Patent ID: 8912302

Claim:
A curable silicone composition for sealing an optical semiconductor element, the composition comprising: (A) an organopolysiloxane that has a viscosity at 25° C. of 50 to 100,000 mPa·s, that has at least two silicon-bonded vinyl groups in one molecule, that has C 1-10 alkyl for the other silicon-bonded organic groups therein, and that lacks a siloxane unit represented by the following formula: SiO 4/2 ; (B) an organopolysiloxane represented by the following average unit formula: (ViR 2 SiO 1/2 ) a (R 3 SiO 1/2 ) b (SiO 4/2 ) c (HO 1/2 ) d wherein Vi is a vinyl group; each R is independently a C 1-10 alkyl group; and a, b, c, and d are each positive numbers that satisfy: a+b+c=1, a/(a+b)=0.15 to 0.35, c/(a+b+c)=0.53 to 0.62, and d/(a+b+c)=0.005 to 0.03, at from 15 to 35 mass % of the total of components (A) and (B); (C) an organopolysiloxane that has at least three silicon-bonded hydrogen atoms in one molecule, that has C 1-10 alkyl for the silicon-bonded organic groups therein, and that contains from 0.7 to 1.6 mass % of silicon-bonded hydrogen atoms, in a quantity that provides from 0.8 to 2.0 moles silicon-bonded hydrogen atoms in this component per 1 mole of the total vinyl groups in components (A) and (B); and (D) a hydrosilylation reaction catalyst, in a quantity sufficient to cure the composition; wherein a viscosity at 25° C. of the composition lacking component (D) is from 3,000 to 10,000 mPa·s and, letting η 25° C. be this viscosity in mPa·s and letting η 100° C. be a viscosity in mPa·s at 100° C. of the composition lacking component (D), a value of the following formula: Log 10 η 100° C. /Log 10 η 25° C. is 0.830 to 0.870.