WO2009139745A1

WO2009139745A1 - Application of viscous material by means of stationary printing head

Info

Publication number: WO2009139745A1
Application number: PCT/US2008/006142
Authority: WO
Inventors: Robert Gray; Robert Rauch
Original assignee: Siemens Electronics Assembly Systems, Llc; Siemens Electronics Assembly Systems Gmbh & Co. Kg
Priority date: 2008-05-14
Filing date: 2008-05-14
Publication date: 2009-11-19
Also published as: CN102027812A; CN102027812B

Abstract

It is described a printing station (110) for applying viscous material (113a) onto a substrate (185) for elec-tronic component assemblies. The printing station (110) comprises a conveying equipment (105), which is adapted to linearly move a workpiece holder (180) along a trans-fer direction (105a) in a continuous manner, wherein the workpiece holder (180) is adapted receive the substrate (185), and a stationary printing head (112), which is located adjacent the conveying equipment (105) in a fixed position. The stationary printing head (112) is adapted to apply the viscous material (113a) onto the substrate (185) while the workpiece holder (180) is moved by the conveying equipment (105). It is further described a printing arrangement comprising the printing station (110) and further working stations (120, 130, 140, 160, 170) and an electronic assembly line (390) comprising the printing station (110) and/or the printing arrangement (100). Furthermore, a method for applying a viscous mate-rial (113a) onto a substrate (185) for electronic compo-nent assemblies is described.

Description

APPLICATION OF VISCOUS MATERIAL¹ BY MEANS OF STATIONARY

PRINTING HEAD

Field of invention

The present invention relates to the field of manufacturing electronic component assemblies by means of electronic assembly lines. In particular, the present invention relates to a printing station and to a printing arrangement for applying a viscous material onto a substrate for electronic component assemblies. Further, the present invention relates to an electronic assembly line, which comprises a printing station and/or a printing arrangement as described above. Furthermore, the present invention relates to a method for applying a viscous material onto a substrate for electronic component assemblies .

Art Background

Electronic assembly lines for manufacturing electronic component assemblies typically comprise (a) a printing machine for applying soldering paste on a substrate, (b) a mounting machine for placing electronic components onto predefined positions on the substrate and (c) an oven for liquidizing the applied soldering paste. After a cool down of the soldering paste the placed electronic components will be firmly attached in a conductive manner to contact surfaces being provided at the substrate. High performance electronic assembly lines may comprise more than one printing machine, mounting machine and/or oven in order to achieve a high throughput. Thereby, the respective devices may be arranged in parallel or in serial with respect to a transfer direction. A known and well established technique for applying soldering paste and/or a conductive adhesive to a substrate such as a Printed Circuit Board (PCB) is the so called screen printing or stencil printing method. Thereby, first a substrate is loaded onto a workpiece holder, which is also called a stencil nest or support block. Second, the nest and substrate are presented to the stencil in an appropriate manner. Thereby, an alignment between the substrate and the stencil may be performed by employing an optical inspection. After having achieved a proper relative alignment the substrate and the stencil are fixed within the stencil nest for instance by means of an appropriate clamping mechanism. Thereafter, the actual printing procedure is accomplished. Before the printed substrates are available for being mounted with electronic components, the fixation within the clamping nest is released and the workpiece holder respectively the stencil nest and the stencil may be used for further printing procedures carried out with other substrates.

The above described measures for performing stencil printing require altogether a length in time which is at least approximately 10 seconds. However additional to this is the need to wipe the stencil which is usually done after a predetermined number of accomplished printing procedures. In many applications the predetermined number is for instance six. Provided that an effective process design is used for wiping the stencil at least another five seconds have to be added to the print cycle time on average. This results in a minimal printer cycle time of at least 15 seconds on average for a single sided substrate .

If a high performance electronic assembly line requires for instance a feed of two substrates every 18.5 seconds, then at least two printing devices are required per line for a single sided product respectively a single sided substrate. However in the case where the substrate has to be provided with a few electronic components also on the other side then the assembly line will need at least three printers. So all "high volume electronic assembly lines" will require at least three printing devices. For assembly lines that will run more complex products the cycle time demands may be less. However, if a known high volume high mix concept is employed, two different electronic component assemblies are produced at the same time. As a consequence, the "high volume high mix" electronic assembly line will require at least four printing devices .

Due to the increased complexity of modern electronic component assemblies more and more electronic assembly lines are converted to the "high volume high mix" concept. Therefore, there may be a need for providing a new concept for applying a viscous material such as soldering paste onto substrates by means of a stencil printing procedure. The new concept should allow for a significant speed up of the printing procedure without decreasing the printing accuracy, which in particular in view of the miniaturization of electronic components is still high.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention there is provided a printing station for applying a viscous mate- rial onto a substrate for electronic component assemblies. The provided printing station comprises (a) a conveying equipment, which is adapted to linearly move a workpiece holder along a transfer direction in a continuous manner, wherein the workpiece holder is adapted receive the substrate, and (b) a stationary printing head, which is located adjacent the conveying equipment in a fixed position with respect to the conveying equipment, wherein the printing head is adapted to apply the viscous material onto the substrate while the workpiece holder is moved by the conveying equipment .

