Semiconductor encapsulating epoxy resin composition and semiconductor device

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This invention relates to an epoxy resin composition for semiconductor encapsulation which is effectively moldable and cures into a product having solder crack resistance, flame retardance and moisture-proof reliability. It also relates to a semiconductor device encapsulated with a cured product of the composition.

BACKGROUND OF THE INVENTION

The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Semiconductor devices are now used in every area of the modern society, for example, in electric appliances and computers. As a guard against accidental fire, the semiconductor encapsulating materials are required to be flame retardant.

Halogenated epoxy resins (typically chlorinated and brominated epoxy resins) combined with antimony trioxide are often blended in epoxy resin compositions in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect.

In a high-temperature environment, however, such flame retardants as halides (typically chlorides and bromides) and antimony compounds are decomposed to give rise to chemical reaction at connections between gold wires and aluminum lines. This results in an increased resistance at the connections or even disconnection, inviting malfunction. In addition, the 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 is desirable to entirely exclude these fire retardants from resin compositions.

In view of the above demand, studies have been conducted on the use of hydroxides such as Al(OH).sub.3 and Mg(OH).sub.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

An object of the invention is to provide an epoxy resin composition for semiconductor encapsulation which is free of harmful halogenated epoxy resins and antimony compounds, is effectively moldable and cures into a product having improved solder crack resistance, flame retardance and reliability. Another object is to provide a semiconductor device encapsulated with a cured product of the composition.

The invention provides a semiconductor encapsulating epoxy resin composition comprising (A) an epoxy resin, (B) a phenolic resin curing agent, (C) a molybdenum compound, (D) a silicon compound, and (E) an inorganic filler. The silicon compound (D) is selected from the group consisting of (D-i) an organopolysiloxane of the following average compositional formula (1): R.sup.1.sub.aSiO.sub.(4-a)/2 (1) wherein R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group, and "a" is a positive number of 0.8 to 3, with the proviso that two R.sup.1 groups, taken together, may form an alkylene group, (D-ii) a cured product of organopolysiloxane, and (D-iii) a block copolymer obtained by reacting an epoxy resin or an alkenyl group-bearing epoxy resin with an organopolysiloxane of the following average compositional formula (2): H.sub.mR.sup.2.sub.nSiO.sub.(4-m-n)/2 (2) wherein R.sup.2 is a substituted or unsubstituted monovalent hydrocarbon group, m is a positive number of 0.001 to 0.2, n is a positive number of 1.8 to 2.1, and m+n is 1.801 to 2.3, the number of silicon atoms in a molecule is an integer of 10 to 1,000, and the number of hydrogen atoms directly attached to silicon atoms is 1 to 5, addition reaction taking place between epoxy groups on the epoxy resin or alkenyl groups on the alkenyl group-bearing epoxy resin and silicon-attached hydrogen atoms (i.e., SiH groups) on the organopolysiloxane.

The semiconductor encapsulating epoxy resin compositions of the invention are effectively molded and cure into products which have an excellent fire retardance, moisture resistance and solder cracking resistance despite the absence of halogenated epoxy resins and antimony compounds (e.g., antimony trioxide).

DETAILED DESCRIPTION OF THE INVENTION

Component (A) is an epoxy resin which is not critical as long as it has at least two epoxy groups per molecule. This epoxy resin is different from the block copolymer (D-iii) to be described later in that it does not contain a siloxane structure in the molecule. 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 such as triphenolmethane type epoxy resin and triphenolpropane type epoxy resin, phenolaralkyl 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. Halogenated epoxy resins are excluded.

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 hydroxy 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 phenolic resins such as triphenolmethane type phenolic resin and triphenolpropane type phenolic resin, phenolaralkyl phenolic resins, biphenyl skeleton-containing aralkyl phenolic resins, biphenyl phenolic resins, alicyclic phenolic resins, heterocyclic 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 epoxy resin composition of the invention contains a molybdenum compound as the flame retardant (C). Exemplary molybdenum compounds are molybdenum oxides, molybdenum borides, molybdenum silicides, molybdenum esters, and molybdic salts such as molybdenum boride, molybdenum disilicide, molybdenum acetylacetonate, molybdenum (IV) oxide, molybdenum (V) oxide, molybdenum (VI) oxide, zinc molybdate, calcium molybdate carbonate, and calcium molybdate. The molybdenum compound by itself is known to have a smoke-reducing and charring effect in burning plastic. Although like antimony trioxide, molybdenum compounds are conventionally used in combination with halogenated resins, it has been found by the inventors that flame retardance is exerted by combining molybdenum compounds with the organopolysiloxane, organopolysiloxane cured product or block copolymer to be described later. Since the molybdenum compounds are free from powder toxicity as found with antimony trioxide, they are quite safe flame retardants. Of these, zinc molybdate is especially preferred since it does not affect the curability of the epoxy resin.

To achieve a satisfactory flame retardant effect, zinc molybdate must be uniformly dispersed in the epoxy resin composition. To improve the dispersibility, zinc molybdate is preferably supported on an inorganic carrier such as silica or talc. Suitable inorganic carriers for supporting zinc molybdate include silicas such as fused silica and crystalline silica, talc, alumina, silicon nitride, aluminum nitride, boron nitride, titanium oxide, and glass fibers. The zinc molybdate-carrying powder should preferably have a mean particle diameter of 0.1 to 40 .mu.m, more preferably 0.2 to 15 .mu.m, and most preferably 0.5 to 5 .mu.m and a specific surface area of 0.5 to 50 m.sup.2/g, and more preferably 0.7 to 10 m.sup.2/g as measured by the BET method.

