Microstructure evolution and grain orientation of IMC in Cu-Sn TLP bonding solder joints

In this work, Cu/Sn/Cu solder joints were fabricated by low temperature transient liquid phase (TLP) bonding. The microstructure evolution and grain orientation of intermetallic compounds (IMC) in Cu-Sn solder joints were systematically investigated by using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). With the bonding time increased, a typically scallop-type Cu₆Sn₅ was formed at the interface between Sn/Cu and then gradually thickened to form a full-IMC solder joints. The higher the temperature, the faster the IMC grew. Besides, the asymmetry growth of IMC at both ends of the solder joints was clearly observed due to the temperature gradients. The corresponding EBSD results indicated that Cu₆Sn₅ and Cu₃Sn grains showed the preferred orientation of (001) and (100), respectively. Measurement of the kinetic exponent of IMC suggested that the growth mechanism of Cu₃Sn is controlled by grain boundary diffusion. In addition, the wettability and mechanical property of the solder joints were also studied. It was found that the wettability and shear strength of the solder joints improved as the bonding temperature and bonding time increased. The shear fracture analysis of the solder joints revealed that most fractures of the Cu-Sn solder joints have the characteristics of brittle fracture.