The described viscous material applying station is based on the idea that a quasi continuous printing procedure may be realized by conveying the substrate in a continuous manner along a predetermined direction with respect to a stationary printing head. Thereby, the workpiece holder is fed towards the stationary printing head from a first direction and removed from the printing head in a second direction being opposite to the first direction. This may allow for a quasi continuous printing because when a first substrate is removed from the printing head a second substrate can already be fed to the printing head. By simultaneously removing a first substrate from and feeding a second substrate to the described viscous material application station the cycle time for process- ing a single substrate can be decreased significantly.

The viscous material may be in particular a soldering paste, which is supposed to be applied at predetermines positions of the substrate. After a defined melting proc- ess for instance by means of an reflow oven and a subsequent cooling down the soldering paste may provide a reliable electrical contact between a conductor path formed on the substrate and an electronic component being mounted adhered to the soldering paste. However, the viscous material may also be for instance a conductive adhesive, which is adapted to provide a reliable electrical contact between a conductor path and an electronic component without a temporarily melting process.

The usage of a stationary printing head may provide the advantage that apart from some moving components of the conveying equipment it is not necessary to employ any movable part in order to realize an appropriate viscous material deposition. This holds in particular in contrast to known techniques using a scraper in order to apply and/or to distribute a viscous material onto a substrate for electronic assemblies.

The usage of a stationary printing head may further pro- vide the advantage that a replenishment of the viscous material can be carried out without stopping the printing procedure. This may allow for an uninterrupted continuation of the printing procedure in case of a complete consumption of the viscous material being originally provided to the stationary printing head.

It has to be mentioned that the term "adjacent" simply means that the stationary printing head is situated in such a close proximity to the conveying equipment that the deposition of viscous material to the substrate can be realized in a spatially precise and defined manner. Specifically, the stationary printing head may be located sidewise, below or above the conveying equipment. However, the printing head preferably is located above the conveying equipment such that the application of viscous material is supported by the gravitational force.

According to an embodiment of the invention the stationary printing head comprises a dispenser opening having an elongated shape. This may provide the advantage that the stationary printing head can be used for simultaneously applying the viscous material along a linear line.

Preferably, the linear line respectively the longitudinal axis of the elongated shape is oriented perpendicular or at least angularly with respect to the transfer direction of the workpiece holder or the substrate. This may provide the advantage that with a single movement of the workpiece holder a two-dimensional area of the substrate can be provided with the viscous material. In particular, if the length of the linear line corresponds at least approximately with the width of the substrate, i.e. the lateral dimension of the substrate perpendicular to the transfer direction, the viscous material can be applied to the whole substrate while the substrate has to pass the printing head only one time.

According to a further embodiment of the invention the stationary printing head comprises a reservoir for the viscous material. The reservoir may represent a storage device for the viscous material . Thereby, the storage can be regularly refilled with the viscous material in order to avoid an exhaustion of the viscous material. This may provide the advantage that it is not necessary to inter- rupt the printing procedure when a predefined amount of viscous material has been used.

According to a further embodiment of the invention the stationary printing head comprises a heating element, which is adapted to heat up the viscous material. This may provide the advantage that by adjusting the temperature of the viscous material the viscosity of the viscous material can selected in an appropriate manner.

In this respect it is mentioned that if a hot viscous material hits the surface of the substrate, the viscous material will be cooled down. This holds at least if the temperature of the substrate is lower than the temperature of the viscous material. Further, a drop of the viscous material temperature will cause an increase of the viscosity of the viscous material such that the form stability of the deposited viscous material increases. Thereby, viscous material depositions having at least approximately a block shape can be realized.

According to a further embodiment of the invention the conveying equipment is adapted for supporting the substrate being received by the workpiece holder such that a bending of the substrate is prevented at least partially.

The support of the substrate may be realized by means of a two-dimensional or at least a quasi two-dimensional support mechanism. The support mechanism may be for instance an inflatable balloon, which may provide for a uniform support pressure being applied to the bottom side of the substrate. In this respect a quasi two-dimensional support may be realized for instance by means of a plurality of supporting pins, which are pressed against the bottom side of the substrate. A bending of the substrate during the printing procedure can be effectively pre- vented if the support pins are distributed over the full area of the substrate.

In order to achieve a temporarily and/or a spatially smooth course of the support force, the support mechanism can be transferred by the conveying equipment in the same way as the workpiece holder.

According to a further embodiment of the invention the workpiece holder is adapted to further receive a stencil. The stencil may be accommodated within the workpiece holder directly above the substrate. Thereby, the stencil can be used in a known manner in order to realize a spatially selective viscous material deposition to the substrate surface. In the corresponding stencil printing procedure the viscous material will only be applied to the substrate within regions, wherein the stencil comprises an opening respectively a recess.

The thickness respectively the height of the stencil may determine the height of the resulting viscous material application. Since the lateral extensions of the openings within the stencil are determined by the layout of the substrate, the height of the stencil also determines the volume of the viscous material deposition.

According to a further embodiment of the invention the workpiece holder comprises a clamping mechanism for de- tachably fixing the substrate and the stencil with respect to each other. The clamping mechanism may be realized by means of for instance a locking screw and/or a resilient element. The resilient element may be for instance a spring element and/or a rubber element.

Preferably, the clamping mechanism is adapted to also secure the workpiece holder to the substrate and/or to the stencil. Thereby, the workpiece holder, the substrate and the stencil represent a mechanically stiff system, which, during the stencil printing procedure, passes the stationary printing head.