It is noted that the mean particle diameter can be determined as the weight average value (or median diameter) based on the laser light diffraction technique, for example.

In the flame retardant comprising zinc molybdate supported on the inorganic carrier, the content of zinc molybdate is preferably 5 to 40% by weight and more preferably 10 to 30% by weight. Less contents of zinc molybdate may fail to provide satisfactory flame retardance whereas excessive contents may detract from flow during molding and curability.

The zinc molybdate on inorganic carrier is commercially available under the trade name of KEMGARD series, such as KEMGARD 1260, 1261, 1270, 1271 and 911C from Sherwin-Williams Co.

An appropriate amount of the flame retardant (C) in the form of zinc molybdate on inorganic carrier is 1 to 120 parts, more preferably 3 to 100 parts, and especially 5 to 100 parts by weight per 100 parts by weight of the epoxy resin (A) and the phenolic resin curing agent (B) combined. The amount of zinc molybdate alone in the flame retardant (when the molybdenum compound is blended without supporting it on an inorganic carrier, the amount of the molybdenum compound itself) is preferably 0.05 to 35 parts, more preferably 0.1 to 30 parts, and especially 0.2 to 20 parts by weight per 100 parts by weight of the epoxy resin (A) and the phenolic resin curing agent (B) combined. Less amounts may fail to provide satisfactory flame retardance whereas excessive amounts may detract from the flow and curability of the composition.

The composition of the invention contains as component (D) at least one silicon compound selected from among:

(D-i) an organopolysiloxane of the following average compositional formula (1): R.sup.1.sub.aSiO.sub.(4-a)/2 (1) wherein R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group, and "a" is a positive number of 0.8 to 3, with the proviso that two R.sup.1 groups, taken together, may form an alkylene group,

(D-ii) a cured product of organopolysiloxane, and

(D-iii) a block copolymer obtained by reacting an epoxy resin or an alkenyl group-bearing epoxy resin with an organopolysiloxane of the following average compositional formula (2): H.sub.mR.sup.2.sub.nSiO.sub.(4-m-n)/2 (2) wherein R.sup.2 is a substituted or unsubstituted monovalent hydrocarbon group, m is a positive number of 0.001 to 0.2, n is a positive number of 1.8 to 2.1, and m+n is 1.801 to 2.3, the number of silicon atoms in a molecule is an integer of 10 to 1,000, and the number of hydrogen atoms directly attached to silicon atoms is 1 to 5, addition reaction taking place between epoxy groups on the epoxy resin or alkenyl groups on the alkenyl group-bearing epoxy resin and silicon-attached hydrogen atoms (i.e., SiH groups) on the organopolysiloxane.

Component (D-i) is an organopolysiloxane of the average compositional formula (1): R.sup.1.sub.aSiO.sub.(4-a)/2 which is typically a non-crosslinkable silicone fluid (oil) or silicone resin (hydrolysis condensate) whereas component (D-ii) is a cured product of organopolysiloxane which is typically a crosslinked silicone rubber or silicone resin. It is known that the addition of a silicone polymer to a plastic material is effective for improving flame retardance because the silicon polymer forms a flame retardant silicon carbide (Si--C) coating upon combustion. According to the invention, component (D-i) and/or (D-ii) is used in combination with the molybdenum compound, especially zinc molybdate, whereby the formation of a silicon carbide (Si-C) coating is promoted to enhance flame retardance.

In formula (1), O represents an oxygen atom forming a siloxane structure (.ident.Si--O--Si.ident.). R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group attached to a silicon atom forming the siloxane structure. The unsubstituted monovalent hydrocarbon groups represented by R.sup.1 are preferably those having 1 to 12 carbon atoms, and more preferably 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, cyclohexyl, and octyl; alkenyl groups such as vinyl, allyl, propenyl, butenyl, and hexenyl; aryl groups such as phenyl, tolyl and naphthyl; and aralkyl groups such as benzyl, phenylethyl, and phenylpropyl. The substituted monovalent hydrocarbon groups include those corresponding to the foregoing unsubstituted monovalent hydrocarbon groups in which some or all hydrogen atoms are replaced by fluorine atoms, cyano, hydroxyl, alkoxy, amino or mercapto groups as well as monovalent hydrocarbon groups containing imino groups, epoxy groups, carboxyl groups, carbinol groups, (methyl)styryl groups, (meth)acrylic groups, polyether groups, higher fatty acid groups, higher fatty acid ester groups, and long-chain alkyl groups of at least 13 carbon atoms. Two R.sup.1 groups, taken together, may form an alkylene group of about 1 to 8 carbon atoms, especially about 2 to 6 carbon atoms, such as methylene, ethylene, trimethylene, tetramethylene, methylethylene, or hexamethylene.

Letter "a" is a positive number of 0.8 to 3, preferably 1 to 2.7. The organopolysiloxane may be a linear or cyclic one composed mainly of R.sup.1.sub.2SiO.sub.2/2 units, or a copolymer of three-dimensional network (or resinous) structure containing essentially R.sup.1SiO.sub.2/3 unit