High reliability gold based solder alloys for micro-electronics packaging for high temperature applications

The performance of the Au-Ge eutectic solder alloy and the Au-Si eutectic solder alloy at 300°C up to 500 h has been extensively reported. Coarsening of the dispersed (Ge) phase as well as the dissolution of the hard (Ge) phase into the soft (Au) matrix is observed during thermal aging. Shear testing and nano-indentation confirmed the loss of strength of the Au-Ge bulk solder during thermal aging at 300°C. However, a fraction of the lost strength was recovered during the final stages of thermal aging at 300°C for 500 h. The coarsening effect was more pre-dominant in the Au-Si eutectic alloy. The pace at which the Au-Si eutectic alloy loses its strength during aging at 300°C is significantly higher, when compared to Au-Ge eutectic alloy. The possibility of averting the grain coarsening in Au-Ge eutectic solder alloy by micro-alloying has also been explored. Among the low melting point metals, Sn has been identified as a potential candidate, since it can dissolve in the (Ge) phase. The reliability of the solder joint is also influenced by the intermetallic compounds (IMC) formed between the bulk solder and the solder wettable layer of the under-bump metallization (UBM). Hence, the interfacial reactions between the Au-Ge and Au-Si eutectic solder alloys and the electroless nickel immersion gold (ENIG) and Cu/Au UBMs have also been extensively studied. ENIG has been identified as a prospective UBM candidate for these eutectic alloys as their consumption during aging and wetting is minimal. Based on the present work´s findings, it can be concluded that among the binary eutectic alloys, Au-Ge eutectic alloy is better suited for high-temperature applications.