Failure analysis of Au-Ge solder in microwave circuit packaging

The Au-Ge eutectic solder has good thermal properties and a melting temperature of 356°C, which is widely used in high power microwave modules. However, there were a few papers investigated the reliability of Au-Ge solder. This paper focused on the failure of Au-Ge eutectic solder in microwave circuit. The interfacial reactions between the Au-Ge eutectic solder alloys and the electroless nickel immersion gold (ENIG) were studied. The microstructure of the samples was observed and analyzed by the scanning electron microscope (SEM) equipped with an energy dispersed X-ray analyzer (EDX). The reliability of the solder joint was influenced by the intermetallic compounds (IMC) formed between the bulk solder and the under-bump metallization (UBM) layer. The soldering temperature, duration and thickness of gold layer affected the IMCs. High soldering temperature or long duration lead to grain coarsening of IMCs and fast consumption of gold layer. As the gold layer dispersed into solder, nickel layer reacted with germanium of solder layer, formed Ni-Ge alloy which sharply decreased the solder shear strength. Based on the failure modes in this work, improving methods were extensively discussed.