Method for manufacturing a plastic encapsulated semiconductor device and a lead frame therefor

A method for manufacturing a plastic encapsulated semiconductor device, comprising the steps of:

• (a) assembling a semiconductor substrate with a lead frame having a first connecting band connected to external leads of a semiconductor device assembly extending from one side of a substrate support serving as a heat sink, and a second connecting band connected to a plurality of strips of said semiconductor device assembly extending from another side of said substrate support and having portions of small cross-sectional areas and of predetermined lengths, the small cross-sectional areas being perpendicular to an extending direction of said plurality of strips, and said plurality of strips being thinner than said substrate support and rear surfaces of said plurality of strips being located above a rear surface of said substrate support;
• (b) clamping said external leads and said plurality of strips between upper and lower molds such that parts of said portions of small cross-sectional areas of said plurality of strips are disposed in a cavity formed by said upper and lower molds and the remainder of said portions of small cross-sectional areas are disposed between said upper and lower molds separate from said cavity and that said substrate support floats in said cavity with the rear surface of said substrate support being spaced apart from a bottom surface defining said cavity by a distance in the range from 0.3 to 0.5 mm;
• (c) injecting an encapsulating plastic into said cavity;
• and
• (d) cutting a connecting portion between said external leads and said first connecting band, and said portions of small cross-sectional areas of said plurality of strips, extending externally from an encapsulating plastic housing formed by said injecting step along a surface of the encapsulating plastic.