SIMS microprofiles of Pb5%Sn solder joints in electronic devices after accelerated life tests

The failure mechanisms of Pb5%Sn (wt%) solder alloy in electronic devices assembled in a plastic package have been investigated. For this purpose dice with Al/Ni/Au 4/1.0/0.05 μm thick and Ti/Ni/Au 0.1/0.4/0.05 μm thick back side metals were prepared. Retrospective SIMS (Secondary Ion Mass Spectrometry) microprofiles obtained in fracture surfaces of the solder layer after thermal cycles, suggest two failure mechanisms. One is based on the formation of regions of the alloy where the Sn concentration is lower than the average. This depletion appears to be due to coupling between defect fluxes and material fluxes in the alloy during thermomechanical fatigue. An additional cause that results in degradation of the alloy is the oxidation of the solder layer. The oxygen source appears to be oxygen species present in the metal layers that diffuse into the alloy during the thermal cycling test.