Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-08-23
2001-02-20
Gerrity, Stephen F. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S740000, C029S832000, C174S260000
Reexamination Certificate
active
06189208
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods for electrically connecting a flip chip to a substrate.
Flip chip mounting is an increasingly popular technique for directly electrically connecting an integrated circuit chip to a substrate such as a circuit board. In this configuration, the active face of the chip is mounted face down, or “flipped” on the substrate. The electrical bond pads on the flip chip are aligned with corresponding electrical bond pads on the substrate, with the chip and substrate bond pads electrically connected by way of an electrically conductive material. The flip chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, resulting in increased reliability of the chip-to-substrate bond.
A wide range of electrically conducting compositions have been proposed for making the interconnection between flip chip and substrate bond pads. Solder balls, gold bumps, gold stud bumps, and other conventional metal bump configurations have been used extensively. Aside from metallic compositions, electrically conducting polymer compositions are gaining wide acceptance as flip chip interconnection bump materials. In a flip chip mounting technique employing polymer interconnections, electrically conductive polymer bumps are formed on the bond pads, typically of the flip chip, and are polymerized or dried during bonding to the substrate bond pads, whereby both an electrical and a mechanical adhesive bond between the flip chip and the substrate bond pads is produced. Electrically conductive polymer materials are particularly well-suited for flip chip mounting techniques because of their ease of application, because they eliminate many of the unwanted characteristics of metallic interconnections, e.g., solder flux, and because for some polymer materials reworkability of faulty flip chips is enabled by simple heating of the material.
Conventionally, once a flip chip is bonded to a substrate, whether by metallic or by polymer bump interconnections between the chip and substrate bond pads, an underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip chip and the substrate, and inhibits environmental attack of chip and substrate surfaces.
SUMMARY OF THE INVENTION
The invention provides a process that exploits the superior bonding capabilities of electrically conductive polymer materials for bonding a flip chip to a substrate while providing a highly efficient and effective technique that eliminates the need for conventional post-bond underfill dispensing operations. In this process, a layer of electrically insulating adhesive paste is applied on a substrate having bond pads, covering the bond pads with the adhesive. Electrically conductive polymer bumps are formed on bond pads of a flip chip to be bonded to the substrate, and the polymer bumps are at least partially hardened. The bond pads of the flip chip are then aligned with the bond pads of the substrate, and the at least partially hardened polymer bumps are pushed through the adhesive on the substrate to contact directly and bond the polymer bumps to the bond pads of the substrate.
This process results in direct electrical and mechanical bonding of the polymer bumps between the chip and substrate bond pads, even though the adhesive film was applied on the substrate in a manner that covered the substrate bond pads. The polymer bumps displace the adhesive as they are pushed through it and expand laterally on the substrate bond pads. As a result, the area around the polymer bumps between the chip and the substrate is filled with the adhesive, in the manner of an underfill. A separate, post-bond underfill process is therefore not required. The particular performance advantages of polymer bump material provided by the invention, in combination with the highly efficient adhesive underfill application process provided by the invention, render this flip chip mounting process superior to conventional mounting techniques.
In embodiments provided by the invention, the adhesive paste applied to the substrate can be at least partially dried or at least partially cured, as appropriate for the selected paste material, before the step of pushing the polymer bumps through the adhesive on the substrate. Similarly, the step of at least partially hardening the polymer bumps can be carried out by at least partially drying or by at least partially polymerizing the polymer bumps, as appropriate for the selected bump material.
In further embodiments provided by the invention, heat is applied to the flip chip as the bumps are pushed through the adhesive on the substrate. Heat can also be applied to the flip chip after the polymer bumps contact the bond pads of the substrate. Pressure is preferably applied to the flip chip during the bonding process for a selected duration, based on material characteristics of the adhesive and of the polymer bumps, that vertically compresses the bumps between the flip chip and the substrate to a compressed height that is less than bump height as-formed; e.g., resulting in a compressed bump height that is less than about 80% of bump height as-formed. The polymer bumps, as-formed, preferably have a bump height that is greater than the adhesive paste thickness as-applied on the substrate, more preferably having a bump height that is at least about 25% greater than the adhesive paste thickness.
The polymer bumps and the adhesive paste can each be formed of, e.g., a thermoplastic material, a thermoset material, or a B-stage thermoset material. The polymer bumps can include hard particles, preferably that have jagged edges which protrude from the bump. Such particles can be electrically conductive or electrically nonconductive. The adhesive material can include a solvent to aid in its application to the substrate.
Both the polymer bump formation and the adhesive application to the substrate can be carried out by, e.g., a stenciling process or a screen printing process. The adhesive can further be applied by, e.g., a dispensing or by a spin-coating process.
The flip chip mounting technique of the invention is widely applicable to a range of substrate materials and flip chip mounting configurations. The flexibility in adhesive application and polymer bump formation methods allow for versatility in material formulation for application-specific considerations. In general, the flip chip mounting technique can be employed as a superior alternative for most conventional flip chip mounting processes that employ solder or other metallic bumps and conventional post-bond underfill processes, resulting in enhanced mounting quality and improved process efficiency.
Other features and advantages of the flip chip mounting method of the invention will be apparent from the following description and accompanying drawing, and from the claims.
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Akita Masanori
Estes Richard H.
Ito Koji
Mori Toshihiro
Wada Minoru
Gerrity Stephen F.
Kim Paul D.
Lober Theresa A.
Polymer Flip Chip Corp.
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