Abstract: While high-performance semiconductor devices encapsulated with a resin composition are subject to the problem of wrong operation due to the alpha-particles emitted from the trace amounts of radioactive impurities, e.g. uranium and thorium, contained in the silica filler incorporated in the resin composition, a means for solving this problem is provided by use of a silicon dioxide powder obtained by the pyrolysis of a volatilizable silicon compound free from radioactive impurities in an oxidizing condition and having an average particle diameter in the range from 0.5 to 100 .mu.m in place of the conventional silica fillers.

Abstract: The invention provides a novel epoxy-based curable resin composition suitable for encapsulation of electronic devices such as transistors, ICs, LSIs and the like. The composition comprises an epoxy compound, a novolac-type phenolic resin as the curing agent, an organophosphine compound such as triphenylphosphine and trilaurylphosphine as a curing accelerator and an inorganic filler. The advantages obtained by the combined use of the novolac-type phenolic resin and the organophosphine compound are very specific and the resin composition is imparted with remarkable flowability in molding and stability in storage in addition to the excellent electric properties at high temperatures and anti-moisture resistance of the cured shaped articles molded therefrom.