + Rpad ) / 3 + 2AcsrCG] • - - ( 30 )
Once the solder joint optimization and lifter pad optimization processes are completed, the inner extension l , outer extension 10, and solder volumes VM/VLP can be used with the given input information to construct the foregoing first embodiment .
A second embodiment of the present invention is shown in FIG. 6B . This embodiment is similar to the first, with the addition of plugged vias 34 and gas pockets 40 filled with gas 42 which assist the lifter pads 30 and solder masses 32 in providing additional upward force FL against the SMD 14. The plugged via 34 consists of a via hole 36 formed through the substrate 10, with the hole 36 being at least partially filled with a plug material 38. Also, the lifter pads 30 have a hole 44 formed therethrough in this embodiment, the hole 44 thus defining an interior wall of the lifter pad 30. A pocket 40 is formed at the top of the plugged via 34, generally bounded on its top by the solder mass 32, on its sides by the solder mass 32 and/or interior wall of the lifter pad 30 and/or via hole 36, and on its bottom by the plug material 38. It is also possible to fill the via hole 36 entirely with plug material 38, so long as some amount of pocket 40 is formed; however, preferably the via 36 is only partially filled with plug material 38, so that a larger pocket 40 is formed and thereby permitting greater lift from the subsequent expansion of gas 42 trapped in the pocket 40.
The plugged vias 34 are designed to trap gas 42 during reflow, and provide gas expansion that tends to increase the solder mass height h beneficially while reducing the solder deposition quantity required for the given increase in solder joint height hσ. The gas 42 may consist of air or other ambient gas trapped in the pocket 40 when the solder mass 32 is deposited atop the lifter pads 30, as well as volatile gases released internally by the solder mass 32 during the high heat of reflow.
Various other modifications can be envisaged. For example, it is also possible to provide plugged vias 34 under the mounting pads 12 in addition to or instead of the plugged vias 34 underneath the lifter pads 30. Also, it is clear that the solder joints 24 and solder masses 32 may be composed of the same solder or solder paste.

Claims

1. A printed circuit board, comprising: a printed circuit board substrate (10); at least one surface mount device (14) having a bottom surface (18) and a bottom edge (21) about said surface (18), said device (14) having terminations (22) on said bottom surface (18) adjacent said bottom edge (21); at least two mounting pads (12) for each of said surface mount devices (14), said mounting pads (12) being disposed on a top surface of said substrate (10) in matched relation with said terminations (22) of said surface mount device ( 14 ) ; a solder joint (24) connecting each of said terminations (22) with its respective mounting pad (12), said solder joint (24) having inner and outer fillets (26,28) and consisting of a predetermined amount of solder; at least one lifter pad (30) for each surface mount device (14), each lifter pad (30) being disposed on said top surface of said substrate (10) amid said mounting pads (12); and a solder mass (32) for each lifter pad (30), wherein each solder mass (32) is disposed between and in contact with its respective lifter pad (30) and said bottom surface (18) of said surface mount device (14) , each solder mass (32) consisting of a predetermined amount of solder; wherein said predetermined amounts of solder (24,32) on said mounting pads (12) and each lifter pad (30) in a molten state provide a net buoyant force on said surface mount device (14) such that said device (14) rises to a predetermined height (ho) above said mounting pads (12), and wherein said solder (24,32) in a solidified state maintains said device (14) at substantially said predetermined height (ho) above said mounting pads (12) .
2. A printed circuit board as claimed in claim 1, wherein an inner extension (li) of each mounting pad (12) is of a predetermined length so as to promote an angle (╬▒) between said bottom surface (18) of said surface mount device (14) and a free surface (27) of said inner fillet )26) to be at least as great as a predetermined minimum angle.
3. A printed circuit board as claimed in claim 2, wherein said predetermined minimum angle is 30 degrees.
4. A printed circuit board as claimed in claim 1, wherein said outer fillet (28) contacts substantially the entire height of a perimeter wall (20) of said surface mount device (14), thereby providing increased crack propagation length.
5. A printed circuit board as claimed in claim 1, wherein said outer fillet (28) of said solder joint (24) is convex in shape.
6. A printed circuit board as claimed in claim 1, wherein an outer extension (10) of each mounting pad (12) is of a predetermined length so as to maximize crack propagation length in said outer fillet (28), said predetermined length being based on a preselected solder deposition strategy.
7. A printed circuit board as claimed in claim 1, wherein each lifter pad (30) is rectangular in shape.
8. A printed circuit board as claimed in claim 1, wherein said substrate (10) includes vias (34) formed therethrough and each lifter pad (30) includes a lifter pad hole (44) formed therethrough, each of said vias (34) being aligned with a respective lifter pad hole (44), said vias (34) being partially plugged with a plug material (38), thereby defining a closed pocket (40) between said solder mass (32) and said plug material (38) where gas (42) may be trapped and allowed to expand during reflow.
9. A printed circuit board as claimed in claim 8, wherein said plug material (38) is thermally conductive.
10. A printed circuit board as claimed in claim 8, wherein said substrate further includes additional vias formed therethrough and said mounting pads include mounting pad holes formed therethrough, said additional vias being aligned with said mounting pad holes, said additional vias being partially plugged with a plug material, thereby defining a closed pocket between said solder joint and said plug material where gas may be trapped and allowed to expand during reflow.
PCT/IB1997/001602 1997-01-16 1997-12-29 Solder joints for surface mount chips WO1998032314A2 (en)

