WO1999056889A1

WO1999056889A1 - Method and apparatus for dispensing small amounts of liquid material

Info

Publication number: WO1999056889A1
Application number: PCT/US1999/009388
Authority: WO
Inventors: James W. Messerly; Laurence B. Saidman; James C. Smith
Original assignee: Nordson Corporation
Priority date: 1998-05-01
Filing date: 1999-04-30
Publication date: 1999-11-11
Also published as: JP3641432B2; CA2330953C; JP2002513674A; US6253957B1; DE69912762T2; AU3873399A; DE69912762D1; EP1073527B1; EP1073527A1; CA2330953A1

Abstract

Apparatus and methods for dispensing droplets of liquid or viscous material. The apparatus generally comprises a valve operated dispenser (10) and a control (128', 129) for moving the valve member (42') with respect to a valve seat (38') in rapid succession. This rapidly accelerates liquid or viscous material in a stream from the dispenser outlet and immediately breaks the stream into a minute droplet. Various embodiments of the valve seat (38') include both rigid valve seats and resilient valve seats. Resilient valve seats are especially useful for dispensing solder pastes as they can prevent material flaking, compacting and clogging conditions. Low friction polymer or plastic material for dispenser components such as the valve member (42') and outlet structure (40') can also prevent such problems.

Description

METHOD AND APPARATUS FOR DISPENSING SMALL AMOUNTS OF LIQUID MATERIAL

Field of the Invention

This invention relates to the field of dispensing liquid materials,

and more particularly, to a method and apparatus for rapidly dispensing

minute amounts of viscous material, such as adhesives, solder fluxes,

solder pastes or other such materials. These materials are generally

dispensed in small quantities during the assembly of, for example, electronic

components and printed circuit boards. It will be appreciated that the

invention has broader applications and may be advantageously employed in

other industries as well.

Background of the Invention

There are three general types of printed circuit (PC) boards. A

surface mount board utilizes components that may be secured to a surface

of the PC board by an adhesive or by a solder paste. Boards with

adhesively secured components are usually sent through a wave solder

machine to complete the electrical connections. When solder paste is used

to secure components to the board, the solder paste is heated, reflowed

and cured to both secure the components to the board and complete the

electrical connections. The second type of board uses through hole

components. As the name implies, these electrical components have leads

that extend through holes or openings in the board. The leads are soldered -2- to complete the electrical connections. In a mixed technology board, a

combination of surface mount components and through hole components

are used and generally manufactured by combining the methods described

above.

In each manufacturing method, a soldering operation is

required on one surface of the board. The entire soldering process is

comprised of three general steps which are normally performed by a single

machine. These steps include (i) flux application, (ii) preheating the board,

and (iii) soldering. Soldering flux is generally defined as a chemically and

physically active formula which promotes wetting of a metal surface by

molten solder, by removing the oxide or other surface films from the base

metals and the solder. The flux also protects the surfaces from reoxidation

during soldering and alters the surface tension of the molten solder and the

base metal. A printed circuit board must be cleaned with flux to effectively

prepare the board for soldering with a lead based or other metal based

solder paste.

In the manufacture of printed circuit boards or other products,

it is frequently necessary to apply minute amounts or droplets of liquid

materials, including solder flux and solder paste, to a substrate or

workpiece. These droplets can be on the order of 0.10 inch diameter and

less. Such materials can generally have a viscosity greater than 25,000

centipoise and in the case of solder pastes, for example, may have a

viscosity of 300,000 centipoise or above. These liquid and viscous -3- materials, besides solder flux and solder paste, include adhesives, solder

mask, grease, oil, encapsulants, potting compounds, inks, and silicones.

Methods of applying minute drops of liquid or viscous material

have, for example, relied on syringes or other positive displacement devices.

Typically, as discussed in U.S. Patent No. 5,320,250, syringe dispensers

place the syringe tip of the dispenser very close to the substrate. This may

be a distance of 0.005 inches for a very small droplet and a distance of

0.060 inches for a larger droplet. The viscous material is pushed out of the

syringe tip and contacts the substrate while it is still connected to the

syringe tip. If the viscous material fails to contact the substrate, it will not

adhere to the substrate and no droplet will result. The contacting of the

viscous material with the substrate is called "wetting." After the viscous

material contacts the surface of the substrate, the tip is pulled back and the

resulting string is broken to form a droplet.

One problem with the prior art systems is the stringing or

sticking of a bead of the viscous material to the nozzle. This can adversely

affect the ability of the delivery system to dispense precise, quantitative

amounts of liquid material. Stringing is most likely to occur at lower

pressures, for instance, when the pressure in the syringe is ramping up or

ramping down. For this reason, stringing also occurs more frequently as

dispensing time decreases. Stringing of the liquid material from the nozzle

tip during the final stage of dispensing may be avoided to some extent by

making the internal pressure of the syringe negative. However, when

dispensing again commences, a build-up of liquid at the nozzle tip almost -4- invariably occurs, thus adversely affecting the stability of the subsequent

extrusion. Also, to facilitate contact between the viscous material and the

workpiece, a robot must constantly move the syringe toward and away

from the workpiece, typically in up and down directions. This can

significantly slow the manufacturing process.

