Temperature Cycling of HIC Thin-Film Solder Connections

Abstract

Soldering is one method used for interconnecting hybrid integrated circuits (HIC's). intermetallic compound formation and cyclic fatigue have been reported as factors which limit solder-joint reliability. Identification of intermetallic compounds which form during constant temperature aging, and which affect HIC thin-film solder-joint reliability, has been previously reported. Temperature cycling effects are described. The investigation consisted of l) preparing test circuits using clip-on terminals, a fixed volume of 60 Sn-40 Pb solder, and resistoi'-capacitor iRC) and resistor HIC terminations; 2) temperature cycling freestanding circuits; 3} measuring solder bond strength; and 4) analyzing weak bond interfaces and solder joint-metallographic cross sections by SEMEDAX (scanning electron microscope with energy dispersive analysis Of X-rays) and EMP (electron microprobe). Solder-joint strength after 1000 temperature cycles from -40 tO 130°C is comparable tO previous measurements after 1000 h at 150°C for both resistor and RC-type terminations. However, terminations after pull testing show a different failure mode after temperature cycling than after constant temperature aging. Interfacial fractures after temperature aging are related to intermetallic formation. Bond degradation during temperature cycling again results from intermetallic compound formation, but there is also weakening at the interface between the Pd and underlying films. Regardless of this failure mode difference, joint strength in the 5- to 10-1b range meets circuit requirements. These results indicate that the thermal expansion coefficients of the solderjoint materials are sufficiently matched to prevent excessive thermal strains.