According to a further aspect of the invention there is provided a printing arrangement for applying a viscous material onto a substrate for electronic component assemblies. The printing arrangement comprises (a) a printing station as described above, (b) a loading station, which is arranged upstream with respect to the printing station and which is adapted to transfer the substrate to the workpiece holder, (c) an unloading station, which is arranged downstream with respect to the printing station and which is adapted to remove the substrate from the workpiece holder, and (d) a re-feeding path, which ex- tends between the unloading station and the loading station and which is adapted to return the workpiece holder from the unloading station to the loading station.

This aspect of the invention is based on the idea that a closed loop process can be established for the workpiece holders. Preferably, the described printing arrangement is equipped with a plurality of workpiece holders such that the printing process can be carried out with a high performance. Thereby, at a certain point in time a first workpiece holder can be situated at the loading station, a second workpiece holder can be situated at the printing station, a third workpiece holder can be situated at the unloading station and a forth workpiece holder can be situated at the re-feeding path. After the printing pro- cedure has been finished, i.e. the respective substrate on the second workpiece holder has completely passed the stationary printing head, all workpiece holders move to the next station of the closed loop for the workpiece holders .

The terms "upstream" and "downstream" are used with respect to the moving direction of the conveying equipment respectively with respect to the moving direction of the workpiece holder. Since each workpiece holder acts as a carrier for the substrate and/or a stencil the workpiece holders could also be named carrier elements.

It has to be mentioned that the re-feeding path might also be adapted to transfer used stencils from the unloading station to the loading station. This may pro- vide the advantage that a closed loop process can also be established for the stencils. In order to provide for a printing procedure with a small cycle time at least one stencil should be available for each workpiece holder. Thereby, also for the stencils a high efficient closed loop transfer can be established.

The described printing arrangement may further provide the advantage that different process steps, which are associated with a reliable printing procedure, can be carried out simultaneously. Thereby, different components of the printing arrangement may perform the different steps in a parallelized manner.

According to a further embodiment of the invention the loading station comprises a means for aligning the substrate with a stencil . This may provide the advantage that an automatic alignment between the substrate and the stencil can be realized. Thereby, any suitable handling equipment for the substrate and/or the stencil can be used. The handling system may comprise for instance a robot arm and/or a gripper arm.

The process of aligning the stencil with the substrate can be monitored by an appropriate vision system. The vision system may evaluate the position of fiducials being provided at the substrate and/or the stencil. The vision system may also be used for performing a closed loop .alignment operation, wherein the vision system pro- vides a feedback control parameter of the relative position between the substrate and the stencil.

It has to be mentioned that the loading unit may further comprise a unit for controlling a clamping mechanism of the workpiece holder. This may provide the advantage that after a successful alignment of the stencil with respect to the substrate the substrate and the stencil can be automatically firmly fixed to the workpiece holder.

In this respect it is further mentioned that the unload- ing station might be equipped with a corresponding un- clamping mechanism, which has to be activated before the substrate can be released from the workpiece holder.

According to a further embodiment of the invention the printing arrangement further comprises a cleaning station, which is located at the re-feeding path and which is adapted for performing a cleaning procedure with a stencil and/or with the workpiece holder. This may provide the advantage that components, which might have been contaminated in particular with the viscous material can be cleaned before being reused for a further printing procedure .

According to a further embodiment of the invention the printing arrangement further comprises an identification unit for identifying an individual stencil from a plurality of stencils and/or for identifying an individual stencil from a plurality of stencils and/or for identifying a workpiece holder from a plurality of workpiece holders and/or for identifying a substrate from a plurality of substrates. This may provide the advantage that the number of accomplished printing procedures can be recorded for each stencil. This can be done for instance by means of a control unit of the printing arrangement, wherein the control unit is coupled with the identification unit.

The identification unit may further be coupled directly or indirectly for instance via the control unit with the cleaning station. Thereby it can be easily realized to perform a cleaning procedure of the stencil only after the stencil has been used for a predetermined number of printing procedures. This may provide the advantage that a cleaning procedure can be carried out automatically only in that case, when it is expected that a cleaning is necessary.

The identification of the individual stencil may be based on any suitable identification element being provided at the respective stencil. The identification element may be for instance a one-dimensional or a two-dimensional barcode, which can be detected by an appropriate barcode reader. The identification element may also be a Radio Frequency Identification (RFID) Chip, which is attached to the respective stencil . In that case the identifica- tion unit should comprise an appropriate RFID reading device . The usage of an RFID Chip may further provide the advantage that each stencil, which is provided with an RFID Chip containing a unique RFID code, can be traced within the described printing arrangement. Such a trace- ability is in particular advantageous if the described printing arrangement interacts with a plurality of stencils.

It has to be mentioned that the described usage of suit- able identification elements may also allow to trace different types of stencils and/or different types of substrates. This may provide the advantage that within a printing arrangement, which is used for a "high mix" electronic assembly line, the usage and/or the current position of each individual stencil can be monitored.

Further, identification elements being provided at different substrates can be used for a traceability of the whole manufacturing process of electronic component assemblies . According to a further embodiment of the invention the printing arrangement further comprises a buffer station, which is located at the re-feeding path and which is adapted for temporarily storing at least one stencil and/or workpiece holder being returned from the unloading station to the loading station. This may provide the advantage that a plurality of stencils and/or workpiece holders might be stored in a stand-by manner. Thereby, the buffer station might be adapted to store different types of stencils and/or workpiece holders. The stencils and/or the workpiece holders might be retrieved from the buffer station when a certain substrate is going to be fed to the loading station, which substrate requires the corresponding stencils and/or the workpiece holders.

