Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-04-25
2003-02-11
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S443000, C523S458000, C257S789000, C257S793000
Reexamination Certificate
active
06518332
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor encapsulating epoxy resin compositions which provide cured products having outstanding fire retardance and free of the toxic substance antimony trioxide. The invention also relates to semiconductor devices encapsulated with these compositions in a cured state.
PRIOR ART
The semiconductor devices in use today are predominantly resin encapsulated diodes, transistors, integrated circuit (IC) chips, large scale integration (LSI) chips, and very large scale integration (VLSI) chips. Resin encapsulation is generally carried out with epoxy resin compositions because epoxy resins offer superior properties, (e.g., moldability, adhesion, electrical characteristics, mechanical characteristics, moisture resistance) compared with other thermosetting resins. Since semiconductor devices are used in all areas of our daily lives, including household appliances and computers, semiconductor encapsulants are required to be fire-retarding in the event that a fire occurs.
Halogenated epoxy resins and antimony trioxide (Sb
2
O
3
) are customarily included in epoxy resin compositions to increase the fire retardance. This combination of a halogenated epoxy resin with antimony trioxide has large radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect. However, halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it would be preferable to entirely exclude these fire retardants from resin compositions.
Not only are resin compositions containing halogenated epoxy resins and antimony trioxide harmful to man and the environment, semiconductor devices encapsulated with these resin compositions have an inferior reliability when exposed to heat and moisture. This poor reliability arises because intermetallic compounds form at the junctions between aluminum electrodes and gold wire on the semiconductor device, causing an increase in electrical resistance and resulting also in wire breaks. The presence of the Br
−
or Sb
+
ions within the resin composition as part of the fire retardant is known to promote the formation of the intermetallic compounds.
In view of the above, studies have been conducted on the use of hydroxides such as Al(OH)
3
and Mg(OH)
2
or phosphorus-containing fire retardants in place of halogenated epoxy resins and antimony trioxide. Unfortunately, because of various problems associated with the use of these alternative compounds, such as inferior curability of the resin composition during molding and poor moisture resistance in the cured product, they are not yet ready for practical application.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide semiconductor encapsulating epoxy resin compositions which contain no halogenated epoxy resins or antimony trioxide, and yet have excellent fire retardance and reliability. Another object of the invention is to provide semiconductor devices encapsulated with these resin compositions in a cured state.
Accordingly, this invention provides semiconductor encapsulating epoxy resin compositions comprising (A) an epoxy resin, (B) a phenolic resin curing agent, (C) a fire retardant comprising zinc molybdate carried on spherical silica having a mean particle diameter of 0.2 to 20 &mgr;m and a specific surface of 1 to 20 m
2
/g, and (D) an inorganic filler. These epoxy resin compositions provide cured products having a high fire retardance and excellent reliability, yet containing no halogenated epoxy resin or antimony trioxide.
DETAILED DESCRIPTION OF THE INVENTION
The epoxy resin used as component (A) in this invention may be any epoxy resin having at least two epoxy groups per molecule, other than halogenated epoxy resins. Illustrative examples of suitable epoxy resins include novolac-type epoxy resins such as phenolic novolac epoxy resins and cresol novolac epoxy resins, triphenolalkane epoxy resins, aralkyl epoxy resins, biphenyl skeleton-containing aralkyl epoxy resins, biphenyl epoxy resins, heterocyclic epoxy resins, naphthalene ring-containing epoxy resins, bisphenol-type epoxy resins such as bisphenol A epoxy compounds and bisphenol F epoxy compounds, and stilbene epoxy resins. Any one or combination of two or more of these epoxy resins may be employed.
No particular limit is imposed on the phenolic resin serving as curing agent (B) in the invention, so long as the phenolic resin has at least two phenolic hydroxyl groups in a molecule. Illustrative examples of typical phenolic resin curing agents include novolac-type phenolic resins such as phenolic novolac resins and cresol novolac resins, naphthalene ring-containing phenolic resins, triphenolalkane resins, aralkyl phenolic resins, biphenyl skeleton-containing aralkyl phenolic resins, biphenyl phenolic resins, alicyclic phenolic resins, heterocyclic phenolic resins, naphthalene ring-containing phenolic resins, and bisphenol-type phenolic resins such as bisphenol A and bisphenol F. Any one or combination of two or more of these phenolic resins may be employed.
The relative proportions of the epoxy resin (A) and the phenolic resin curing agent (B) used in the epoxy resin compositions are not subject to any particular limits, although it is preferred that the amount of phenolic hydroxyl groups in the curing agent (B) be from 0.5 to 1.5 moles, and especially 0.8 to 1.2 moles, per mole of epoxy groups in the epoxy resin (A).
The semiconductor encapsulating epoxy resin compositions of the invention do not contain conventional fire retardants such as antimony trioxide and brominated or otherwise halogenated epoxy resins. Instead, the inventive compositions use as the fire retardant (C) a substance prepared by supporting zinc molybdate on spherical silica having a mean particle diameter of 0.2 to 20 &mgr;m and a specific surface of 1 to 20 m
2
/g. Zinc molybdate by itself is known to have a smoke-reducing and charring effect in burning plastic, but it exists in the form of very fine particles and so cannot easily be dispersed in a resin composition. However, by supporting zinc molybdate on spherical silica having a mean particle diameter of 0.2 to 20 &mgr;m and a specific surface of 1 to 20 m
2
/g, there is obtained a fire retardant which is well dispersible in resin compositions. This fire retardant does not cause any loss in flow or curability during molding, and makes it possible to obtain epoxy resin compositions having sufficient fire retardance and excellent reliability in the cured state without using a halogenated epoxy resin or antimony trioxide.
The shape, particle diameter, and distribution of the supporting filler (spherical silica) are crucial for achieving fire retardance using as little zinc molybdate as possible, and for maintaining or enhancing the moldability of the epoxy resin composition.
Therefore, the spherical silica used as the zinc molybdate carrier should have a mean particle diameter of 0.2 to 20 &mgr;m, and preferably 0.3 to 10 &mgr;m. One of several ways in which the mean particle diameter can be determined is as the weight average value (median diameter) using a particle size distribution measurement apparatus based on the laser light diffraction technique. Particles with a mean particle diameter smaller than 0.2 &mgr;m are less dispersible within the resin compositions. A mean particle diameter greater than 20 &mgr;m discourages uniform dispersion and support of the zinc molybdate, lowering the fire retardance. This in turn necessitates the use of a larger amount of the fire retardant, which is economically undesirable. The specific surface, as obtained by a suitable technique such as BET adsorption, is from 1 to 20 m
2
/g, and preferably from 2 to 18 m
2
/g. Particles with a specific surface of less than 1 m
2
/g retard the uniform support of zinc molybdate, resulting in a lower fire retardance. On the other hand, at a specific surface above 20 m
2
/g, dispersibility within the resin com
Asano Eiichi
Osada Shoichi
Shiobara Toshio
Tomiyoshi Kazutoshi
Yoshino Masachika
Aylward D.
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
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