Another approach to dispensing fluid from a syringe is

disclosed in U.S. Pat No. 5,320,250. This dispensing apparatus includes a

reservoir or syringe of a viscous material which communicates with a

chamber that continuously receives the viscous material. A flexible resilient

diaphragm forms an exterior wall of the chamber. An impact mechanism

applies a predetermined momentum to the diaphragm to propel a

predetermined, minute quantity of the viscous material from the chamber

through a nozzle at a high velocity. This minute quantity takes the form of

a very small jet of viscous material. As the impact energy is removed by

means of a stop, the sudden decrease of the chamber pressure and the

forward momentum of the jet "pinches" or stops the jet. For many viscous

materials, the chamber is heated to controLthe viscosity of the material.

The reservoir is preferably pressurized with gas to force the viscous material

into the chamber. One problem with this type of design is that the high

velocity imparted to form the jet of viscous material causes the jet tail to

break into smaller droplets forming satellites.

Specific problems are encountered when dispensing solder

pastes. Solder pastes typically comprise lead, tin or other metallic particles

contained in a viscous material. One problem experienced with these -5- pastes is that they tend to adhere to metallic parts of a dispenser. For

example, adherence to metallic parts at the outlet, such as the outlet

nozzle, can cause clogging problems over time. Also, when dispensing

solder pastes in accordance with the descriptions set forth in the above

incorporated applications, the constant impact of the valve member or valve

shaft against the metal valve seat compacts the solder paste and causes it

to flake, conglomerate and create clogging problems.

To overcome some of the problems of the prior art devices, a

two-stage delivery system has been used where the viscous material

resides in a syringe under a constant air pressure of about 4 psi to about 1 2

psi, depending on the viscosity. This insures steady flow of the material

into a chamber of a rotary positive displacement pump. The pump

dispenses as many as 25,000 dots of the viscous fluid per hour onto a high

density, printed circuit (PC) board. Since the viscous material is pushed out

of the syringe tip and contacts the substrate while it is still connected to

the tip, however, the same problems exist as those described above relating

to delivery from a syringe.

For at least these reasons, it would be desirable to provide a

dispenser that can more rapidly and effectively apply minute amounts of

viscous material to a substrate or workpiece.

Summary of the invention

The present invention therefore generally provides apparatus

for effectively and rapidly dispensing minute amounts of viscous material, -6- such as solder flux, solder paste or other materials discussed above, in a

non-contact manner. That is, the apparatus need not be moved toward and

away from the workpiece during the dispensing operation. Various other

disadvantages of prior apparatus in this area are overcome through the

provision of a dispenser body generally having a valve member mounted for

movement therein with respect to a valve seat. The valve member

selectively allows viscous material to be discharged from an outlet

downstream of the valve seat. In accordance with the invention, a control

is operatively connected to the valve member to move the valve member

from the closed position to an open position and then rapidly back to the

closed position. This rapid succession of movements accelerates the

viscous material from the outlet in a thin stream and then positively stops

the of material so that the stream breaks away rapidly from the outlet to

form a minute droplet of the viscous material.

The dispenser is preferably an air operated dispenser in which

the valve member is connected to a piston. The piston and attached valve

member are rapidly moved under the force of applied air pressure,

preferably from a control valve directly mounting against the dispenser.

This direct mounting minimizes the distance between the air outlet of the

control valve and the piston. Thus, air pressure can rapidly move the piston

and the attached valve member away from the valve seat and, optionally,

also move the valve member quickly against the valve seat. In the preferred

embodiment, a spring return mechanism is also used to close the valve

member against the valve seat. Preferably, to dispense the minute droplets -7- of viscous material in accordance with the invention, air pressure is supplied

to the dispenser such that the valve member is opened for a time period of

less than about 25 milliseconds. For dispensing solder pastes in accordance

with the invention, for example, the time period may be approximately 20

milliseconds. This time period will vary depending on viscosity and pressure

characteristics of the viscous material and outlet orifice dimensions of the

dispenser. Also in accordance with the invention, a heating element is

connected to the dispenser adjacent the outlet. Since only localized heating

of the dispenser occurs by this heating element, the control valve may be

directly connected to another area of the dispenser without being adversely

affected by the heat.

The valve seat may be formed of a rigid material, such as

tungsten carbide, when dispensing most viscous materials. However, in

accordance with another aspect of the invention, significant benefits are

realized if the valve seat is formed of a resilient material, especially when

dispensing solder pastes. Solder pastes contain lead, tin or other metallic

particles that can cause the paste to compact, flake and potentially clog the

dispenser in the vicinity of the valve seat. This is caused by the constant

impacts on the material by the valve member against a rigid valve seat. The

resilient valve seat of this invention helps prevent these problems and may

also contribute a suctioning effect at the end of each dispensing cycle. This

suctioning or suck-back effect can prevent accumulation of excess viscous

material at the dispenser outlet. -8-

One resilient valve seat of this invention comprises a generally

flat, natural or synthetic rubber valve seat member having an outlet bore

extending coaxially with the valve member. The material of the valve seat

is most preferably a polyisoprene rubber, although many types of resilient

materials may be suitable. When dispensing solder pastes, for example,

having a viscosity in the range of 300,000 to 450,000 centipoise, the

outlet bore may have a diameter of about 10 - 30 mils or about 0.010 inch

to about 0.030 inch.