The buffer station might also be realized as a so called first in, first out (FIFO) buffer which releases the stencils and/or workpiece holders in the same order as they are put in storage. A FIFO buffer may provide the advantage that the buffer station can be realized in a comparatively simple mechanical design.

The FIFO buffer can be arranged between the above described cleaning station and the loading unit. This may provide the advantage that after a cleaning procedure the stencils and/or workpiece holders are kept within the buffer station at least for a predetermined period of time, during which for instance cleaning chemicals can be evaporated.

According to a further embodiment of the invention the printing arrangement further comprises an inspection station, which is located downstream with respect to the unloading station and which is adapted to perform an inspection of the viscous material application. The inspection station might comprise an optical system, which is capable of measuring the lateral size and/or the height of the viscous material application. The optical system might be for instance a so call Automatic Optic Inspection system, which is capable of performing two- dimensional and/or three-dimensional measurements of the viscous material deposition.

It has to be mentioned that apart from being capable to evaluate the quality of the viscous material application the provision of the described inspection station allows for a cooling down of the applied viscous material in case the viscous material is hot when it is applied to the substrate.

According to a further aspect of the invention there is provided an electronic assembly line for manufacturing electronic component assemblies. The provided electronic assembly line comprises (a) a printing station as de- scribed above and/or a printing arrangement as described above .

This aspect of the invention is based on the idea that the above described printing station and/or the above described printing arrangement can be used in order to speed up the manufacturing process for electronic component assemblies. Thereby, the printing process, which in known electronic assembly lines typically exhibits a limiting factor for reducing the cycle time in the elec- tronic component assembly manufacturing process, can be speeded up significantly without using a complex and costly parallel arrangement of a plurality of individual substrate printing stations.

The electronic assembly line may further comprise at least one automatic mounting machine for placing elec- tronic components onto predefined positions on the substrate. In case the electronic assembly line comprises a plurality of different mounting machines these mounting machines can be arranged in a sequential manner along the conveying equipment and/or along a successive conveying equipment of the conveying equipment described above with reference to the described printing station. However, at least some of the different mounting machines can also be arranged in a parallel manner. This means that the printed substrates, which are provided by the above described printing station and/or by the above described printing arrangement, are distributed along different conveying paths respectively different conveying equipments .

The electronic assembly line may further comprise an oven such as a reflow oven for liquidizing the applied soldering paste, such that after a cool down of the soldering paste the placed electronic components will be firmly attached in a conductive manner to contact surfaces being provided at the substrate.

According to a further aspect of the invention there is provided a method for applying a viscous material onto a substrate for electronic component assemblies. The provided method comprises (a) linearly moving a workpiece holder along a transfer direction in a continuous manner by means of a conveying equipment, wherein the substrate is received by the workpiece holder, and (b) applying the viscous material onto the substrate while the workpiece holder is moved by the conveying equipment by means of a stationary printing head, which is located adjacent the conveying equipment in a fixed position with respect to the conveying equipment . Also this aspect of the invention is based on the idea that a quasi continuous printing procedure can be realized by conveying the substrate in a continuous manner along a predetermined direction with respect to a sta- tionary printing head. Thereby, the workpiece holder is fed towards the stationary printing head from a first direction and removed from the printing head in a second direction being opposite to the first direction. This may allow for a quasi continuous printing because, when a first substrate is removed from the printing head, a second substrate can already be fed to the printing head. By simultaneously (a) removing a first substrate from the described viscous material applying station and (b) feeding a second substrate to the described viscous material applying station the cycle time for processing a single substrate can be decreased significantly.

In case the viscous material is applied to the substrate under heated conditions the usage of a stationary print - ing head may provide the advantage that the substrate is exposed to precisely defined thermal treatment. Specifically, when an arbitrary surface point of the substrate is moved towards the stationary printing head the temperature of this surface point will increase as the sur- face point approaches the printing head. When the viscous material is applied to the surface point of the substrate the temperature will reach a maximum value. Thereafter, when the surface point departs from the printing head the temperature will again drop. This means that by contrast to known solder paste application techniques, which require a repeated backward and forward movement of a movable scraper, the temperature behavior of an arbitrary substrate surface point simply shows at first a single temperature increase and later a single temperature de- crease. This may provide the advantage that due to pre- cisely defined thermal conditions the whole printing process can be performed under defined conditions.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method type claims whereas other embodiments have been described with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the method type claims and features of the apparatus type claims is considered as to be disclosed with this application.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment . The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

Brief Description of the Drawings

Figure 1 shows in a schematic representation a printing arrangement comprising a printing station with a stationary printing head for applying solder paste onto a substrate being carried by a workpiece holder.

Figure 2 shows in a sectional view a workpiece holder comprising a clamping mechanism for detachably securing a substrate, a stencil and the workpiece holder relative to each other.

Figure 3 shows an electronic assembly line comprising the printing arrangement as shown in Figure 1.

Figure 4 shows a diagram depicting a substrate printing procedure with a feedback loop for workpiece holders and stencils, wherein the feedback loop optionally includes a stencil cleaning procedure.