Priority Applications (5)

Application Number Priority Date Filing Date Title
CA002278019A CA2278019A1 (en) 1997-01-16 1997-12-29 Solder joints for surface mount chips
JP53403498A JP2001508949A (en) 1997-01-16 1997-12-29 Optimal brazing for surface mounting chips
BR9714289-1A BR9714289A (en) 1997-01-16 1997-12-29 Weld joints optimized for surface-mounted chips
EP97947200A EP0953277B1 (en) 1997-01-16 1997-12-29 Optimized solder joints for surface mount chips
DE69709172T DE69709172T2 (en) 1997-01-16 1997-12-29 OPTIMIZED SOLDERING CONNECTIONS FOR SURFACE MOUNTED CHIPS

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US08/786,389 US5936846A (en) 1997-01-16 1997-01-16 Optimized solder joints and lifter pads for improving the solder joint life of surface mount chips
US08/786,389 1997-01-16

Publications (2)

Publication Number Publication Date
WO1998032314A2 true WO1998032314A2 (en) 1998-07-23
WO1998032314A3 WO1998032314A3 (en) 1999-06-03

Family

ID=25138437

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/IB1997/001602 WO1998032314A2 (en) 1997-01-16 1997-12-29 Solder joints for surface mount chips

Country Status (10)

Country Link
US (1) US5936846A (en)
EP (1) EP0953277B1 (en)
JP (1) JP2001508949A (en)
CN (1) CN1245007A (en)
BR (1) BR9714289A (en)
CA (1) CA2278019A1 (en)
DE (1) DE69709172T2 (en)
ES (1) ES2167797T3 (en)
PT (1) PT953277E (en)
WO (1) WO1998032314A2 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1014767A1 (en) * 1998-12-23 2000-06-28 Lucent Technologies Inc. Thermal stress relief for surface mount components using via filling

Families Citing this family (23)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR100843737B1 (en) * 2002-05-10 2008-07-04 페어차일드코리아반도체 주식회사 Semiconductor package having improved reliability of solder joint
US7049051B2 (en) * 2003-01-23 2006-05-23 Akustica, Inc. Process for forming and acoustically connecting structures on a substrate
TWI243462B (en) * 2004-05-14 2005-11-11 Advanced Semiconductor Eng Semiconductor package including passive component
JP4557676B2 (en) * 2004-10-27 2010-10-06 京セラ株式会社 Mounting structure of semiconductor device
DE102005017527A1 (en) * 2005-04-15 2006-11-02 Osram Opto Semiconductors Gmbh Surface-mountable optoelectronic component
DE102006054085A1 (en) * 2006-11-16 2008-05-29 Epcos Ag Component arrangement
US7893545B2 (en) * 2007-07-18 2011-02-22 Infineon Technologies Ag Semiconductor device
US7830022B2 (en) * 2007-10-22 2010-11-09 Infineon Technologies Ag Semiconductor package
US8399995B2 (en) * 2009-01-16 2013-03-19 Infineon Technologies Ag Semiconductor device including single circuit element for soldering
DE102009060060A1 (en) * 2009-12-22 2011-06-30 Rohde & Schwarz GmbH & Co. KG, 81671 Retaining element for captive assembly of a screw
JP5552882B2 (en) * 2010-04-26 2014-07-16 株式会社デンソー Mounting structure of surface mount semiconductor package
JP5389748B2 (en) * 2010-06-18 2014-01-15 日本メクトロン株式会社 Electronic component surface mounting method and printed circuit board manufactured using the method
US8604356B1 (en) * 2010-11-12 2013-12-10 Amkor Technology, Inc. Electronic assembly having increased standoff height
CN102522347B (en) * 2011-12-23 2015-04-29 清华大学 Method for manufacturing solder bump
JP2014110370A (en) * 2012-12-04 2014-06-12 Seiko Epson Corp Base substrate, mounting structure, module, electronic equipment, and mobile object
JP5646021B2 (en) * 2012-12-18 2014-12-24 積水化学工業株式会社 Semiconductor package
US9622356B2 (en) * 2013-03-14 2017-04-11 Lockheed Martin Corporation Electronic package mounting
US9237655B1 (en) 2013-03-15 2016-01-12 Lockheed Martin Corporation Material deposition on circuit card assemblies
AT515071B1 (en) * 2013-09-03 2019-03-15 Zkw Group Gmbh Method for positionally stable soldering
JP6481446B2 (en) * 2014-06-13 2019-03-13 株式会社村田製作所 Multilayer capacitor mounting structure
US9634053B2 (en) 2014-12-09 2017-04-25 Taiwan Semiconductor Manufacturing Co., Ltd. Image sensor chip sidewall interconnection
CN113694796B (en) * 2021-10-14 2022-02-08 深圳市澳华集团股份有限公司 Dissolving device for producing intestinal immunopotentiator and use method thereof
US11839031B2 (en) * 2022-04-06 2023-12-05 Western Digital Technologies, Inc. Micro solder joint and stencil aperture design

