High-Temperature Cycle Durability of Superplastic Al–Zn Eutectoid Solder Joints With Stress Relaxation Characteristics for SiC Power Semiconductor Devices

We have developed a new high-temperature Al-Zn lead-free soldering process that utilizes superplasticity to realize SiC power devices with high-temperature cycle durability. The joining process consists of an Al-78wt.%Zn preparation being sandwiched by a SiC die and an insulation substrate, an interfacial cleaning process at approximately 250-270 °C, a heating stage to reach the solid-liquid coexisting temperature of 420-430 °C, the ejection of low-melting-temperature β(Zn) from the joining area by press stress, and the transformation to a superplastic composition, i.e., Al-70 wt.% Zn at 270-310 °C. Many lamellar phases that enable superplasticity can be formed in this microstructure. This solder joint composition was proven to have a better stress-relaxation effect than eutectic Al-95wt.%Zn, and the composition shows a much higher damping capability at the maximum operating temperature of SiC devices (200 °C) than that of other joining candidates. The outstanding temperature cycle durability was verified in temperature cycle tests from -40 °C to 300 °C for 5000 cycles. This durability is due to the high-stress-relaxation effect from the superplasticity transformation realized by the lamellar structures in the Al-Zn alloy solder.