Detailed Description

The illustration in the drawing is schematically and often not to scale. It is noted that in different figures, similar or identical elements are provided with the same reference signs .

Figure 1 shows a printing arrangement 100, which com- prises a plurality of working stations 120, 110, 130, 140, 170 and 160. The core of the printing arrangement 100 is a printing station 110 and a conveying equipment 105. The conveying equipment 105 is adapted to linearly move workpiece holders 180 along a transfer direction 105a in a continuous manner. Each workpiece holder 180 is adapted to receive a substrate respectively a PCB 185, which is supposed to be printed with a solder paste 113a. The workpiece holder 180 is adapted to receive a stencil, which is located directly above the substrate 185. The stencil comprises a plurality of openings, which allow solder paste to pass through and to be deposited within predetermined regions on the upper surface of the substrates 185.

The printing procedure is carried out by means of a stationary printing head 112, which is located adjacent the conveying equipment 105 in a fixed position with respect to the conveying equipment 105. The stationary printing head 112 is adapted to apply solder paste 113a representing a viscous material 113a onto the respective substrate 185 while the workpiece holder 180 is moved by the conveying equipment 105.

The stationary printing head 112 comprises a reservoir 113 for solder paste 113a. The reservoir 113 represents a storage tank for the solder paste 113a. The stationary printing head 112 further comprises a heating element 115. According to the embodiment described here the heating element is a heating coil 115, which is adapted to heat up the solder paste 113a before it is transferred to the upper surface of the respective substrate 185. Since the viscosity of the solder paste 113a exhibits a distinct temperature dependency, the whole solder paste transfer process can be adjusted by activating the heating element 115 in an appropriate manner. In particular, when the hot solder paste 113a gets into contact with the cooler substrate 185, a better printing image can be achieved. This is based on the fact that in the hot state the solder paste 113a comprises a lower viscosity. When the solder paste 113a gets in touch with the cooler sub- strate 185, the viscosity of the solder paste 113a will quickly increase such that the form stability increases. Thereby, block shaped solder paste depositions can be achieved.

According to the embodiment described here the solder paste 113a is transferred from the printing head 112 to the substrate 185 via a dispenser opening 114, which has an elongated shape. The elongated shape is oriented perpendicular to the transfer direction 105a. The length of the elongated shape corresponds approximately with the lateral width of the substrate 185, such that the viscous material can be applied over the whole substrate surface while the substrate is passing the printing head 112.

In order to establish a closed loop for the workpiece holders 180 and the stencils, a re-feeding path respectively a return path comprising a plurality of sections 150a-f extends from an unloading station 130 to a loading station 120. The loading station 120 is arranged upstream with respect to the printing station 110. The unloading station 130 is arranged downstream with respect to the printing station 110.

The loading station 120 is used for transferring a sub- strate 185, which has been fed to the loading station 120 along a transfer direction 185a, to a workpiece holder 180, which has been fed to the loading station 120 along the path section 150c. The workpiece holder is equipped with a stencil, which for the sake of clarity is not depicted in Figure 1. The loading station 120 comprises an alignment means 122 for aligning the substrate with a stencil. According to the embodiment described here the alignment means comprises a handling system 122 having a robot arm 122a. The robot arm 122a is adapted for placing the stencil and the substrate 185 relative to each other in a predefined spatial position, which position is determined by the openings of the stencil and the regions on the substrate surface, on which the solder paste 113a is supposed to be deposited. For aligning the stencil with the substrate 185 an optical inspection system may be used in order to precisely determine the relative position between the stencil and the substrate 185. Thereby, the current linear and angular positions of the substrate 185 and the stencil are measured by the optical inspection system. The robot arm 122a may further be adapted to activate a clamping mechanism, which will be described in more detail later with reference to Figure 2. With the clamping mechanism the stencil, the substrate 185 and the work- piece holder 180 can be fixed to each other in a detachable manner. When being fixed to each other the workpiece holder 180, the substrate 185 and the stencil represent a mechanically stiff system, which, during the stencil printing procedure, passes the stationary printing head 112.

Downstream with respect to the loading station 120 is located the above described printing station 110, in which the stencil printing procedure is accomplished.

Downstream with respect to the printing station 110 is located the unloading station 130. The unloading station 130 is equipped with a non depicted mechanism for un- clamping the substrate 185 from the workpiece holder 180. After having released the connection between the substrate 185 and the workpiece holder 180, the substrate is transferred to an inspection station 170 along a transfer direction 185b. According to the embodiment described here the transfer direction 1485b is the same as the transfer direction 105a of the conveying equipment 105.

The inspection station 170 comprises an optical inspection system 172 for performing an optical inspection of the size, the form and/or the volume of the solder paste deposition (s) . In case the solder paste depositions complies with given requirements, the corresponding substrate 185, which is now ready for receiving electronic components, is output from the described printing arrangement 100 along a transfer direction 185c. If the solder paste depositions do not comply with given re- quirements, the corresponding substrate 185 can be discarded.

According to the embodiment described here the unloading station 130 comprises an identification unit 132 for identifying an individual stencil from a plurality of stencils. Thereby, an identification element such as a barcode and/or a Radio Frequency Identification (RFID) Chip can be recognized by the identification unit 132. The recognition of a stencil can be used for recording the number of accomplished printing procedures for each stencil. Thereby, a not depicted control unit of the printing arrangement 100 can be employed.