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3536431A1 (en) * 1985-10-12 1987-04-16 Standard Elektrik Lorenz Ag Soldering of surface mounted devices (SMDs)
US5051339A (en) * 1988-03-29 1991-09-24 Dieter Friedrich Method and apparatus for applying solder to printed wiring boards by immersion
DE4137045A1 (en) * 1991-11-11 1993-05-13 Siemens Ag METHOD FOR PRODUCING SOLDER AREAS ON A CIRCUIT BOARD AND SOLDER PASTE FILM FOR CARRYING OUT THE METHOD
WO1993023981A1 (en) * 1992-05-12 1993-11-25 Mask Technology, Inc. Method, apparatus and product for surface mount solder joints
DE4402545A1 (en) * 1993-02-05 1994-08-11 Ncr Int Inc Method for the formation of discrete solder points on corresponding contact connection surfaces on a printed circuit board
WO1996020580A1 (en) * 1994-12-23 1996-07-04 Ford Motor Company Optimally shaped solder joints

Family Cites Families (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0263222B1 (en) * 1986-10-08 1992-03-25 International Business Machines Corporation Method of forming solder terminals for a pinless ceramic module
US4749120A (en) * 1986-12-18 1988-06-07 Matsushita Electric Industrial Co., Ltd. Method of connecting a semiconductor device to a wiring board
US4760948A (en) * 1986-12-23 1988-08-02 Rca Corporation Leadless chip carrier assembly and method
JPH01251789A (en) * 1988-03-31 1989-10-06 Toshiba Corp Printed board
JP2761113B2 (en) * 1991-02-25 1998-06-04 松下電工株式会社 Printed wiring board
US5315070A (en) * 1991-12-02 1994-05-24 Siemens Aktiengesellschaft Printed wiring board to which solder has been applied
US5311402A (en) * 1992-02-14 1994-05-10 Nec Corporation Semiconductor device package having locating mechanism for properly positioning semiconductor device within package
KR100280762B1 (en) * 1992-11-03 2001-03-02 비센트 비.인그라시아 Thermally Reinforced Semiconductor Devices Having Exposed Backsides and Methods of Manufacturing the Same
JP3152834B2 (en) * 1993-06-24 2001-04-03 株式会社東芝 Electronic circuit device
US5400950A (en) * 1994-02-22 1995-03-28 Delco Electronics Corporation Method for controlling solder bump height for flip chip integrated circuit devices
US5726861A (en) * 1995-01-03 1998-03-10 Ostrem; Fred E. Surface mount component height control

