Properties of Die Bond Alloys Relating to Thermal Fatigue

The differing performances of hard soldered and soft soldered power devices under cycling stresses are well understood. Hard soldered dice are often under considerable stress and may even fracture in extreme cases, but the solder itself does not degrade by fatigue after many temperature cycles. This behavior is due to the very high mechanical strengths of the hard solders between -55 and 150°C. On the other hand, soft solders transmit very little stress to the die but exhibit considerable degradation from thermal fatigue during temperature or power cycling. Both behaviors are related to the very low mechanical strengths and high ductilities of the soft solders. In this study the mechanical properties of the eutectic hard solders, Au-Sn, Au-Ge, and Au-Si, and two common soft solders, Pb-5%Sn and Sn-3.5%Ag-l.5%Sb, have been measured between -55 and 200°C. Several physical properties were also measured. An ahoy was then developed with mechanical properties intermediate to those of hard and soft solders and with a power cycling performance that approaches that of the Au eutectics. A model is presented which relates the fatigue behavior of hard, soft, and intermediate solders to their mechanical properties.