Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1999-05-11
2002-07-02
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S039000, C228S041000, C029S832000
Reexamination Certificate
active
06412685
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 the release and optional inspection of a pattern of conductive preforms from a locating mechanism onto electronic pads that are on a receiving substrate such as for a ball grid array (BGA) applicator. The general concept of a release mechanism for a conductive preform placement and/or locating apparatus is taught in the parent patent application.
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 conductive preforms on electronic pads on a substrate utilizes a locating plate placed over the electronic pads on the substrate to guide the conductive preforms to drop through a pattern of openings within the locating plate onto the electronic pads. The electronic pads having been pre-deposited with solder paste, the conductive preforms then adhere to the electronic pads via the solder paste. It is also recognized in the art that flux, conductive epoxy, or other tacky mediums may be used to temporarily secure the conductive preforms to the electronic pads prior to permanent attachment. During a reflow operation, the conductive preforms fuse to the electronic pads on the substrate.
The first known method of releasing the conductive preforms from the locating plate is using the tackiness of the tacky medium to pull the conductive preforms from the openings in the locating plate during separation of the locating plate and the substrate. This method does not ensure the conductive preforms are dislodged from the locating plate.
A second known method utilizes a hot-air knife to reflow the conductive preforms while using the locating plate to maintain alignment of the conductive preforms to the electronic pads of the substrate. 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 conductive preforms are in the locating plate may cause the conductive preforms to melt and adhere to the locating plate. Further, the conductive preforms may change shape to geometrically couple themselves to the locating plate and cause manufacturing defects with missing solder spheres or long term reliability risks with lifted pads. Furthermore, a heating-knife motion control mechanism can be expensive.
A third 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. This is actually moving the tube, not applying an externally sourced vibrational energy to the tube.
The apparatus for this third method tends to be complicated and can be expensive to produce and maintain. Since the conductive preforms 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. The vibrations are used to break any static energy which would hold the solder sphere to the tube, but does not apply forces to positively separate the conductive preforms from the tube.
A fourth known method for placing conductive preforms on electronic pads on a substrate is taught in the original application which utilizes a vacuum system coupled to conductive preform locating apparatus sometimes referred to as a placement head. The placement head consists of a vacuum block and a locating plate, where the locating plate includes a pattern of openings. Each opening provides a vacuum force to lift a conductive preform into each opening. After locating the pattern of openings holding the conductive preforms proximate the pattern of electronic pads, the conductive preforms are placed on the electronic pads by removing the vacuum force from the tubes and using a tacky media to release the solder balls onto the electronic pads. A fifth known method for placing conductive preforms onto electronic pads uses preforms, where the preforms temporarily hold a pattern of conductive preforms within a removable holder. The use of preforms is not conducive to high volume production of these devices. Additionally, the current removable holder is of water soluble material and requires exposure to water for removal. It is not conducive to subject the said components to moisture, as this yields manufacturing defects and long term reliability risks. The entrapped moisture expands during the reflow process and the expansion of moisture trapped between the Integrated Circuit (IC) and the die attach pad causes them to separate.
It is known that tacky medium such as flux paste is used to remove the conductive preforms from the locating apparatus and temporarily couple the conductive preforms proximate the electronic pads. Since the forces tend to separate the conductive preform from the electronic pad, the conductive preform is generally suspended within the tacky medium. When the tacky medium is heated, as required by the securing process, the conductive preforms are known to float off the electronic pads and sometimes fuse with adjacent conductive preforms creating a defect. The use of Ball Grid Array technology is increasing as the advantages of the interconnect process are recognized. Two primary methods are used to assemble the components: 1) placing the spheres through a locating plate and 2) picking up the conductive preforms using vacuum, and generally placing them within a locating plate then transferring them to the component. A recognized problem in the high volume manufacturing process of these components is the assurance of the completeness of the pattern of conductive preforms. It is known that the completeness of the pattern is dependent upon ensuring a conductive preform is positioned within each of the locating mechanisms and subsequently transferred to the respective electronic pad. The current inspection systems utilize vision which is costly and time consuming. It is also known that the conductive preforms tend to float off the electronic pads when simply placed upon the tacky media during the curing process. The preferred separation process would include a means to position the conductive preforms against the electronic pads, where the conductive preform would not float off the electronic pad during heating.
What is desirable is a mechanism to ensure completeness of a pattern of conductive preforms, the separation between the conductive preform locating apparatus (locating plate, etc.) and the conductive preforms, depositing the conductive preforms onto a tacky media, and preferably locating the conductive preform proximate the electronic pad.
SUMMARY OF THE INVENTION
One aspect of the present invention is to provide an apparatus for placing and/or locating a pattern of conductive preforms onto a corresponding pattern of electronic pads. The apparatus comprises a conductive preform locating mechanism which includes a means of creating a pattern of conductive preforms from a unorganized volume of conductive preforms. It can be recognized that one skilled in the art may apply the invention to any of the above mentioned or other methods for the location and/or placement of conductive preforms recognizing the spirit and intent of the present invention.
Another aspect of the present inve
Hertz Allen D.
Hertz Eric Lee
Edmondson L.
Galahad, Co.
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