Coating apparatus – Immersion or work-confined pool type – Mask or stencil
Reexamination Certificate
1999-07-22
2002-05-28
Edwards, Laura (Department: 1734)
Coating apparatus
Immersion or work-confined pool type
Mask or stencil
C118S410000, C118S413000, C425S461000
Reexamination Certificate
active
06395087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. 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.
2. 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.
Moreover, these conventional methodologies and assemblies do not substantially ensure that the viscous material is dispensed and selectively placed upon the various portions of the circuit board, through the perforated stencil, at a substantially equal velocity. These conventional methodologies and assemblies also do not substantially ensure that the “backpressure” formed within the compression head and/or within or through those portions of the exit aperture overlaying solid or “non-perforated” portions of the stencil and/or overlaying those perforated stencil portions which are filled with paste or viscous material, is substantially identical or uniform. Hence, these conventional methodologies and assemblies provide an undesirable and “non-uniform” velocity and pressure or “backpressure” distribution or “profile”. Particularly, a velocity “profile” is the numerical value of the velocity of the emitted viscous material at various locations within the exit aperture which overlay perforated portions of the stencil. A pressure “profile” is the numerical value of the pressure or “backpressure” created through and/or within the various locations of the exit aperture by the exiting viscous material which encounters “solid” stencil portions (e.g., paste filled openings or portions of the stencil having no perforations).
Particularly, the viscous material or solder “paste” is typically and selectively received within a top portion of a compression head through a material reception aperture. The received viscous material is made to travel through the compression head before exiting through an exit aperture or a slotted opening which is usually formed within the bottom portion of the compression head.
Most of the received viscous material traverses through the compression head along a path which is substantially aligned with the reception aperture. This path, in the above-described arrangement, typically lies along the center of the compression head and, more particularly, typically lies between the viscous material reception aperture (e.g., the location where the viscous material enters the head) and the exit opening or exit aperture. The viscous material traveling along this aperture-aligned path selectively emanates from or is selectively emitted from the compression head at a substantially higher velocity than the material which traverses along the various ends or outer wall portions of the compression head, and forms greater amounts of pressure or “backpressure” than the viscous material traveling along these other paths. This non-uniform velocity and pressure distribution or profile causes uneven amounts of the viscous material to be deposited upon the stencil, thereby causing much of the deposited paste or viscous material to be undesirably wasted and concomitantly reducing the overall quality of the printed and created circuit board.
To provide improved pressure and velocity profile uniformity of the viscous material, a series of perforated plates or islands are oftentimes deployed and used within the compression head. These objects are somewhat effective to redistribute the flow of the received viscous material in a manner which causes more of the received viscous material to flow along the end or wall portions of the compression head and to increase the flow resistance along the previously described aperture-aligned path of and/or within the compression head. While these plates or islands do reduce some of the foregoing non-uniformity, they suffer from some drawbacks.
By way of example and without limitat
Ha Vinh Van
Jairazbhoy Vivek Amir
Lin Jeff (Jin Her)
Trublowski John
Yang Zhaoji
Brinks Hofer Gilson & Lione
Edwards Laura
Visteon Global Technologies Inc.
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