As can be seen from Figure 1, the described printing arrangement further comprises a cleaning station 140. If for a particular stencil, which has been transferred from the printing station 110 to the unloading station 130, the number of so far accomplished printing procedures exceeds a predetermined number of for instance six, the particular stencil together with the workpiece holder 180 is preliminarily transferred to the cleaning station 140 along the transfer direction 140a. In the cleaning station 140, in particular solder paste material, which adheres to the stencil and/or the workpiece holder 180, is removed. Thereby, appropriate chemical and/or mechanical cleaning procedures can be carried out. After having finished the described cleaning procedure, the particular stencil together with the workpiece holder 180 is for- warded along a transfer direction 140b to a buffer station 160.

For the sake of completeness it is mentioned that in case the number of accomplished printing procedures carried out after a preceding cleaning procedure does not exceed the predetermined number, a cleaning procedure is considered to be not necessary and the particular stencil together with the workpiece holder 180 is directly transferred to the buffer station 160 along the re- feeding path section 150a.

In the buffer station 160 the workpiece holder 180 together with the stencil is stored into a first in, first out (FIFO) buffer 165. The corresponding sections of the return path into the FIFO buffer 165 are indicated with reference numerals 150b and 150c. As can be seen from Figure 1, the FIFO buffer is adapted to receive a plurality of workpiece holders 180 in a vertical stack.

In order to close the re-feeding path the workpiece holders 180 are removed from the FIFO buffer 165 and transferred to the loading station 120 along the sections 15Oe and 15Of of the re-feeding path.

It has to be mentioned that it is also possible to arrange a buffer and/or a further buffer at other locations within the printing arrangement 100. In particular, if a buffer is arranged between the printing station 110 and the unloading station 130, the time span between printing the substrate 185 and separating the substrate 185 from the stencil and/or from the workpiece holder can be extended. Thereby, a better printing result may be achieved.

It has to be further mentioned that the described printing arrangement 100 can also be used in an advantageous manner if different types of stencils and/or substrates are simultaneously treated. In that case the above described technique of identifying the individual stencils for instance by means of RFID chips can be extended to an identification of individual substrates. Thereby, also a traceability of substrates and/or circuit arrangements being later mounted on the substrates can be achieved.

The described printing arrangement may provide the advan- tage that by using a plurality of workpiece holders 185 the operatic cycles in the various working stations 110, 120, 130, 140, 170 and 160 can be performed simultaneously. In other words, by changing the process flow of a standard printer in such a manner that a mass paralleli- zation of the process is allowed, different steps being associated with the printing procedure can be carried out in parallel such that a much faster cycle time can be achieved. Further, by applying the described concept of a stationary printing head, a forward and backward movement of a scraper and/or the substrate together with the stencil can be avoided. Thereby, a minimal batch size of one can be achieved without loosing performance of the described printing procedure.

Figure 2 shows in a sectional view the workpiece holder, which is now denominated with reference numeral 280. The workpiece holder 280 comprises a chassis 281. The chassis 281 comprises a lower flange 281b and an upper flange 281a. The workpiece holder 280 is adapted to receive a substrate 285 and a stencil 283. As can be seen in Figure 2, the substrate 285 and the stencil 283 can be de- tachably affixed to the upper flange 281a by means of a clamping element 288. According to the embodiment described here the clamping element is a clamping plate 288.

In the fixed state, the clamping plate 288 is pressed upwards against the substrate 285. The upwards directed pressure is generated by a resilient clamping mechanism 287. According to the embodiment described here the clamping mechanism comprises springs 287.

In order to unclamp the substrate 285 and/or the stencil 283 from the workpiece holder 280 an appropriate handling mechanism can be used in the unloading station 130. This handling mechanism might engage with the clamping plate 288 and moves the clamping plate 288 against the spring force downward in order to deblock the substrate 285 and/or the stencil 283.

Figure 3 shows an electronic assembly line 390. The electronic assembly line 390 comprises the printing arrangement shown in Figure 1, which is now denominated with reference numeral 300. Downstream with respect to the printing arrangement 300 there are arranged a plurality of electronic component mounting machines 392. Following the mounting machines 392 there is provided a reflow oven 394. Following the reflow oven 394 there is provided an output station 396 for outputting the electronic component assemblies, which have been manufactured by the printing station 300, the electronic component mounting machines 392 and the reflow oven 394.

Figure 4 shows a diagram depicting a substrate printing procedure according to a preferred embodiment of the present invention. The substrate printing procedure comprises a feedback loop for workpiece holders and stencils. The feedback loop optionally includes a stencil cleaning procedure.

From the diagram one can see that the whole printing process has undergone a "Taylorism" division of labor. In the diagram one can see on the left hand margin the dif- ferent working stations of Figure 1, which are now denominated with the reference numerals 420, 410, 430, 470, 460 and 440. The different process steps, which will be described below and which for a particular workpiece holder are performed one after the other, are respectively assigned to one of the working stations 420, 410, 430, 470, 460 or 440. However, since a plurality of work- piece holders together with the corresponding substrates and stencils can be employed, the process steps can also be carried out parallel to each other since each major process area is assigned to its own working station rep- resenting a mini machine of the printing arrangement 100. Further, due to the described "Taylorism" in the printing arrangement a replenishment of other consumables for cleaning are possible without stopping the printing process .

