Curable organopolysiloxane composition, encapsulant and semiconductor device

A curable organopolysiloxane composition, consisting of:

• (A) a branched organopolysiloxane having at least one silicon-bonded alkenyl group and at least one silicon-bonded aryl group per molecule, and having siloxane units of the formula:
• RSiO3/2 where R is a substituted or unsubstituted monovalent hydrocarbon group;
• (B) a linear organopolysiloxane with both terminal ends of the linear organopolysiloxane bearing silicon-bonded hydrogen atoms and having at least one silicon-bonded aryl group per molecule;
• (C) a hydrosilylation catalyst;
• and,
• (D) at least 1 parts by weight, relative to 100 parts by weight of the total of components (A) and (B), of a low molecular weight siloxane selected from the group consisting of DVi4, MVi4Q, MVi6Q2, MVi3T, and mixtures thereof, and
• optionally, a crosslinking agent, a phosphor, an inorganic or organic filler, a heat stabilizer, a dye, a pigment, a flame retardant, or a solvent, or a mixture of one or more of these optional ingredients,
• wherein the curable organopolysiloxane composition does not comprise a linear organopolysiloxane having at least two silicon-bonded alkenyl groups and at least one silicon-bonded aryl group per molecule,
• wherein the weight ratio of component (A) to component (B) is 70:30 to 90:10.