Temperature Aging of External Connections Condensation Soldered to Ti-Pd-Au Thin Films

Soldering has a number of advantages for making external connections to HIC's (hybrid integrated circuits). However, recent investigations report a potential reliability problem due to the formation of brittle intermetallic compounds. An investigation of the mechanism of solder joint strength degradation due to the temperature aging of clip-on terminals to HIC Ti-Pd-Au thin films is described. The investigation consisted of aging circuits at elevated tempeatures, measuring bond strength, and assigning the failure mode. Important variables evaluated included gold thickness, conductor thermal stabilization .prior to soldering, and solder reflow time. Activation energies in the range of 1.1 to 1.7 eV were calculated from the observed solder bond strength decrease. From an Arrhenius-type extrapolation the acceptable mean solder bond strength for all films tested exceeds a 40year 50°C requirement. These included thermally stabilized films with both 2- and 5-µm thick gold, and unstabilized film with 5-µm thick gold. Metallographic cross sectioning and chemical analysis of solder bonds were used to identify the intermetallic compounds and the resultant weak bond interfaces. Activation energies for the solid state diffusion and intermetallic formation of solder with gold, palladium, and copper are in the range of 0.6 to 1.0 eV. These are significantly less than the effective activation energies measured for the degradation of the solder bond strength. Several rate-limiting processes are proposed to explain this. A model consistent with the analytical results and bond strength measurements is included.