As can be seen from Figure 4, the printing process starts at the circle denominated with "begin" . The following steps are carried out by the loading station 420:

A first step S21 comprises an unclamping of the workpiece holder and a loading of a substrate respectively a

Printed Circuit Board (PCB) into the workpiece holder. In the following, the workpiece holder, which is adapted to receive both a substrate and a stencil, will also be denominated a "nest". Next, in a step S22, the substrate and a stencil both being received by the nest are aligned with respect to each other. Further, the substrate and the stencil are clamped to the nest. The subsequent step S23 comprises a transfer of the nest assembly to an input buffer (not depicted in Figure 1) of the printing station 410.

The following steps are carried out by the printing station 410:

In a step SIl, the assembled nest is loaded into the input buffer of the printing station 410. In a subsequent step S12, the assembled nest is mated to a downstream nest, which is already present in the printing station. The assembled nest is following the downstream nest along the transfer direction of the conveying equipment with the print speed of the downstream nest. In a subsequent step S13, the assembled nest is passed under the printing head at a predetermined speed. Thereby, the solder paste deposition is accomplished. In a subsequent step S14, the assembled nest with the printed substrate is transferred to the unloading station 430.

The following steps are carried out by the unloading station 430:

In a step S31, the assembled nest with the printed sub- strate is loaded from the printing station 410 into the unloading station 430. In a subsequent step S32, the substrate is undamped from the nest. This is done at a determined snap off speed. In a subsequent step S33, the printed substrate is transferred to the inspection sta- tion 470. In a subsequent step S34, the nest (together with the stencil) is transferred to the buffer station 460, which in the following might also be denominated a return station.

In the following paragraph steps will be described, which are carried out with the printed substrate at the inspection station 470:

In a step S71, the printed substrate is loaded by the inspection station 470 from the unloading station 430. In a subsequent step S72, the deposited solder paste in inspected. Thereby, an automatic optical inspection system for measuring the lateral and the vertical extension of various solder paste deposits can be used. In a step S73, the measurement results are stored in an inspection database. In a step S74, the measured printed substrate is transferred to a machine being arranged downstream. This machine may be in particular an electronic component mounting machine. Therewith, the printing procedure of the substrate ends. This is indicated in Figure 4 with "end".

In order to establish a closed loop for the nest respectively for the workpiece holder, the buffer station 460 becomes active. The following steps described in the next and in the next but one paragraph are carried out by the buffer station 460:

In a step S61, the nest together with the stencil and released from the substrate is loaded by the buffer station 460 from the unloading station 430. In a subsequent step S62 it is determined whether after a preceding cleaning procedure the stencil has already been used six times for further printing procedures.

If the stencil has been used less than six times, in a step S63 the nest together with the stencil is transferred to the nest buffer, which is denominated in Figure 1 with the reference numeral 165. In a subsequent step S64, if the waiting time in the FIFO buffer 165 has been elapsed, the nest together with the stencil is trans- ferred to the loading station 420.

In a corresponding subsequent step S24, which is carried out by the loading station 420, the nest together with the stencil is loaded into the loading station 420.

If in the above described step S62 it is determined, that since the last cleaning procedure the stencil has already been used for six times, the nest together with the stencil is transferred to the cleaning station 440. This step is indicated with S41. In a subsequent step S42, the stencil is cleaned by the cleaning station 440. After the cleaning procedure, wherein in particular residual solder paste is removed from the stencil, in a step S43, the nest together with the cleaned stencil is inserted into a further buffer. This buffer, which is not depicted in Figure 1, is assigned to the cleaning station 440.

Thereby, the nest together with the cleaned stencil is inserted at a correct position with respect to other nests being stored in the buffer. Thereafter, the nest together with the cleaned stencil is transferred to the FIFO buffer 165. This is indicated with the step S63, which has already been described above.

Due to the described parallelization of different tasks with the working stations 420, 410, 430, 470 and 460, within each station a short cycle time of less than six seconds can be achieved. In case a cleaning procedure has to be performed, the resulting cycle time is extended to about 8.5 seconds. This is indicated in the upper margin of Figure 4.

The substrate printing procedure described within this application may provide for various advantages, which are listed in the following:

• The overall printing process is divided into different procedures, which are carried out in different working stations .

• These different working stations can perform their functions in parallel to all other working stations.

• The substrate is loaded into a stencil nest. Thereby, a "support block" can be used, which is an integral part of the stencil. The support block may comprise an actuated "spring loading" mechanism.

• The loaded stencil nest is conveyed under the stationary printing head in a predetermined manner to optimize process performance, which allows for a continued flow of printing.

• Al nests are cycled through the printing arrangement. Thereby it is possible to allow nests to be added or removed from a nest buffer system. Thereby, even a batch size of only one may be realized.

• Stencil cleaning can be performed in parallel to the major substrate printing process.

• Individual working stations can be replaced independ- ently allowing for high uptime through offline preventative and corrective maintenance.

• It is possible to arrange a loading of two substrates per stencil nest.

• With the described substrate printing concept it is possible to print two substrates within six seconds.

Thereby, also multiple stencil images are allowed to be produced at one time. This can satisfy the requirements of a high volume and high mix concept for manufacturing electronic component assemblies.

