A New Lead-Free Solder Joint Utilizing Superplastic Al-Zn Eutectoid Alloy for Next Generation SiC Power Semiconductor Devices

A new high-temperature lead-free solder joint which withstands up to 300°C utilizing superplasticity in the Al-Zn eutectoid alloy has been developed to realize SiC power semiconductor devices. The new solid state joining process consists of interfacial cleaning of joints utilizing superplasticity of the Al-Zn-eutectoid alloy at 250°C followed by diffusion bonding between 350 and 390°C. The bonding strength of the new joints exhibits almost the same value at the temperature range from RT to 300°C, above which it decreases slightly with increasing temperature. It is also found that the bonding strength of the new joints is 8 times as high as those of a high-temperature Pb-5wt%Sn-1.5wt%Ag solder and the Al-Zn eutectoid alloy solder without utilizing superplasticity at 250°C. The Al-Zn eutectoid alloy solder joint has shown high reliability in the temperature cycle testing between 50°C and 300°C up to 300 cycles.

Effect of Sn Addition on Superplastic Properties of Zn-22mass%Al Eutectoid Alloy

Superplastic behavior of a Zn 22 mass % Al eutectoid alloy (SPZ) with small addition of Sn (SPZSn) was investigated. Granular grain size of about 0.3 μm was obtained by water quench after annealing SPZ and SPZ05Sn (addition of 0.05 mass % Sn into the SPZ) at 653 K for 2 h. The fundamental microstructure of the SPZ05Sn was similar to that of the SPZ, but, microstructure observation by STEM showed additive Sn was present at the α’ grain boundary in the SPZ05Sn. Excellent high strain rate superplasticity was achieved in the SPZ05Sn, with elongation of more than 1300 % at 523 K at strain rate of 10-1 S-1. Furthermore, large elongation of about 1100 % was recorded at 473 K at strain rate of 10-1 S-1. The large elongation and high strain rate sensitivity value of the SPZ05Sn tend to shift to higher strain rate region as compared to those of the SPZ. It was considered that the small addition of Sn into the SPZ effectively suppressed the grain growth of α and β phase during the superplastic deformation, because granular grains less than 2 μm is maintained after superplastic deformation at 523 K.