Evolution of Microstructure and Mechanical Properties of Copper Pillar Solder Joints Under Ultrasound

The method of introducing ultrasonic action was used to bond the flip chip with copper pillar solder joints, and the influence of ultrasonic vibration on the structure evolution and mechanical properties of the solder joint was studied. The three-dimensional finite element analysis method was used to simulate the vibration distribution of the chip and the sound pressure distribution in the solder joints. The results indicate that the introduction of ultrasound will cause the intermetallic compound (IMCs) at the upper interface of the solder joint to be broken and thin, and the IMCs at the lower interface will grow and thicken on the original basis. At the same time, the shear strength of the solder joints first increases and then decreases with the increase of the ultrasonic power, and has little relationship with the ultrasonic action time. The introduction of ultrasound in the middle stage of the bonding process will increase the strength of the solder joint, and, in the final stage of the bonding process, will cause larger cracks in the solder joints, which will affect their strength. Simultaneously, the simulation results show that the amplitude at the edge of the upper surface of the chip is larger than that in the middle, and that the sound pressure of the solder joints at the boundary is larger than that of the middle solder joints.