Solder preform

Abstract

A solder preform for hermetically sealing a cover to a container for a semiconductor package comprises a relatively thick flat ring of an alloy consisting of substantially 95% lead, 2.5% tin, and 2.5% silver, which alloy has the characteristic that it solidifies from the fluid state as a homogeneous mixture without substantial separation of the minority element crystals. The solder preform further comprises a relatively thin coating clad on each surface of the flat ring of an oxidation-resistant alloy, preferably an alloy consisting of substantially 96.5% tin and 3.5% silver.

What is claimed is: 
     
       1. A solder preform .[.for hermetically sealing a cover to a container for a semiconductor package.]. comprising a relatively thick flat ring of an alloy consisting of 90-97% lead, 1-4% tin, and 1-4% silver and having the characteristic that it solidifies from the fluid state as a homogeneous mixture and a relatively thin coating clad on each surface thereof of an oxidation-resistant alloy consisting of 92-98% tin and 2-8% silver. 
     
     
       2. A solder preform in accordance with claim 1 in which said ring is an alloy consisting of substantially 95% lead, 2.5% tin, and 2.5% silver. 
     
     
       3. A solder preform in accordance with claim 1 in which said coating is an alloy consisting of substantially 96.5% tin and 3.5% silver. .Iadd. 4. A solder preform comprising a relatively thick flat ring of an alloy comprising preponderantly lead and tin and having the characteristic that it solidifies from the fluid state as a homogeneous mixture and a relatively thin coating clad on each surface thereof of an oxidation-resistant alloy consisting of 92-98% tin and 2-8% silver..Iaddend.