Another form of the resilient valve seat of this invention

includes an outlet bore having a tapered width from a larger dimension

closest to the valve member to a smaller dimension closest to the dispenser

outlet. Yet another resilient valve seat embodiment includes a plurality of

outlet bores extending through the resilient valve seat at an angle toward an

outlet. In this embodiment, when the valve member is in the closed

position, the valve seat is deformed and pinches or cuts off any flow of

viscous material through the plurality of outlet bores. Finally, another

embodiment of the resilient valve seat includes an outlet bore offset from

the valve member axis. In this embodiment as well, the valve member

pinches or cuts off the flow of viscous material through the aperture. In

each of the resilient valve seat embodiments, therefore, the valve member

will preferably compress the valve seat material and block the outlet bore as

a minute droplet is dispensed.

When dispensing minute drops of solder paste, for example,

from a resilient valve seat of the invention, the viscous material will enter -9- the outlet bore when the valve member is open. Then, when the valve

member is closed preferably within less than about 25 milliseconds, for

example, this sudden impact and compression of the valve seat will eject

the drop of viscous material. Upon lifting of the valve member from the

resilient valve seat and decompression of the valve seat, an advantageous

suck-back effect at the dispenser outlet can occur.

In the case of dispensing certain viscous materials, especially

solder paste, it has been found highly advantageous to use a low friction

polymeric material for dispenser components located generally at the

dispenser outlet. Solder pastes can adhere and accumulate on metallic

valve shaft and seat structures and on nozzle components. This is believed

to be due to the nature of the lead, tin or other metallic particles contained

in the paste. The use of a low friction plastic or polymer for such

components as the end of the valve member or shaft and the dispenser

outlet nozzle has especially aided in preventing significant adherence of

solder pastes and resulting clogging problems.

Additional objects and advantages of the various inventive

aspects will be realized by those of ordinary skill after reviewing this

disclosure.

Brief Description of the Drawings

The structure, operation, and advantages of the presently

preferred embodiment of the invention will become further apparent upon -10- consideration of the following description taken in conjunction with the

accompanying drawings, wherein:

Fig. 1 is a side view, in cross section, of a preferred

embodiment of a liquid or viscous material dispensing apparatus disposed

above a workpiece;

Fig. 2 is a enlarged view of the valve seat area of Fig. 1

showing the valve member in an open position;

Fig. 2A is an enlarged view of an alternative nozzle;

Figs. 3-5 are views similar to Fig. 2 but showing the valve

member progressively moving to a fully closed position to dispense a minute

droplet of viscous material;

Fig. 6 is a view similar to Fig. 2, but showing the valve

member in the open position and for a suck-back effect at the nozzle;

Fig. 7 is a top plan view of another alternative resilient valve

seat having a plurality of angled outlet bores;

Fig. 8 is a side elevational view of the embodiment shown in

Fig. 7;

Fig. 9 is a bottom view of the embodiment shown in Fig. 7;

Fig. 10 illustrates an alternative valve seat embodiment, similar

to Fig. 2, but eliminating the nozzle and showing a tapered outlet bore in

the valve seat member;

Figs. 1 1 -14 illustrate another resilient valve seat with an

alternative valve member location and movement while dispensing of a

minute droplet of viscous material; and -1 1 -

Fig. 1 5 illustrates another embodiment of a resilient valve seat

and valve actuating structure.

Detailed Description of the Preferred Embodiments

In this description, like reference numerals refer to like

structure shown and described in the above incorporated related

applications. Like numerals having prime marks (') or double prime marks

(") herein refer to analogous structure in the incorporated applications

which has been somewhat modified as will be apparent by comparison.

Also, it will be appreciated that the principles of the invention may be

practiced with respect to each alternative dispenser described in the

incorporated applications, or with still other dispensers.

Fig. 1 illustrates dispensing apparatus 10' for dispensing small

amounts of liquid or viscous material initially contained in a syringe 12.

