Coating apparatus – Immersion or work-confined pool type – Work-confined pool
Reexamination Certificate
1999-07-22
2002-03-26
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Immersion or work-confined pool type
Work-confined pool
C118S413000, C118S693000
Reexamination Certificate
active
06361606
ABSTRACT:
FIELD OF THE INVENTION
Embodiments of the present invention relate generally to methods and devices for depositing viscous materials onto a printed wiring board. In one aspect, the present invention relates to methods and devices for compressing viscous materials, such as solder paste, through openings in a perforated substrate, such as a patterned screen or stencil.
DESCRIPTION OF RELATED ART
Surface Mount Technology (SMT) involves placing circuit components onto circuit paths embedded on the upper surface of a printed wiring board and then soldering the components in place by a process called “reflow soldering”. Before the circuit component is placed on the printed wiring board, however, it is desirable to apply solder paste to the area on the printed wiring board where the component is to be soldered into place.
Conventional methods do exist to deposit (“print”) solder paste onto desired areas of a printed wiring board by forcing the paste through openings in a substrate (e.g., a stencil) placed in intimate contact with the printed wiring board.
U.S. Pat. No. 4,622,239 describes such a method and device for dispensing viscous materials. The method includes forcing a viscous material from a housing through an opening and depositing it onto a stencil between a pair of flexible members (parallel squeegee blades) which depend from the housing on either side of the opening and are in contact with the stencil. The ends of the flexible members are not connected and remain open ended. The viscous material, accordingly, is not contained within an enclosed area when it is deposited on the surface of the stencil. Movement of the housing and the flexible members horizontally across the stencil causes the trailing flexible member to force the viscous material through the openings in the stencil. U.S. Pat. No. 4,720,402 describes a similar method and device except that the leading flexible member is raised off of the stencil during movement of the housing.
U.S. Pat. Nos. 5,133,120 and 5,191,709 describe methods for filling through-holes of a printed wiring board via a mask with pressurized conductive filler material by means of a nozzle assembly unit having a nozzle tip member. The nozzle tip member, however, is designed only to dispense the pressurized conductive filler material through the mask to a single through-hole. The nozzle tip member then “scans” the printed wiring board for a second through-hole to fill. The nozzle tip member has a blunt end section which rests on the mask and a circular exit, the diameter of which may be increased or decreased by changing the nozzle tip member. The nozzle tip member dispenses the filler material without controlling unwanted flow of “excessive” filler material back through the stencil. Additionally, the nozzle tip member does not define a contained environment where “compression” of the filler material takes place through the mask followed by the immediate shearing off of the filler material within that contained environment from the surface of the stencil. In fact, the nozzle tip member itself provides no effective means for shearing off filler material from the top of the stencil, rather, after the through hole is filled and filler material “backs up” through the stencil, the nozzle tip member moves forward whereupon the “excessive” filler material is then wiped off by a separate, single, flexible squeegee member which is designed for unidirectional use only.
Unfortunately, these conventional efforts do not provide a contained environment for compression of viscous material through holes in a stencil and shearing of viscous material within the contained environment from the upper surface of the stencil. Reliance upon squeegee movement to force the viscous material, such as solder paste, through the stencil openings can lead to damage and eventual failure of both the squeegee blades and the stencil due to repeated friction. Since conventional efforts do not provide a contained environment in which compression and shearing is accomplished, waste of the viscous material is frequently encountered.
Conventional efforts, therefore, (1) fail to maximize the efficiency of printing solder paste onto a desired area of a printed wiring board and (2) fail to minimize waste of the solder paste during the printing process. A need therefore exists to develop a method for printing solder paste onto a printed wiring board and a device suitable for use therewith which overcomes the deficiencies of the conventional efforts.
Other methods and assemblies utilize a pneumatically driven piston assembly which typically resides upon the compression head and which typically includes a container having a certain amount of viscous material and at least one piston which is selectively and movably deployed within the container and which is effective to cause a certain amount of the contained viscous material to be selectively dispensed and communicated to the compression head.
These prior assemblies and methodologies suffer from at least several drawbacks. First, the piston-viscous material containment assemblies are relatively heavy and bulky, thereby requiring undesirable and relatively costly structural or “support-type” modifications be made to the existing compression head assembly. These pneumatically driven piston assemblies are also relatively difficult to service and undesirably interrupt the printing process each time that maintenance is performed upon them or additional viscous material is added to the container. Moreover, these prior piston-viscous material containment assemblies usually inject or communicate at least a portion of the contained viscous material into a top portion of the head. The communicated viscous material is typically made to traverse through the compression head and is made to exit at or through a bottom portion or viscous material dispensation aperture which is typically and integrally formed within the compression head.
This arrangement typically causes the velocity of the emitted or exiting viscous material to be non-uniform and to have a substantially non-uniform velocity profile. That is, the velocity of the viscous material which exits the aperture and which enters the stencil perforations is non-uniform and varying. The exiting viscous material therefore provides or has a non-uniform velocity profile. Moreover, that portion of the dispensed viscous material, which encounters a solid or non-perforated portion of the stencil, such as a “paste-filled” hole, causes a certain amount of “backpressure” to be created within the compression head and/or within or through the dispensation aperture. The created “backpressure” is substantially non-uniform and varies from location to location within the compression head and varies from location to location within or through the viscous material dispensation aperture. The selectively dispensed viscous material thereby provides or has a non-uniform pressure profile.
Particularly, most of the received viscous material flows through a path or paths which is (are) substantially and axially aligned with the material reception aperture or that location through which the viscous material is communicated into the compression head. This flow pattern is due to the fact that this path or (these paths) has (have) a relatively lower flow resistance than other flow paths which are not substantially aligned with (are remotely positioned from) the material reception aperture. Accordingly, material traversing this path (or paths) exits the compression head at a substantially higher velocity than the material flowing through the various other paths. It is this dissimilarity in flow resistance which causes the viscous material to be dispensed into the stencil perforations at velocities which vary from “location to location” within the material dispensation aperture, thereby forming a substantially non-uniform velocity dispensation profile (e.g., the “velocity dispensation profile” or “velocity profile” comprises the velocity values associated with the viscous material which is communicated to the stencil perforat
Lamb Brenda A.
Visteon Global Tech. Inc.
Visteon Global Technologies Inc
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