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3536431A1 (en) * 1985-10-12 1987-04-16 Standard Elektrik Lorenz Ag Soldering of surface mounted devices (SMDs)
US5051339A (en) * 1988-03-29 1991-09-24 Dieter Friedrich Method and apparatus for applying solder to printed wiring boards by immersion
DE4137045A1 (en) * 1991-11-11 1993-05-13 Siemens Ag METHOD FOR PRODUCING SOLDER AREAS ON A CIRCUIT BOARD AND SOLDER PASTE FILM FOR CARRYING OUT THE METHOD
WO1993023981A1 (en) * 1992-05-12 1993-11-25 Mask Technology, Inc. Method, apparatus and product for surface mount solder joints
DE4402545A1 (en) * 1993-02-05 1994-08-11 Ncr Int Inc Method for the formation of discrete solder points on corresponding contact connection surfaces on a printed circuit board
WO1996020580A1 (en) * 1994-12-23 1996-07-04 Ford Motor Company Optimally shaped solder joints

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1014767A1 (en) * 1998-12-23 2000-06-28 Lucent Technologies Inc. Thermal stress relief for surface mount components using via filling

Also Published As

Publication number Publication date
ES2167797T3 (en) 2002-05-16
BR9714289A (en) 2000-04-25
WO1998032314A3 (en) 1999-06-03
EP0953277A2 (en) 1999-11-03
JP2001508949A (en) 2001-07-03
US5936846A (en) 1999-08-10
CN1245007A (en) 2000-02-16
EP0953277B1 (en) 2001-12-12
DE69709172D1 (en) 2002-01-24
CA2278019A1 (en) 1998-07-23
DE69709172T2 (en) 2002-05-02
PT953277E (en) 2002-06-28

Similar Documents

Publication Publication Date Title
EP0953277B1 (en) Optimized solder joints for surface mount chips
US6623283B1 (en) Connector with base having channels to facilitate surface mount solder attachment
US7186926B2 (en) Surface mounting structure for surface mounting an electronic component
EP0971569A1 (en) Enhanced mounting pads for printed circuit boards
US20030056975A1 (en) Anti-tombstoning structures and methods of manufacture
EP2051376A1 (en) Quartz crystal device
US6410861B1 (en) Low profile interconnect structure
JPH0677632A (en) Circuit board
US7506438B1 (en) Low profile integrated module interconnects and method of fabrication
JP2002359103A (en) Chip type thermistor
US6114769A (en) Solder paste brick
JP6489037B2 (en) Electronic device and manufacturing method thereof
KR100338167B1 (en) Electrical contactor and its manufacturing method
CA2412030C (en) Perimeter anchored thick film pad
KR20180089407A (en) Stress Reduction Interposer for Ceramic Lead-Free Surface Mounted Electronic Devices
US6610430B1 (en) Method of attaching a device to a circuit board
MXPA99006496A (en) Optimized solder joints for surface mount chips
JP3823881B2 (en) Connection structure between circuit boards, formation method thereof, circuit board, and electronic component surface-mounted on mounting board
JPH08264928A (en) Pad for forming solder bump
JP2003007510A (en) Chip thermistor
JP2000195982A (en) Compact semiconductor device, its packaging structure, and manufacture of ceramic substrate
JPH11307309A (en) Chip thermister
JPH1140918A (en) Ceramics element, component-mounting board and wiring board
EP0888037B1 (en) Anti-tombstoning solder joints
JPH02114595A (en) Mounting of chip component

Legal Events

Date Code Title Description
WWE Wipo information: entry into national phase

Ref document number: 97181418.X

Country of ref document: CN

AK Designated states

Kind code of ref document: A2

Designated state(s): BR CA CN JP MX

AL Designated countries for regional patents

Kind code of ref document: A2

Designated state(s): AT BE CH DE DK ES FI FR GB GR IE IT LU MC NL PT SE

DFPE Request for preliminary examination filed prior to expiration of 19th month from priority date (pct application filed before 20040101)
121 Ep: the epo has been informed by wipo that ep was designated in this application
AK Designated states

Kind code of ref document: A3

Designated state(s): BR CA CN JP MX

AL Designated countries for regional patents

Kind code of ref document: A3

Designated state(s): AT BE CH DE DK ES FI FR GB GR IE IT LU MC NL PT SE

WWE Wipo information: entry into national phase

Ref document number: 1997947200

Country of ref document: EP

WWE Wipo information: entry into national phase

Ref document number: PA/a/1999/006496

Country of ref document: MX

ENP Entry into the national phase

Ref document number: 1998 534034

Country of ref document: JP

Kind code of ref document: A

ENP Entry into the national phase

Ref document number: 2278019

Country of ref document: CA

Ref document number: 2278019

Country of ref document: CA

Kind code of ref document: A

WWP Wipo information: published in national office

Ref document number: 1997947200

Country of ref document: EP

WWG Wipo information: grant in national office

Ref document number: 1997947200

Country of ref document: EP