Syringe 12 may be a standard, commercially available syringe filled, for

example, with solder flux or solder paste 13. The viscous material may be

dispensed in minute droplets on a substrate or workpiece 14, such as a

printed circuit (PC) board. A dispenser housing 20' of apparatus 10' has an

inlet 18 into which is mounted an outlet 16 of syringe 12. The term

"housing" is not intended to convey any particular integral or assembled

structure but to broadly define the overall support and containment

structure of apparatus 10'. Inlet 18 is connected by a bore 22 to an inlet

opening 23 of a flow bore 24 forming a flow passage 25. An outlet 26 of

flow bore 24 is connected to a first end 28 of a bore 35 extending through -12- an outlet tube 30 and forming a flow passage 31 from which the

pressurized liquid or viscous material is dispensed. A valve seat assembly

32' is mounted to a second free end 34 of outlet tube 30. Valve seat

assembly 32' has a flow passage 36 extending therethrough with a valve

seat 38' disposed therein. The inlet end of flow passage 36 is in flow

communication with the flow passage 31 of outlet tube 30 and the

opposite outlet end of passageway 36 has a nozzle 40' mounted thereto.

Nozzle 40' is advantageously formed of a low friction polymer material,

such as polyetheretherketone (PEEK) tubing available from Small Parts, Inc.

Fig. 2A illustrates an alternative nozzle 40", also formed from

PEEK tubing. However, nozzle 40" includes a tapered bore 100" from the

inlet end to the outlet end thereof. For example, this bore may taper from a

diameter of 0.0625 inch at the inlet end to between 0.006 - 0.030 inch at

the outlet end based on a length of 0.206 inch. The use of PEEK material

or other comparable plastics presents a non-stick surface, especially useful

when dispensing solder paste 13, while the taper of bore 100" prevents

crowding of particles at the nozzle inlet and generally allows for a smoother

flow path.

A valve shaft or valve member 42' extends through flow bore

24 of housing assembly 20', through bore 35 of outlet tube 30 and into

flow passage 36 of valve seat assembly 32'. Valve member 42' has a

lower end 44' adapted for sealing engagement with valve seat 38' to close

passageway 36. As discussed generally above, this may advantageously

include a rounded valve end member 44a formed of a low friction polymer -13- material, such as Vespel™ or PEEK. Vespel™ is currently preferred and is

available from E.I. du Pont de Nemours and Company, Wilmington,

Delaware and is a polyimide material. This material inhibits adherence and

accumulation of materials, such as solder paste, and therefore can prevent

clogging or other dispensing problems. An opposite upper end 46 of valve

member 42' is engaged with the control mechanism 48 of dispensing

apparatus 10'. Control mechanism 48 reciprocates valve member 42' out

of and into seating engagement with valve seat 38'.

Valve seat 38' may be formed of a rigid material, such as

tungsten carbide, as is disclosed in the incorporated applications or may be

formed of a resilient material, such as a natural or synthetic rubber.

Polyisoprene rubber has allowed significantly greater numbers of dispensing

cycles without noticeable wear or degradation of the rubber. In general, it

is best to use rubbers that exhibit low heat build-up during repeated

compression by valve member 42', low reversion and high strength and

modulus properties. The preferred polyisoprene has the following

formulation, with all components listed in parts per hundred rubber (PHR):

A) A master non-productive batch is first produced with the

following components:

polyisoprene rubber 100.00 pentachlorodiphenol 0.20 hydrated precipitated silica 0.50 stearic acid 2.00 zinc oxide 6.00 antioxidant system 1 .00 phthalic anhydride 0.50 high abrasion furnace (HAF) black 35.90

master batch 146.10 -14-

A preferred antioxidant system includes Goodrite Stalite (0.75

PHR) and N,N'-Diphenyl-p-phenylenediamine (DPPD) (0.25 PHR).

B) The master batch is then mixed again immediately before

adding the curing agents to form the total compound according to the

following formula:

master batch 146.10

Santagard™ post-vulcanization 0.20 inhibitor cobalt stearate 2.00

2,2'-Dithiobis[benzothiazole] 0.20

(MBTS) sulfur 3.75 hexamethylenetetraamine 50%/ 1 .20 styrene-butadiene rubber 50%

total compound 153.45

Also according to the invention, a heating element 50 is

disposed adjacent valve seat assembly 32' to heat a very small volume of

the liquid or viscous material in the valve seat assembly as discussed in

more detail below. A seal ring 52 is disposed in sealing relation about valve

member 42' and is located above inlet 23 of flow passage 24 to insure that

the viscous fluid flowing through bore 22 and into flow passage 24 does

not leak past valve member 42' and into the control mechanism 48. Seal

ring 52 is secured in place by a ring 54 which in turn is held in place by the

bottom surface of the housing block 56 of control mechanism 48.

As further shown in Fig. 1 , outlet tube 30 has a first end

secured to dispenser housing 20' by conventional means, such as a

mounting plate 58, so that the outlet 26 of flow passage 24 is aligned with

an inlet opening 60 of bore 35 extending through outlet tube 30. The -15- outlet tube 30 has a second end 34 onto which valve seat assembly 32' is

secured by conventional means such as by a threaded connection (not

shown). A valve seat component 78' is disposed within valve seat

assembly 32' and carries valve seat 38'. Also, a temperature controller

102 is connected by leads 104, 106 to heating element 50 in order to

selectively and locally heat valve seat assembly 32'.

As seen in Fig. 1 , the control mechanism 48 includes housing

block 56. A centrally disposed longitudinal bore 1 10 extends through

housing block 56 and is coaxially formed about valve member axis 87.

