Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-07-21
2001-05-15
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C523S466000
Reexamination Certificate
active
06231997
ABSTRACT:
This invention relates to epoxy resin compositions for semiconductor encapsulation and more particularly, to epoxy resin compositions suited for the encapsulation of ball grid array (BGA) packages because of minimized package warp, minimized wire flow and good adhesion. It relates also to semiconductor devices encapsulated with the compositions in a cured state.
BACKGROUND OF THE INVENTION
Resin-encapsulated devices currently predominate in the semiconductor industry. Epoxy resins are generally superior to other thermosetting resins in terms of moldability, adhesion, electrical characteristics, mechanical characteristics, and moisture resistance. Epoxy resin compositions are commonly used for the encapsulation of semiconductor devices.
Ball grid array (BGA) packages, developed fairly recently by Motorola, have a distinctive structure in which the chip is mounted directly onto the circuit board substrate, and the top of the chip is encapsulated in plastic. In BGA packages, only one side of the substrate is resin encapsulated. Hence, the difference in shrinkage factor between the substrate and the resin leads to warping of the package, which is a major problem.
A number of attempts have been made to overcome this problem by increasing the glass transition temperature (Tg) and lowering the thermal expansion coefficient of the resin so as to reduce the difference in shrinkage between the substrate and the resin, and thus minimize package warp. One specific solution involves using a polyfunctional epoxy resin, a polyfunctional phenolic resin as the curing agent, and an imidazole compound as the curing accelerator in order to increase the glass transition temperature, and including also a large quantity of silica filler to lower thermal expansion. In order to enable high loading of silica filler while maintaining good flow characteristics, it is known to use all spherical silica particles free of fragments so as to optimize the particle size distribution of the filler. A method of treating silica with a coupling agent to optimize its surface state is also known. Another important property when evaluating device reliability is the adhesion of the resin to the solder mask covering the substrate surface. It is well known in the art that adhesion of the resin to the solder mask can be dramatically enhanced by the judicious selection of an epoxysilane or mercaptosilane coupling agent.
Yet, the prior art described above was found to have a number of serious drawbacks. For instance, the use of an epoxy resin and a phenolic resin both of the polyfunctional type results in a cured product having an increased water absorption because of an increased free volume within the molecular structure. As a result, the cured product becomes low in soldering heat resistance after moisture absorption and susceptible to popcorn cracks. Also, all non-crystalline epoxy resins including polyfunctional ones have a relatively high viscosity. When an epoxy resin composition is loaded with a large quantity of an inorganic filler such as silica, the composition has an increased melt viscosity which when a BGA package is encapsulated therewith, causes molding defects such as wire flow and breakage. Additionally, epoxy resin compositions using an imidazole compound as the curing accelerator have a shelf stability inferior to that of compositions using a phosphorus-containing accelerator, so that wire flow and incomplete filling due to a rapid rise in viscosity in the resin encapsulation step are more likely to arise unless the resin encapsulating step is strictly managed. In addition, hydrolyzable chlorine within the epoxy resin is more readily extracted, which can be detrimental to device reliability in the presence of moisture. Furthermore, the steady increase in package size within the industry requires that further reductions be made in thermal expansion, but the high loadings of silica currently in use increase the viscosity of the composition, resulting in frequent wire flow. A certain type of coupling agent slows the curing speed of the epoxy resin composition at the time of resin encapsulation, which can lead to an increase in package warp.
Effective solutions have not previously been found to these and other problems associated with prior-art resin compositions for BGA encapsulation.
SUMMARY OF THE INVENTION
An object of the invention is to provide a semiconductor-encapsulating epoxy resin composition which has improved working efficiency and reliability upon encapsulation of semiconductor devices because of minimized package warp, minimized wire flow, good adhesion to a solder mask, and shelf stability. Another object of the invention is to provide a semiconductor device encapsulated with the composition in a cured state.
The inventor has found that by combining (A) a crystalline epoxy resin, (B) a polyfunctional phenolic resin, (C) an organic phosphorus curing accelerator, and (D) an aminosilane coupling agent, and blending (E) a large amount of an inorganic filler therein, there is obtained a semiconductor-encapsulating epoxy resin composition featuring minimized package warp, improved adhesion, cured product's low water absorption, minimized wire flow, and storage stability, and enabling highly reliable encapsulation of semiconductor devices, especially BGA.
Specifically, the invention provides a semiconductor-encapsulating epoxy resin composition comprising as essential components, (A) a crystalline epoxy resin, (B) a polyfunctional phenolic resin, (C) an organic phosphorus curing accelerator, (D) an aminosilane coupling agent, and (E) an inorganic filler. The inorganic filler (E) is at least 88% by weight based on the composition. The polyfunctional phenolic resin (B) is of the following general formula (1):
wherein R is hydrogen, methyl or ethyl, R′ is hydrogen or an alkyl group having 1 to 4 carbon atoms, and m is an integer of 1 to 4.
DETAILED DESCRIPTION OF THE INVENTION
The epoxy resin used herein is a crystalline epoxy resin. It is preferably selected from epoxy resins having the molecular structure represented by the following general formulae (2), (3), (4) and (5). These epoxy resins have a high melting point (or softening point) of at least 100° C. so that they maintain a robust crystal structure until the temperature reaches the melting point (or softening point). Once the melting point (or softening point) is reached, they quickly melt into a very low viscosity liquid. This attribute allows the epoxy resin composition to be loaded with a large amount of inorganic filler, which enables to reduce the expansion coefficient of a cured product thereof.
In formula (5), R
1
to R
10
are independently selected from among hydrogen and alkyl groups of 1 to 6 carbon atoms, and n is an integer of 0 to 4.
The alkyl groups may be straight, branched or cyclic and include, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, and cyclohexyl. The letter n is an integer of 0 to 4, preferably equal to 0 or 1.
Also useful as the crystalline epoxy resin (A) is a mixture of at least two of the epoxy resins of formulae (2) to (5) wherein n is an integer of 0 to 4, in any desired ratio.
In the practice of the invention, another epoxy resin may be used in combination with the crystalline epoxy resin insofar as the objects of the invention are not impaired. Examples of the other epoxy resins include bisphenol A type epoxy resins, novolac type epoxy resins such as phenol novolac epoxy resins and cresol novolac epoxy resins, triphenolalkane type epoxy resins such as triphenolmethane epoxy resins and triphenolpropane epoxy resins, naphthalene ring-containing epoxy resins, phenolaralkyl type epoxy resins, alicyclic type epoxy resins, and dicyclopentadiene type epoxy resins. In this embodiment, the crystalline epoxy resin should preferably account for at least 50% by weight (i.e., 50 to 100% by weight) and more preferably 75 to 95% by weight of the entire epoxy resins.
Component (B) is a polyfunctional phenolic resin of the following general formula (1) whic
Arai Kazuhiro
Shiobara Toshio
Tomiyoshi Kazutoshi
Aylward D.
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
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