Design and Control of a Piezoelectric Driven Fatigue Testing System for Electronic Packaging Applications

Failure of solder joints for electronic packaging is an important issue for controlling the reliability of semiconductor devices. However, the complicated coupling between mechanical stressing and temperature and time dependent material properties makes it difficult to explore the fundamental failure control mechanisms using the existing accelerated thermal cycling methods. In addition, the testing speed is also severely restricted by the thermal time constant of the characterization system. In order to decouple the mechanical stress effect from other factors as the first step toward exploring the control mechanisms of failure, a piezoelectric-based fatigue characterization system is developed to replace the thermal cycling and provide fast and purely mechanical stressing cycles. A self-tuning based (STR) adaptive controller is also developed to provide accurate process control during experiments for compensating stiffness variation due to fatigue crack growth. It is found that this STR regulator is more robust than the traditional PID controller. The bandwidth of the system is approximately 70 Hz and is currently restricted by the equivalent time constant of the piezoelectric material. Nevertheless, this speed is sufficient for conducting a successful fatigue testing of solder joints. Finally, preliminary fatigue experiments have been performed on Sn₆₃Pb ₃₇ solders and the reduction of stiffness due to crack growth is clearly visible while the actuation performance is consistent and stable during the entire testing period. In the future, it is possible to operate in conjunction with a temperature control unit and a creep testing scheme to explore both the temperature and time dependent nature of solders in order to fully understand the failure control mechanisms of packaging