Valve member 42' extends through bore 1 10 and projects from the upper

end of bore 1 10 into a stepped bore chamber 1 12 of an air chamber block

1 13 having a lower bore 1 14 which intersects an upper bore 1 16 having a

larger diameter than lower bore 1 14. Two sealing discs 1 18, 1 18a formed

of glass filled polytetrafluorethylene (PTFE), are mounted onto a support

structure 120 which, in turn, has a central bore 122 through which valve

member 42' extends and is fixedly attached thereto. Respective air inlets

124, 124a are connected to a source of pressurized air (not shown) by a

tube 126. An air solenoid valve 128' operated by a conventional controller

129, and located between tube 126 and inlets 124, 124a, controls the

pressurized air used to operate dispenser 10'. As shown in Fig. 1 , valve

128' is directly connected against housing assembly 20' adjacent chambers

1 12, 1 15. This allows a quicker responsive movement by valve member

42' to the introduction of pressurized air into chamber 1 12 or 1 15 than

would be possible if solenoid valve 128' had to be mounted away from -16- housing 20'. Such stand-off mountings are practiced, for example, with hot

melt dispensers due to the fact that the entire hot melt dispensing gun is

heated to a temperature that would adversely affect a directly mounted

solenoid valve. Specifically, valve 128' controls air flow into chamber 1 12

formed below disc 1 18 in lower bore 1 14 and a chamber 1 15 formed by

upper bore 1 1 6 above seal 1 18a. An air seal ring 1 19 about member 42 is

located in a counterbore 121 between bore 1 10 and bore chamber 1 12 to

prevent air leakage into bore 1 10. Solenoid valve 128' is advantageously

mounted directly to dispenser housing 20'. Typical hot melt adhesive guns,

for example, use solenoid valves mounted away from the gun body due to

the more extreme heat conditions thereof that would adversely affect the

solenoid. As solenoid valve 128' is mounted directly to gun body 20', the

cycle times of dispensing gun or apparatus 10' are quicker than the same

size solenoid valve mounted in a stand-off fashion. One particular solenoid

valve 128' useful for this invention is Model 35A-B00-DDFA-1 BA,

Modification M599 from MAC Valve Co. in Wixsom, Michigan.

A spring housing 130 is mounted against the top surface of air

chamber block 1 13 and is formed with a central bore 132. A spring

retainer 134 is securely mounted onto the upper end of valve member 42'

and abuts against the support structure 120. A cup-shaped spring

adjustment component 136 is threadably secured to spring housing 130

and has an elongated bore 138 open at one end and closed at the other end

by a base 139 with a bore 141 extending therethrough and an interior

bottom surface 140 about bore 141 . A compression spring 142 extends -17- between spring retainer 134 and the bottom surface 140 of spring

adjustment component 136. A lock nut 144 is threadably secured to spring

adjustment component 136 by threads so that the component 136 can be

locked into position closer to or farther away from spring retainer 134. The

compression of spring 142 is increased as the spring component 136 is

moved towards spring retainer 134 and decreased as the spring component

1 36 is moved away from spring retainer 134.

One feature of the invention relates to the closure force

exerted by compression spring 142 on spring retainer 134 and, ultimately,

by valve end 44a on valve seat 38'. Preferably, compression spring 142

has a free length of 1 .15 inches and exerts a closure force of between

about 17 pounds and about 28 /2 pounds. The compression of spring 142

can be adjusted by positioning spring adjustment component 136, as

previously discussed. To add to the quickness of the spring return shut-off,

pressurized air may be directed into chamber 1 15, as discussed below.

Another feature of the control mechanism 48 is a knob 146

which is attached to a rod 148 that is threadably secured in bore 141 and

which passes through compression spring 142 to bear against the top end

of valve member 42' extending above the spring retainer 134. By moving

the rod 148 up or down, the stroke of the valve member 42' can be

adjusted with respect to the valve seat 38'.

To further appreciate the advantages of the present invention,

a description of the operation is appropriate. First, a syringe 12 of liquid or

viscous material, typically having a viscosity of between about 25,000 and -18- about 500,000 centipoise, is mounted to the inlet opening 18 of a

dispenser housing 20'. An air tube 150 connected to a pressure regulator

152 and a source of low pressure air (not shown) is coupled to the inlet of

syringe 12 to force the liquid or viscous material into bore 22 and flow

passage 24 about the valve member 42' at a constant pressure of about 4

psi to about 30 psi. In the default closed position, as shown in Fig. 1 , the

valve seat component 78' above valve seat 38' is filled with a small amount

of the liquid or viscous material while valve end 44a is seated against valve

seat 38'. The mounting body 70 is formed of a heat conducting material,

such as brass, to locally transfer heat from heating element 50. Heating

element 50 is disposed around and secured to mounting body 70 and

therefore transfers heat through body 70 into valve seat component 78',

which may be constructed of tungsten carbide, to heat the liquid or viscous

material in valve seat component 78' which surrounds valve member 42'.

