Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1997-04-03
2001-05-15
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S041000
Reexamination Certificate
active
06230963
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to the field of conductive preform placement systems for surface mount technology, and in particular to a method and apparatus for placing solder balls on electronic pads that are on a substrate such as for a ball grid array (BGA) applicator.
BACKGROUND OF THE INVENTION
Conventional methods for manufacturing surface mount components, or for manufacturing circuit supporting substrates for surface mount components, typically include methods for placing conductive preforms, e.g., solder balls, solder spheres, and preformed solder bumps, on electronic pads arranged in a predetermined placement pattern that is sometimes called a ball grid array (BGA).
A known method for placing solder bumps on electronic pads on a substrate utilizes a stencil placed over the electronic pads on the substrate to guide solder paste to flow through openings in the stencil plate onto the electronic pads. The solder paste is typically spread over the stencil using a squeegee to remove the excess solder paste. After the stencil is removed from the substrate, solder bumps are formed on, and remain attached to, the electronic pads. This method technically forms the solder bumps on the electronic pads and does not place solder that has been preformed on the electronic pads.
The solder paste, as formed in this method, has a tendency to develop internal structural defects, such as voids, or variation of fused solder volumes during the fusing process, thereby introducing potential defects to the manufacturing process and/or risk of failure during the life of the product. This is an undesirable consequence of this method.
A first known method for placing solder balls on electronic pads on a substrate utilizes a stencil plate placed over the electronic pads on the substrate to guide solder balls to drop through openings in the stencil plate onto the electronic pads. The electronic pads having been pre-printed with solder paste, the solder balls then adhere to the electronic pads via the solder paste. During a reflow operation, the solder balls fuse to the electronic pads on the substrate.
Besides requiring a guiding force to reliably introduce the solder balls into the openings in the stencil plate, this method additionally suffers from a hot-air knife reflow heating step that unevenly distributes heat over the solder balls in the stencil plate. Further, the heating step applied while the solder balls are in the stencil may cause the solder to melt and adhere to the stencil. Furthermore, a heating-knife motion control mechanism can be expensive.
A second known method for placing solder balls on electronic pads on a substrate utilizes tubes to hold the solder balls over the electronic pads. Each tube applies a vacuum force to hold a solder ball to the end of the tube. After locating the tubes holding the solder balls over the electronic pads, the solder balls are placed on the electronic pads by removing the vacuum force from the tubes and vertically vibrating the tubes to release the solder balls onto the electronic pads.
The apparatus for this second method tends to be complicated and can be expensive to produce and maintain. Since the solder balls are placed sequentially, the process is not conducive to cycle time. It also may not be suitable for micro-BGA placement where the pitch of the pads is very fine and requires tight tolerances in locating the solder spheres.
A third known method for placing solder balls on electronic pads on a substrate utilizes a plate with solder bumps attached to the plate in a pattern corresponding to the pattern of the electronic pads on the substrate. The solder bumps are attached to the plate by etching a pattern of openings in a photoresist mask over the plate according to a predefined artwork, and then depositing solder composition on the plate at the openings (where the plate surface is exposed) by an electroplating operation. Lastly, after removing the photoresist layer, the solder bumps remain attached to plate. The solder bumps are then placed on the electronic pads on the substrate by positioning the plate over the electronic pads to allow the solder bumps to contact the electronic pads. By heating the entire assembly, the solder bumps melt and transfer onto the electronic pads.
Besides constituting a relatively expensive process to implement in a mass production environment or use for occasional rework, this method requires trained operators to perform numerous steps, including chemical processing steps that can subject an operator to environmental hazards. The overall process, therefore, can be environmentally unfriendly, time consuming, expensive, and generally requiring trained operators to be effective.
Foils can be of various materials, including stainless steel, Brass, Nickel, and the like. The means of colorizing a foil can be respective to the particular material, and accomplished by inquiring to one skilled in the art.
One known method for processing metals is used by the automotive industry with one example taught by Gay, et al (U.S. Pat. No. 4,764,260, dated Aug. 16, 1988) “Process for Electroplating Nickel Over Stainless Steel”, where Gay teaches a method of plating nickel to stainless steel. Gay additionally teaches the ability to use conventional plating methods onto the base Nickel surface omre specifically Chrome. Gay further mentions, whereby once the nickel is plated onto the stainless steel, other conventional plating processes may be used to apply other materials. Once plated with nickel, the foil can then be plated in a zinc bath, and lastly subject the plate to a black chromate bath.
A second known method of colorizing metal is to apply tool maker's die such as red tool makers dye.
A plurality of pins for use as a releasing and/or placing mechanism was disclosed in the parent application, Ser. No. 08/789,883, dated Jan. 28, 1997.
The use of Ball Grid Array technology is increasing as the advantages of the interconnect process are recognized. The disadvantage of this technology is where rework or salvage of components using Ball Grid Array technology is required; once the component is removed a portion of the solder preforms remains on the component and a portion of the solder preforms remains on the Printed Circuit Board (PCB). Thus, what is necessary is a low cost and efficient method and apparatus for placing conductive preforms on pads on a component, or on a substrate.
SUMMARY OF THE INVENTION
One aspect of the present invention is to provide a low cost tool for locating and placing the conductive preforms onto the pads of substrates or components. The tool preferably comprises a foil structure that includes a plurality of openings that are used to locate, hold, and place the conductive preforms onto the pads.
Another aspect of the present invention is the use of current state of the art technology, including artwork and a photodeveloping and etching process on the foil to create the openings. This process eliminates significant variation in locating and forming the openings in the foil while maintaining a low cost for the tool. As the distance between the centers of the pads (pitch) decreases, such as for fine pitch, or micro BGA (PBGA) manufacturing, the variation in locating and shaping the openings becomes significantly more critical for maintaining an accurate and reliable conductive preform placement process.
Another aspect of the present invention is the ability to facilitate changing a pattern of openings on a foil for placing conductive preforms on different arrangements (patterns) of pads. By using different foils with different etched patterns (different patterns of openings etched in the foils), the low cost tool can efficiently place conductive preforms on different patterns of pads on a substrate.
Another aspect of the present invention is the ability to utilize one aperture pattern and modify the placed pattern of preforms by filling or covering the undesirable apertures. The material partially covering the first foil aperture can increase the reliability of filler mat
Knapp Jeffrey T.
Ryan Patrick
LandOfFree
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