Patent ID: 8519063

Claim:
An underfill composition for filling a gap, said gap ranging in size from 10 to 200 μm, between a substrate and an optical semiconductor element to thereby form an optical semiconductor device, said composition comprising (A) 60 to 99 parts by weight of a branched organopolysiloxane having a weight average molecular weight of from 500 to 20000, reduced to polystyrene, and represented by the following average compositional formula (1), (CH3)aSi(OR1)b(OH)cO(4-a-b-c)/2 (1) wherein R1 is an organic group having 1 to 4 carbon atoms and a, b, and c are numbers satisfying the equations: 0.8 <=a<= 1.5, 0 <=b<= 0.3, 0.001 <=c<= 0.5, and 0.801 <=a+b+c< 2, (B) 250 to 1000 parts by weight of an inorganic filler, wherein not more than 0.1 weight-% of the particles of said inorganic filler have a particle size that is greater than or equal to half the size of said gap, (C) 0.01 to 10 parts by weight of a condensation catalyst, (D) 1 to 40 parts by weight of an organopolysiloxane comprising R 2 SiO units and RSiO 1.5 units wherein R is, independently of each other, a group selected from the group consisting of hydroxyl groups, methyl groups, ethyl groups, propyl groups, cyclohexyl groups, phenyl groups, vinyl groups, and allyl groups, provided that at least a portion of the R 2 SiO units are a linear diorganopolysiloxane moiety represented by the following formula (2), wherein R2 and R3 are, independently of each other, selected from the group consisting of a hydroxyl group, alkyl groups having 1 to 3 carbon atoms, a cyclohexyl group, a vinyl group, a phenyl group and an allyl group, and m is an integer of from 5 to 50, and (E) 0.2 to 2.0 parts by weight of a silane coupling agent provided that a total amount of component (A) and component (D) in said composition is 100 parts by weight.