During this stage of operation, the liquid or viscous material,

such as an adhesive, a solder flux or a solder paste, is heated to a

temperature range (depending on the material) of between about 22°C to

about 90°C. For example, solder pastes having a viscosity of between

300,000 centipoise and 450,000 centipoise are preferably heated at about

160°F (88°C). Solder flux may be heated at about 40°C to about 65°C.

Therefore, while the viscous material is briefly located in valve seat

component 78', the material is briefly heated prior to dispensing.

Although the present description of the operation is generally

applicable to all embodiments of this invention which incorporate various -19- types of valves and valve seats, the description will now be more

specifically described while referring to Figs. 2-6. The same description

below applies to Fig. 2A as well. After the valve end 44a raises from seat

38' as shown in Fig. 2, the viscous material 13, such as solder paste, is

very briefly pushed through orifice 38a of seat 38' and orifice 100 of nozzle

40' as a thin stream (Fig. 3). Then, after valve end 44a impacts and closes

against valve seat 38' as shown progressively in Figs. 4 and 5, the sudden

deceleration of the flowing material 13 overcomes the material yield stress

and breaks the stream into a minute droplet 200A'. This causes the

viscous material to break off from the nozzle 40' rather than flow into a

string. Referring to Figs. 4 and 5, when valve seat 38' is a resilient

material, such as the polyisoprene describe above, it will compress as

shown. In the preferred embodiment, when dispensing solder paste 13, the

total stroke length of valve member 42 is about 0.100 inch. The depth of

penetration into valve seat 38' is about .025 inch. However, these

distances may be changed, for example, to alter the droplet size. It is

important to maintain the material at the selected temperature range for

only a brief period of time and not to exceed the temperature where the

catalyst or solid particles melt and/or cure. For this reason, only the valve

seat component 78' is heated and not the remainder of dispensing

apparatus 10'. As also discussed above, this localized heating has the

benefit of allowing shorter cycle times due to the direct mounting of

solenoid valve 128'. As shown in Fig. 6, when a resilient valve seat 38' is

used, the initial movement of valve end 44a away from valve seat 38' will -20- decompress the resilient material and may thereby contribute a material

suck-back effect to prevent accumulation, stringing or drooling of the

material 13 at the outer end of nozzle 40'.

Specifically, to open dispenser 10', valve member 42' is

retracted to withdraw valve end 44a from valve seat 38'. More specifically

referring to Fig. 1 , this step is accomplished by introducing pressurized air

from air solenoid 128' through air inlet 124 and into the air chamber 1 12

located below diaphragm seal 1 18. Air pressure applied against seal 1 18

moves valve member 42' in a direction away from valve seat 38' and

towards compression spring 142. During this period of operation, the

heated viscous material flows as directed below.

To almost immediately break the flowing string of liquid or

viscous material, air solenoid 128' is activated by controller 129 to switch

the flow of air from passage 124 to passage 124a. Then, pressurized air

applied against seal 1 18a as well as the force of spring 142 will move valve

end 44a against valve seat 38'. Switching air pressure to passage 124 off

should occur in a very short period of time, i.e., less than about 25

milliseconds between respective "on" and "off" signals sent to solenoid 128

by controller 129. In the solder paste example given above, a time period

of about 20 milliseconds worked well. It is believed that this time period

could range from about 5 milliseconds to about 50 milliseconds depending

on factors such as material viscosity, material pressure and orifice sizes. In

the embodiment shown in Fig. 1 , solenoid valve 128 is a so-called four-way

electromagnetically actuated valve which allows air in chamber 1 12 to be -21 - exhausted and pressurized air from line 126 to be redirected immediately to

chamber 1 15. Although less preferable, especially when dispensing solder

pastes, a three-way valve can also be used in the manner disclosed in the

applications incorporated herein. When either a three-way valve or a four-

way valve is used, compression spring 142 rapidly moves valve end 44a to

a seated position against valve seat 38' when pressurized air in chamber

1 12 is exhausted. This is a positive displacement step which pushes the

heated liquid or viscous material out of the outlet end of nozzle 40'.

One aspect of the invention is to deform viscous material at a

high frequency so that the material acts as a solid for a very brief period of

time and then returns to a more fluid state when it breaks away from the

outlet end 101 ' of orifice 100'. With orifices 38a and 100' having a

diameter of between about 0.010 inch and about 0.030 inch, single solder

paste droplets were produced with about 0.025 inch to about 0.090 inch

diameters. In these cases, the syringe pressures ranged from about 10 - 25

psi. Of course, larger droplets may be produced according to the methods

described in the above incorporated applications. Orifice diameters could

also be lower as described in the above incorporated applications or could

be larger as when dispensing solder pastes, due mainly to the increased

viscosity. To address the range of materials and droplet sizes mainly of

concern to this invention, the outlet orifice diameters, such as of orifices

38a and 100', may be from about 0.005 inch to about 0.050 inch.

