DEGRADATION OF TAB OUTER LEAD CONTACTS DUE TO THE AU-CONCENTRATION INEUTECTIC TIN-LEAD SOLDER

The influence of the Au-concentration in OLB-solder fillets on the con tact reliability is shown. Quantitative analysis of the Au content sho w a good accordance with the estimated concentrations using the geomet rical data. The Au-concentration is the major influence factor on the thermal aging behavior of OLB contacts. A change in the failure mechan ism due to Kirkendall porosity is observed if a Au-concentration of 9 wt% in the solder fillet is reached. The pores formed during diffusion at the interface between copper and the ternary intermetallic compoun d Cu3Au3Sn5 cause a strong degradation of pull forces. For low Au-conc entrations, the degradation of pull-forces and the growth of the terna ry compounds CuAu5Sn5 and CuAu4Sn5 have comparable activation energies in the range of 0.3 eV. For higher Au-concentrations the two effects show different values. The activation energy of pull-force degradation increases significantly up to 0.67 eV, whereas the growth constant of the ternary compound shows a small increase to 0.4 eV. The investigat ions show evidence of a critical Au-concentration in OLB contacts. Kir kendall pore formation causes higher activation energies for the pull- test degradation compared to the activation energy for the growth rate of the ternary compound.