It should be noted that the term "comprising" does not exclude other elements or steps and "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims . List of reference signs:

100 printing arrangement

105 conveying equipment 105a transfer direction

110 printing station

112 stationary printing head

113 reservoir / storage tank

113a viscous material / solder paste 114 dispenser opening

115 heating element / heating coil 120 loading station

122 alignment means / handling system 122a robot arm 130 unloading station

132 identification unit 140 cleaning station

140a transfer direction (before cleaning procedure) 140b transfer direction (after cleaning procedure) 150a-f sections of re-feeding path / sections of return path 160 buffer station / return station

165 first in, first out buffer

170 inspection station

172 optical inspection system

180 workpiece holder / carrier

185 substrate / PCB

185a transfer direction (loading substrate)

185b transfer direction (unloading substrate)

185c transfer direction (output substrate)

280 workpiece holder / carrier

281 chassis

281a upper flange

281b lower flange 283 stencil

285 substrate / PCB

287 clamping mechanism / spring

288 clamping element / clamping plate

300 printing arrangement

390 electronic assembly line

392 mounting machines 394 reflow oven 396 output station

410 printing station

420 loading station

430 unloading station 440 cleaning station

460 buffer station / return station

470 inspection station

Claims

CLAIMS :

1. A printing station for applying a viscous material (113a) onto a substrate (185) for electronic component assemblies, the printing station (110) comprising

• a conveying equipment (105) , which is adapted to linearly move a workpiece holder (180) along a transfer direction (105a) in a continuous manner, wherein the workpiece holder (180) is adapted receive the substrate (185) , and

• a stationary printing head (112), which is located adjacent the conveying equipment (105) in a fixed position with respect to the conveying equipment (105) , wherein the stationary printing head (112) is adapted to apply the viscous material (113a) onto the substrate (185) while the workpiece holder (180) is moved by the conveying equipment (105) .

2. The printing station as set forth in the preceding claim, wherein the stationary printing head (112) comprises a dispenser opening (114) having an elongated shape.

3. The printing station as set forth in any one of the preceding claims, wherein the stationary printing head (112) comprises a reservoir (113) for the viscous material (113a) .

4. The printing station as set forth in any one of the preceding claims, wherein the stationary printing head (112) comprises a heating element (115) , which is adapted to heat up the viscous material (113a) .

5. The printing station as set forth in any one of the preceding claims, wherein the conveying equipment (105) is adapted for supporting the substrate (185) being received by the workpiece holder (180) such that a bending of the substrate (185) is prevented at least partially.

6. The printing station as set forth in any one of the preceding claims, wherein the workpiece holder (180, 280) is adapted to further receive a stencil (283) .

7. The printing station as set forth in the preceding claim, wherein the workpiece holder (180, 280) comprises a clamping mechanism (287, 288) for detachably fixing the substrate (185, 285) and the stencil (283) with respect to each other.

8. A printing arrangement for applying a viscous material (113a) onto a substrate (185) for electronic component assemblies, the printing arrangement (100) comprising

• a printing station (110) as set forth in any one of the preceding claims,

• a loading station (120) , which is arranged upstream with respect to the printing station (110) and which is adapted to transfer the substrate (185) to the workpiece holder (180) ,

• an unloading station (130) , which is arranged downstream with respect to the printing station (110) and which is adapted to remove the substrate (185) from the workpiece holder (180) , and

• a re-feeding path (150a-150f ) , which extends between the unloading station (130) and the loading station (120) and which is adapted to return the workpiece holder (180) from the unloading station (130) to the loading station (120) .

9. The printing arrangement as set forth in claim 8, wherein the loading station (120) comprises means (122) for aligning the substrate (185) with a stencil (283) .

10. The printing arrangement as set forth in any one of the claims 8 to 9, further comprising

• a cleaning station (140) , which is located at the re- feeding path (150a-150f) and which is adapted for per- forming a cleaning procedure with a stencil (283) and/or with the workpiece holder (180) .

11. The printing arrangement as set forth in any one of the claims 8 to 10, further comprising, • an identification unit (132)

- for identifying an individual stencil (283) from a plurality of stencils and/or

- for identifying a workpiece holder (180, 280) from a plurality of workpiece holders and/or - for identifying a substrate (185, 285) from a plurality of substrates .

12. The printing arrangement as set forth in any one of the claims 8 to 11, further comprising • a buffer station (160) , which is located at the re- feeding path (150a- 15Of) and which is adapted for temporarily storing at least one stencil (283) and/or workpiece holder (180, 280) being returned from the unloading station (130) to the loading station (120) .

13. The printing arrangement as set forth in any one of the claims 8 to 12, further comprising

• an inspection station (170) , which is located downstream with respect to the unloading station (130) and which is adapted to perform an inspection of the viscous material application.

14. An electronic assembly line for manufacturing elec- tronic component assemblies, the electronic assembly line (390) comprising

• a printing station (110) as set forth in any one of the claims 1 to 7 and/or

• a printing arrangement (100) as set forth in any one of the claims 8 to 13.

15. A method for applying a viscous material (113a) onto a substrate (185) for electronic component assemblies, the method comprising • linearly moving a workpiece holder (180) along a transfer direction (105a) in a continuous manner by means of a conveying equipment (105) , wherein the substrate (185) is received by the workpiece holder (180), and

• applying the viscous material (113a) onto the substrate (185) while the workpiece holder (180) is moved by the conveying equipment (105) by means of a stationary printing head (112) , which is located adjacent the conveying equipment (105) in a fixed position with respect to the conveying equipment (105) .