Figs. 7-9 illustrate yet another embodiment of a resilient valve

seat member 700. Valve seat 700 may also be formed of natural or -22- synthetic rubber, such as polyisoprene, as described above for valve seat

38'. Also, valve seat 700 may be acted upon by valve member end 44a in

the same manner as generally described above with respect to Figs. 1 -6.

However, valve seat 700 instead includes a plurality of angled orifices 702,

704, 706. Orifices 702, 704, 706 angle toward one another from an upper

side 700a of valve seat 700 to a lower side 700b thereof as shown best in

Fig. 8. Orifices 702, 704, 706 preferably meet at a single outlet 708 on

the lower side 700b of valve seat 700. In all other general respects, the

embodiment of Figs. 7-9 will operate generally as described above with

respect to Figs. 2-6.

Fig. 10 illustrates an alternative valve seat 710 which, like the

embodiment shown in Figs. 1 -6, may be formed of a resilient material such

as a synthetic or natural rubber as described above. For example, valve

seat 710 may also be formed of polyisoprene as described above with

respect to valve seat 38'. The main difference between the embodiment

shown in Fig. 10 and the embodiment of Figs. 1 -6 is that nozzle 40' has

been eliminated and valve seat component 78" has been modified to hold

valve seat 710. Optionally, a nozzle as described or incorporated herein

may be used with valve seat 710. Valve seat 710 includes an orifice 712

which is tapered in diameter from an upper end 712a to a lower end 712b.

Otherwise, valve seat 710 operates in the same general manner as

described above with respect to valve seat 38' to dispense a minute droplet

714 of viscous material 13. Preferably, the lower end 712b of orifice 712

has a diameter of a size similar to the orifice or outlet bore sizes mentioned -23- above. The taper is contemplated to be of such a nature that the upper end

712a has a diameter of roughly two to three times the diameter of the

lower end 712b.

Figs. 1 1 -14 illustrate yet another valve seat and valve

actuating embodiment. In this embodiment, a valve seat 720, again

preferably formed of a natural or synthetic rubber, such as polyisoprene,

includes an orifice 722 aligned with another orifice 723 of a nozzle 724. In

this case, however, valve member axis 87 is offset from, but parallel to, an

axis 725 generally defining the aligned orifices 722, 723. Valve end 44a is

caused to move against valve seat 720 using a control, apparatus and

method as generally described above with respect to Figs. 1 -6. However,

as valve end 44a is offset from orifice 722, valve seat material will be

deformed into orifice 722 to pinch off and close orifice 722 as shown in

Fig. 12 and, at the same time, push out a thin stream of viscous material

726. In the same sudden manner as described above, this thin stream 726

will break off into a minute droplet 728 as shown in Fig. 13 when valve end

44a fully impacts against valve seat 720. As further shown in Fig. 14,

when valve end 44a is retracted, and resilient valve seat 720 decompresses

into its normal state, a suck-back effect may occur within nozzle 724 to

prevent accumulation of viscous material 13 and/or dripping or stringing at

nozzle outlet 724a.

Fig. 15 illustrates an alternative valve seat 730 and valve

actuating structure 732 which operates similarly to the embodiment

depicted in Figs. 1 1 -14. The main difference is that the valve member or -24- valve shaft 734 is contained within a movable actuator member 736 which

is sealed off or isolated from the viscous material 13, such as the solder

paste. This prevents any solder paste 13 from being impacted between the

valve seat 730 and the valve shaft or valve member 734. Specifically, the

valve seat actuator member 736 may simply be a cylindrical member that

may be reciprocated by the action of valve member or valve shaft 734. The

valve seat actuator member 736 is mounted for reciprocation within a seal

738. Thus, when the valve member or valve shaft 734 is at the bottom of

its stroke, the valve seat 730 will be deformed as shown in Fig. 14 and a

drop 740 of solder paste 13 may be dispensed in the same manner as

described above. In this embodiment, however, as there is no contact

between the valve member 734 and the solder paste 13, wear on the valve

seat 730 by the solder paste may be prevented.

While the invention has been described in combination with

various embodiments thereof, it is evident that many alternatives,

modifications, and variations and combinations of features will be apparent

to those skilled in the art in light of the disclosure specifically contained

herein or incorporated herein or otherwise available in the art. Accordingly,

the invention is intended to embrace all such alternatives, modifications and

variations as fall within the spirit and scope of the appended claims.

Claims

-25-We claim:

1 . Apparatus for dispensing small amounts of viscous material,

the apparatus comprising:

a dispenser housing having an inlet for receiving a supply of

the viscous material and an outlet for discharging the viscous material,

a valve member having a piston disposed for movement within

a chamber and operative to move the valve member within the dispenser

housing in response to pressurized air being directed into the chamber,

a valve seat disposed proximate the outlet and having an

orifice communicating with the outlet, wherein an end of the valve member

moves with respect to the valve seat to define open and closed positions of

said orifice, and

a control valve mounted against the housing for intermittently

directing pressurized air into the chamber, and thereby moving the valve

member quickly between the open and closed positions during the

dispensing of small droplets of viscous material, and

a heating element disposed proximate the dispenser outlet for

locally heating the viscous material prior to dispensing the small droplets.

