Effect of pre-soldering temperatures on the microstructures and shear behaviors of SnBi-SAC/ENIG solder joints

​Thermal deformation during soldering between the chip and the substrate is an important factor affecting the quality of high-density packaging of organic circuit boards. This study proposes a low-temperature pre-connection, package reinforcement, and secondary reflow process based on SAC305 and Sn-58Bi solders, which melts SnBi solder paste at a minimum temperature of 150 °C to connect the SAC305 solder ball and the substrates initially. In the subsequent packaging process, the composite solder joint undergoes a second high-temperature soldering process. Due to the reinforcement of the package structures, the deformation of the substrate at high temperatures is limited. This study investigates the effects of different pre-soldering temperatures on the microstructure and shear behavior of composite solder joints before and after secondary soldering. The results show that the boundary between the SAC region and the mixed region can be clearly observed in the pre-soldered joint. As the temperature increases, the melting degree of the SAC305 solder ball intensifies, and the area of the mixed microstructure increases. In the shear test, the average shear strength of the pre-soldered joints was about 50 MPa. As the pre-soldering temperature increased, the fracture mode changed from composite fracture to brittle fracture. After secondary reflow at 260 °C, SAC305 and Sn-58Bi were completely melted and fully mixed, and each phase was evenly distributed in the solder joints. The average shear strength increased to about 60 MPa.