Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1997-01-28
2001-03-20
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S041000
Reexamination Certificate
active
06202918
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 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, 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.
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 hold and place the conductive preforms onto the pads.
Another aspect of the present invention is the use of artwork and a photodeveloping and etching process on the foil to create the openings. This eliminates significant variation in locating and forming the openings in the foil while maintaining a low cost for the tool. As the distance between pads (pitch) decreases, such as for fine pitch BGA 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 include a mechanism to assist with releasing the conductive preforms from the openings in the foil to reliably place the conductive preforms on the pads while overcoming friction or tack at the openings in the foil.
Another aspect of the present invention is the ability to include a mechanism to hold the conductive preforms at the openings in the foil and then remove the holding force to place the conductive preforms on the pads.
Another aspect of the present invention is the ability to create a taper in the openings of the foil to attain a better process for accounting for tolerances, securing, and releasing the conductive preforms. The taper can preferably be varied by modifying the artwork for the two opposing sides of the foil.
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Edmondson Lynn
Ryan Patrick
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