Metal fusion bonding – Including means to apply flux or filler to work or applicator – By partial or total immersion of work or applicator into liquid
Reexamination Certificate
2000-06-07
2001-10-02
Dunn, Tom (Department: 1725)
Metal fusion bonding
Including means to apply flux or filler to work or applicator
By partial or total immersion of work or applicator into liquid
C228S033000, C228S207000
Reexamination Certificate
active
06296169
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of semiconductor manufacturing including surface mount technologies (SMT), and pertains more particularly to apparatus for placing solder flux on an array of die pads on a substrate in preparation for BGA assembly.
BACKGROUND OF THE INVENTION
The field of integrated circuit packaging is one of the most evolutionary fields connected to semiconductor manufacturing. As demand for devices that are smaller and more powerful continues to increase, pressures are put on manufacturers to develop better and more efficient ways to assemble and package IC products. One of the more recently developed methods for assembling and packaging IC products is known as Ball-Grid-array (BGA) technology. Motorola™ inc. is one of the more noted pioneers of BGA technology. Currently there are many companies that license BGA technology developed by Motorola™.
BGA technology, which uses solder balls instead of interconnect leads, offers several advantages over more mainstream technologies such as Fine-Pitch-Technology (FTP), and Pin-Grid-Array (PGA). One obvious advantage is that there are no leads that can be damaged during handling. Another advantage is that the solder balls are self-centering on die pads. Still other advantages include smaller size, better thermal and electrical performances, better package yields, and so on.
In typical BGA assembly, an array of metallic die pads provided in a substrate or wafer is fluxed during a fluxing operation in preparation for insertion of individual solder balls one per pad. After the solder balls are placed into the array of pads, a next operation involving a heat process causes the solder balls to be soldered to the individual pads performing the connecting mechanisms for surface mounting.
The above-described fluxing operation conventionally involves the use of a plate having a specific array of openings placed therein for the purpose of retaining an identical array of same-length pins, which are mounted into the openings. The resulting fixture is then placed into a source of flux material and touched to the die-pad array thus transferring flux material to each individual die pad simultaneously. Generally speaking, the arrangement of openings and mounted pins specific to a flux fixture is identical to a specific arrangement of die pads on a given substrate.
Problems can accrue with respect to the method described above, specifically regarding the physical aspects of and vulnerability of the fluxing fixture itself. For example, it is a difficult process to ensure that all pins specific to a fixture are mounted such that they are protruding at a same length. If some pins are shorter than others in a complete fixture, then the associated die pads on a given substrate will not receive the proper amount of fluxing. In addition to length variances, perpendicularity variances with regard to individual pins and their positions of mounting within the provided openings of the flux plate cause the pin surfaces of the non-perpendicular pins to form a non-coplanar relationship with their associated die pads. This results in improper delivery of the flux material to the die pads. It is further noted that prior-art pins and openings are restricted by practicality to being round, which is another disadvantage in flux application.
What is clearly needed is a fluxing fixture that is formed of one-piece such that all of the pins are physically identical to each in cross-section and length and retain identical positions with respect to the surface of the fixture. A fixture such as this would streamline the fluxing operation in BGA assembly by providing a physically consistent interface between the surfaces of each pin and their associated die pads when transferring flux material.
SUMMARY OF THE INVENTION
In a preferred embodiment of the present invention a flux-application fixture for applying flux to die pads in a ball-gild-array assembly process is provided, comprising a solid plate having a lower surface and an upper surface; and a plurality of flux pins formed in the upper surface of the solid plate. The fixture is characterized in that the flux pins are contiguous with the solid plate and are formed by removing material from the upper surface of the plate such that the remaining material not removed forms the plurality of flux pins.
The material removed from the upper surface of plate is removed by a machine-directed grinding process in a preferred embodiment, wherein grooves are ground into the plate in an onthogonal arrangement. The material may be stainless steel, nickel-steel, or some other hardened alloy. Typically the flux pins in embodiments of the invention are rectangular in shape having upper surfaces that are rounded by virtue of smoothing the rectangular comers and edges.
In another aspect of the invention a method for manufacturing a flux-application fixture is provided, comprising steps of (a) placing a flat plate on a grinding machine having a grinding wheel for removing material from the plate; (b) removing material from the plate in a series of equally spaced and parallel grooves across the plate; (c) reorienting the plate on the grinding machine at a 90 degree rotation from the original orientation; and (d) removing material from the plate in a series of equally spaced and parallel grooves across the plate. Preferably the flat plate is of rectangular shape and is ground flat before removing material.
Now, for the first time, a fluxing fixture is provided, wherein all of the pins are physically identical to each other and retain identical positions with respect to the surface of the fixture. A fixture such as this streamlines the fluxing operation in BGA assembly and provides a physically consistent interface between the surfaces of each pin and their associated die pads when transferring flux material.
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patent: 5816481 (1998-10-01), Economy et al.
patent: 5834062 (1998-10-01), Johnson et al.
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patent: 6056190 (2000-05-01), Foulke et al.
Advanced Interconnect Solutions
Boys Donald R
Central Coast Patent Agency Inc.
Cooke Colleen P.
Dunn Tom
LandOfFree
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