Abstract:(Sn_1-xZn_x)₅₇(In_0.78Bi_0.22)₄₃ (x=0.10, 0.15, 0.20, at.%) quaternary alloys were designed and characterized based on the multi-principal-element mixing concept in order to develop a lead-free solder suitable for low temperature soldering in three-dimensional integrated circuit. The microstructure, thermal properties and wetting properties of the alloys were investigated using scanning electron microscope, X-ray diffractometer and differential scanning calorimeter. The results show that the alloys consist of intermetallic compounds (IMCs), BiIn₂ and In_0.2Sn_0.8, and Zn-rich solid solution. With the increase of Zn content, the volume fractions of the BiIn₂ and Zn-rich phases increase accordingly. The alloys exhibit low melting points of about 70 °C, due to the formation of the low-melting-point In_0.2Sn_0.8 and BiIn₂ IMCs. The alloys show good wetting properties with wetting angle of about 40°. At the solder joints between the alloys and Cu layer, a thin and tough Cu₅Zn₈ layer is formed, which is supposed to improve the joint reliability. It is demonstrated that the welding performance of solders can be effectively enhanced by designing solders through the multi-principal-element mixing concept.

Key words: lead-free solder; multi-principal-element mixing; low melting point; microstructure; thermal properties