Fatigue Life Prediction of Die-Attach Joint in Power Semiconductors Subjected to Biaxial Stress by High-Speed Thermal Cycling

A method for predicting the lifetime of fatigue crack network formation in die-attach joints is considered through experiments on highspeed thermal cycling using a Si/solder/Si joint specimen and the mechanism is identified. Equibiaxial stresses are generated in the solder layer because thermal deformation of the solder is constrained by the Si, which causes continuous initiation and propagation of crisscross-shaped cracks. When the crack density is sufficiently high, crack growth is arrested by collisions between cracks, and the formation of the fatigue crack network is completed. Based on these results, development of the damaged area and arrest of the development by collisions between the cracks is expressed in terms of extended volume theory incorporating crack initiation and propagation functions for solder as well as considering the damage rate equation. The experimental result for the relationship between the damage ratio in the die-attach joint and the number of cycles under each thermal condition are reproduced by the damage rate equation.