Dispensing – Processes of dispensing
Reexamination Certificate
1999-09-24
2001-07-31
Kaufman, Joseph A. (Department: 3754)
Dispensing
Processes of dispensing
C222S061000, C222S389000, C222S496000, C222S504000, C222S518000
Reexamination Certificate
active
06267266
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to an apparatus and method for dispensing liquid material, and more particularly, to a non-contact dispenser or applicator having a valve assembly for ejecting liquid material onto a substrate wherein the valve assembly includes a compressible dispensing element.
BACKGROUND OF THE INVENTION
It is often desired to dispense a small amount of material to a precise location, such as when dispensing liquid material including solder fluxes and adhesive materials during the fabrication of a printed circuit board. Throughout this application the term “liquid material” or “material” is intended to mean any substance including but not limited to semisolid or highly filled materials such as adhesives and coatings having by volume or weight fifty percent (50%) or above solids, epoxy, silver filled solder paste, thermogreases, etc., and low viscosity materials such as adhesives and coatings having by volume or by weight below fifty percent (50%) solids, solder flux, etc. Contact type dispensers are often used for such applications. These contact type dispensers use syringes with a tip usually located very close to the substrate; i.e. from about 0.005 inches to 0.060 inches away. The liquid material is held within the syringe and is pushed out of the syringe tip. The liquid material contacts the substrate while it is still in contact with the syringe tip. After the liquid material contacts the substrate, the syringe tip is pulled back from the substrate, thereby drawing out a string of material. Upon sufficient retraction of the syringe, the string is broken and the liquid material forms a droplet on the substrate. One problem with the contact type dispenser is that it requires significant movement of the dispenser away from the substrate to separate the liquid material from the syringe, thereby adding to the time and complexity of the dispensing operation. Another problem with the contact type dispensers is that the string of liquid material can break in a manner such that material ends up on the dispenser instead of the substrate or the droplet does not comprise all the dispensed liquid material, but rather a portion of the material separates from the droplet and lands at an undesignated portion of the substrate.
Non-contact dispensers have been developed to overcome the problems associated with the contact type dispenser. In known non-contact type dispensers a reciprocal plunger or piston engages a valve seat that is immersed in the liquid material to be dispensed. In order to dispense a droplet, the plunger is retracted from the valve seat, allowing the liquid material to flow into the area surrounding the valve seat and into an orifice formed in the seat. The plunger is then urged downward into contact with the seat, thereby forcing the liquid material through the orifice. The plunger is accelerated toward the seat to impart sufficient kinetic energy such that a droplet of the liquid material is formed. The droplet breaks away from the end of the dispenser and is propelled downward toward the substrate.
Specific problems arise when dispensing liquid material from known non-contact dispensers. For example, if solder paste is used, the repeated impact of a plunger against the valve seat compacts and “flattens” the solder balls in the paste. The flattened or deformed solder balls cluster and conglomerate, and block or impair the flow of material through the dispenser. In addition, the surface-to-surface contact between the plunger and the seat, when the contacting surfaces are surrounded by solder paste, creates wear due to the abrasive characteristic of the solder paste. Accordingly, it would be desirable to have a non-contact liquid material dispenser which does not cause the liquid material being dispensed to cluster and block the flow of the liquid material and which does not have an abrasive action between the plunger and valve.
SUMMARY OF THE INVENTION
The present invention is a dispenser or applicator having a bellows valve assembly and method for applying liquid material to a substrate which overcomes the deficiencies of the known dispensers. In the preferred embodiment, the invention is described as a dispenser for ejecting small amounts of material onto a printed circuit board. An exemplary material dispensed from the apparatus is solder paste. However, it should be understood that while the present invention is particularly useful and described herein with reference to dispensing small amounts of solder paste material, the invention is applicable to the dispensing of any liquid material in any amount onto any substrate or other surface.
The dispenser comprises in one embodiment a housing, a force applying element or piston positioned within the housing and having a central or flow bore formed therein, and a valve assembly positioned below and in liquid material communication with the piston. In the preferred embodiment, the valve assembly comprises an upper plate, a resilient and compressible core dispensing element and a nozzle. Herein, the valve assembly with a compressible dispensing element is referred to as a bellows valve because the compressible dispensing element is preferably axially compressed to cause a radial stricture or closure of a liquid passageway there through. Material to be dispensed is drawn or forced into the liquid passageway in the compressible element and forced out through the nozzle by closure or compression of the core element. It is further within the scope of the invention to provide a compressible dispensing element that is axially compressed and in effect thereby radially compressed due to constrained movement, caused by an associated housing that surrounds the compressible dispensing element, to close the liquid passageway there through.
The bellows valve assembly in the preferred embodiment includes three elements: a compressible core dispensing element, an upper or backing plate, and a nozzle. These three parts of the bellows valve are preferably adhered together by any process such as a vulcanized metal to rubber process well known to those of ordinary skill in the art. The bellows valve assembly is positioned partially within the housing below the piston and may have at least a portion of the nozzle extending beyond the housing. The bellows valve assembly has a bore in liquid communication with the central bore of the piston. The liquid material to be ejected flows through the central bore of the force-applying piston and is fed directly to the bellows valve assembly. The piston is reciprocated to selectively apply a force and remove the force from the bellows valve assembly which causes an amount of the liquid material to be dispensed. The upper plate of the bellows valve assembly remains in contact with the piston during operation to prevent the liquid material being dispensed from getting between the piston and the upper plate. In this manner, the liquid material being dispensed does not accumulate or pack between the piston and the bellows valve assembly.
In accordance with another aspect of the invention, the nozzle has a central liquid passage or orifice of diameter “'d”, the compressible dispensing element has a central bore of diameter “c”, the piston has a restricted portion of a central bore of diameter “b” and a main portion of the bore having a diameter “a”. In accordance with one aspect of the invention, the relationships b≦c and b≦d are preferably used to reduce back flow of the liquid when the bellows valve is closed. This is useful, for example, when the liquid material is solder paste or other materials having similar characteristics of solder paste. For such liquid materials such as solder paste, it may also be desirable to use the relationship d≧c. However, the relationship d<c may be useful for some liquid materials such as adhesives and other materials having similar characteristics of adhesives. The specific relationship used for any particular application will be defined in part, therefore, by the type of material being dispensed. In most cases,
Messerly James W.
Smith James C.
Calfee Halter & Griswold LLP
Kaufman Joseph A.
Nordson Corporation
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