Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-02-07
2001-09-25
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C257S793000, C523S429000, C523S440000, C523S443000, C523S466000
Reexamination Certificate
active
06294271
ABSTRACT:
This invention relates to a flip-chip type semiconductor device sealing material and a flip-chip type semiconductor device encapsulated therewith.
BACKGROUND OF THE INVENTION
With the advance of electric equipment toward smaller size, lighter weight and higher performance, the semiconductor mounting technology has changed from the pin mating type to the surface mounting which now becomes the mainstream. One bare chip mounting technology is flip-chip (FC) mounting. The flip-chip mounting is a technique of providing an LSI chip on its circuit pattern-bearing surface with several to several thousands of electrodes, known as bumps, of about 10 to 100 microns high and joining the chip to electrodes on a substrate with a conductive paste or solder. Then the sealing material used for the protection of FC devices must penetrate into gaps of several tens of microns defined by bumps between the substrate and the LSI chip. Conventional liquid epoxy resin compositions used as the underfill material for flip-chip devices are generally composed of a liquid epoxy resin, a curing agent and an inorganic filler. Of these, the most predominant is a composition in which a large amount of inorganic filler is blended in order to provide a matching coefficient of linear expansion with those of semiconductor chips, substrates and bumps for increased reliability.
With respect to stress properties, the flip-chip underfill materials with high loading of filler give rise to no problem. However, they suffer from very low productivity since they have a high viscosity due to the high filler loading so that they may penetrate into the gap between chip and substrate at a very slow rate. There is a desire to overcome this problem.
In conventional liquid epoxy resin compositions used as underfill material, acid anhydrides are often employed as the curing agent. Since the acid anhydrides are hygroscopic, there arises prior to curing a phenomenon that a viscosity increase due to moisture absorption can cause varying infiltration or interrupt infiltration. Also, the acid anhydrides are likely to pick up moisture prior to curing and even after curing, subject to enhanced hydrolysis by virtue of ester bonds, both of which in turn can cause volumetric expansion, increasing the resistance at the interface between solder bumps and leads. This gives rise to a reliability problem.
SUMMARY OF THE INVENTION
An object of the invention is to provide a flip-chip type semiconductor device sealing material which maintains a low enough viscosity to ensure interstitial infiltration even when filled with a large amount of inorganic filler. Another object is to provide a flip-chip type semiconductor device encapsulated with the sealing material and having improved reliability.
It has been found that a liquid epoxy resin composition comprising a liquid epoxy resin and an inorganic filler is improved using as a reaction or curing accelerator a specific imidazole compound of the general formula (1) to be defined below having a solubility of up to 1% by weight in the epoxy resin, a melting point of at least 170° C., a mean particle diameter of 1 to 5 &mgr;m, and a maximum particle diameter of up to 20 &mgr;m. The resulting composition remains highly stable at elevated temperatures and when used for the encapsulation of flip-chip semiconductor devices, can infiltrate through narrow gaps at a low viscosity even with high loading of the inorganic filler, as long as the semiconductor device is heated to elevated temperatures. The composition thus provides a sealing material suitable as an underfill material for flip-chip semiconductor devices.
As described above, acid anhydrides are widely employed as the curing agent in conventional liquid epoxy resin compositions used as underfill material. Since the acid anhydrides are hygroscopic, there arises prior to curing a phenomenon that a viscosity increase due to moisture absorption can diversify the infiltration capability or interrupt infiltration. Also, the acid anhydrides are likely to pick up moisture prior to curing and even after curing, subject to enhanced hydrolysis by virtue of ester bonds, both of which in turn can cause volumetric expansion, increasing the resistance at the interface between solder bumps and leads. This gives rise to a reliability problem. It is thus believed that how to reduce the saturation moisture pickup is a key to reliability improvement. A study is made on a resin composition having epoxy self-condensation or ether bonds which are less liable to hydrolysis than ester bonds. Use of this curing system can minimize the deterioration of properties by humidity, for example, lowering of bonding force and glass transition temperature (Tg) after water absorption. However, the epoxy self-condensation system has a drawback that it tends to be highly viscous because of the resin's viscosity and thus has an inferior infiltration capability, as compared with the acid anhydride curing system. This drawback can be overcome by using the curing accelerator which is stable at elevated temperatures and keeping the device at elevated temperatures so that the sealing material may become low viscous and have a high infiltration capability. In addition, as opposed to the acid anhydride system which is unstable during storage and must be kept at very low temperatures for storage, the composition of the invention using a potential catalyst is improved in storage stability and its viscosity remains unchanged over 168 hours at room temperature.
Accordingly, a sealing material for flip-chip type semiconductor devices in the form of a liquid epoxy resin composition as defined herein has improved thin-film infiltration capability and storage stability as an underfill material. A semiconductor device encapsulated with the sealing material remains highly reliable.
Thus the invention provides a sealing material for flip-chip type semiconductor devices in the form of a liquid epoxy resin composition comprising (A) a liquid epoxy resin, (B) an inorganic filler, and (C) a curing accelerator having a solubility of up to 1% by weight in the epoxy resin, a melting point of at least 170° C., a mean particle diameter of 1 to 5 &mgr;m, and a maximum particle diameter of up to 20 &mgr;m. The curing accelerator is represented by the following general formula (1):
wherein R
1
and R
2
each are hydrogen, methyl, ethyl, hydroxymethyl or phenyl, R
3
is methyl, ethyl, phenyl or allyl, and R
4
is hydrogen or a group of the following formula (2):
Also contemplated herein is a flip-chip type semiconductor device sealed with the sealing material.
REFERENCES:
patent: 4999699 (1991-03-01), Christie
patent: 5089440 (1992-02-01), Christie
patent: 5248710 (1993-09-01), Shiobara et al.
patent: 5292688 (1994-03-01), Hsiao
patent: 5928595 (1999-07-01), Knapp
Kumagae Kimitaka
Shiobara Toshio
Sumita Kazuaki
Wakao Miyuki
Aylward D.
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
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