Casting compound for semiconductor devices

Claim 1

A curable composition which is liquid at room temperature or at a slightly elevated temperature and which comprises a. a major proportion of a polyepoxide A--(X).sub.a --B wherein A and B are selected from the class of (1) cycloaliphatic groups, (2) open chain aliphatic groups and (3) segments of a cycloaliphatic single ring or fused ring systems, A and B each containing an epoxide group, X is a linking entity selected from the class of a valence bond and an atom or group of atoms and a is unity except where A and B belong to group (3) in which case a is zero; b. a cross-linking agent in the form of an anhydride of a polycarboxylic acid ##STR25## wherein R is a cycloaliphatic group, an open chain aliphatic group or an aromatic group c. a catalyst capable of promoting polymerizing and cross-linking of (a) and (b); and d. a filler in the form of a finely divided fused silica the surface of which has been treated with a substance to convert silanol groups to groups ##STR26## wherein the R 3 's are the same or different hydrocarbon groups said components (a) and (b) being present in amounts and proportions to enable the composition to cure at temperatures not greater than about 175° C, to a hard product; said components (a), (b) and (d) being selected and their proportions being such that the composition is a pourable liquid at temperatures not greater than about 40° C and is free of significant quantities of chloride and sodium ions.