2. The apparatus of claim 1 wherein the valve seat member is

formed of a rigid material. -26-

3. The apparatus of claim 1 wherein the valve seat is at least

partially formed of a resilient material and the valve member deforms the

valve seat in the closed position.

4. The apparatus of claim 3, wherein the resilient material is a

rubber material.

5. The apparatus of claim 3, wherein the resilient material is

polyisoprene rubber.

6. The apparatus of claim 1 further comprising a source of

pressurized viscous material operatively connected to the inlet, wherein the

viscous material is supplied by the source at a pressure of less than about

30 psi.

7. The apparatus of claim 1 wherein the heating element is

mounted to a nozzle assembly which includes the valve seat therein and is

disposed at one end of the dispenser housing, and the control valve is

mounted closer to an opposite end of the housing. -27-

8. The apparatus of claim 7, wherein the heating element is

maintained at a temperature of about 22┬░C to about 90┬░C.

9. The apparatus of claim 1 wherein the dispenser outlet is

formed in a nozzle member disposed adjacent to the valve seat member,

and wherein the nozzle member is formed from a low friction polymer

material.

10. The apparatus of claim 9, wherein the low friction polymer

material is polyetheretherketone.

1 1 . The apparatus of claim 1 wherein the valve seat member is at

least partially formed of a resilient material and the valve member is offset

from the valve seat orifice such that, in the closed position, the valve

member deforms the resilient material to close the outlet bore.

12. The apparatus of claim 1 , wherein the control valve is mounted

against a wall of the chamber.

13. The apparatus of claim 1 wherein at least the end of the valve

member that engages the valve seat is formed from a low friction polymer

material. -28-

1 4. Apparatus for dispensing small amounts of viscous material,

the apparatus comprising:

a dispenser housing having an inlet for receiving a supply of

the viscous material and an outlet for discharging the viscous material,

a valve member mounted for movement within the dispenser

housing between open and closed positions, and

a valve seat disposed proximate the outlet, the valve seat

having a resilient portion with an outlet orifice, wherein an end of the valve

engages and deforms the resilient portion of the valve seat in the closed

position to dispense a droplet of the viscous material from the outlet orifice.

1 5. The apparatus of claim 1 4, wherein the valve member is

aligned with a first end of the outlet orifice such that the valve member

obstructs the first end thereof in the closed position.

1 6. The apparatus of claim 14, wherein the outlet orifice is tapered

to decrease in diameter from the first end toward a second end.

1 7. The apparatus of claim 14, wherein the valve member is offset

from the outlet orifice and deforms the resilient material to pinch off the

outlet orifice in the closed position. -29-

18. The apparatus of claim 14 further comprising a nozzle member

disposed adjacent the valve seat member and having an outlet formed

therein, wherein the nozzle member is formed from a low friction polymer

material.

19. The apparatus of claim 18, wherein the low friction polymer

material is polyetheretherketone.

20. The apparatus of claim 14, wherein the valve seat includes a

plurality of outlet orifices and each outlet orifice is blocked when the valve

member is in the closed position.

21 . The apparatus of claim 20, wherein the valve seat has an inlet

side and an outlet side and the plurality of outlet orifices angle toward each

other between the inlet and outlet sides.

22. The apparatus of claim 14, wherein the resilient portion is a

rubber material.

23. The apparatus of claim 14, wherein the resilient portion is

formed of polyisoprene rubber. -30-

24. The apparatus of claim 1 4, wherein at least the end of the

valve member that engages the valve seat is formed of a low friction

polymer material.

25. A method for rapidly dispensing a small quantity of a viscous

material from a dispenser including a valve member, a viscous material

passage and a resilient valve seat with an outlet orifice, the method

comprising the steps of:

moving the valve member out of engagement with the resilient

valve seat,

introducing viscous material into the outlet orifice from the

viscous material passage, and

compressing the valve seat with the valve member to close the

outlet orifice and urge a small droplet of the viscous material from the outlet

orifice.

26. The method of claim 25, wherein the valve member is moved

coaxially with respect to the outlet orifice.

27. The method of claim 25, wherein the valve member is moved

along an axis which is offset from an axis of the outlet orifice. -31 -

28. The method of claim 25 further comprising the step of

decompressing the valve seat and thereby suctioning excess viscous

material into the outlet orifice.

29. The method of claim 25, wherein the viscous material is a

solder paste.

30. The method of claim 25, wherein the viscous material is a

adhesive.

31 . The method of claim 25, wherein the viscous material is a

solder flux.

32. The method of claim 25, wherein an end of the valve member

directly contacts the valve seat during the compressing step, and wherein

at least the valve member is formed of a low friction polymer material.

33. The method of claim 25, wherein an end of the valve member

indirectly engages the valve seat through an actuator member.

34. The method of claim 25, wherein the viscous material is

contained in the viscous material passage at a pressure of about 4 psi to

about 30 psi.