Degradation of TAB outer lead contacts due to the Au-concentration in eutectic tin/lead solder

The influence of the Au-concentration in OLB solder fillets on the contact reliability is shown. Quantitative analysis of the Au content shows a good accordance with the estimated concentrations using the geometrical data. The Au-concentration is the major influence factor on the thermal aging behavior of OLB contacts. A change in the failure mechanism 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 compound Cu₃Au₃Sn₅ cause a strong degradation of pull forces. For low Au-concentrations, the degradation of pull-forces and the growth of the ternary 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 investigations show evidence of a critical Au-concentration in OLB contacts. Kirkendall pore formation causes higher activation energies for the pull-test degradation compared to the activation energy for the growth rate of the ternary compound