Microstructure characterization of (Sn1−xZnx)57(In0.78Bi0.22)43 low melting point lead-free solder materials

(Sn1−xZnx)57(In0.78Bi0.22)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 In0.2Sn0.8, 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 In0.2Sn0.8 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.