Design for reliability with AuSn interconnects

AuSn is a special purpose interconnect material with advantages concerning its high temperature resistance, used for fluxless soldering for optoelectronic and RF devices as well as in fine pitch flip chip technology. Finite element (FE) analyses were conducted on assemblies with AuSn interconnects. Stress analyses require data on the materials stress-strain behavior, however, in case of AuSn solder joints material characteristics are not easily available. In the paper investigations on material characterization concerning the elastic and plastic behaviour as well as the fatigue properties of electroplated Au and AuSn are reported. It is shown that properties of these materials depend strongly on their composition and their microstructure, the latter being fundamentally affected by processing. By application of thermal lap shear tests the low cycle fatigue behavior of AuSn joints is studied. The results show that inter-phase cracking is the dominant fatigue failure mechanism, which is only indirectly linked to plastic straining calculated under homogeneity assumption. Computational analyses concerning the reliability of AuSn flip chip joints for GaAs pixel detectors are finally reported in the paper.