Correlation Between Solder Joint Fatigue Life and Accumulated Work in Isothermal Cycling

The fatigue behavior of solder joints in realistic service applications is still not well understood. Service life prediction based on conducting accelerated tests and extrapolating test results therefore involves a high potential for error. Understanding both the evolution of solder joint properties and the damage accumulation has proved to be critical to reliability modeling. Damage accumulation in isothermal cycling is shown to scale with the accumulated inelastic work even in complex cycling scenarios, so that the life of a solder joint ends upon accumulation of a given amount of work. Individual ball grid array solder joints were cycled in shear fatigue experiments with different load amplitudes and strain rates. The effects of load amplitudes and strain rates on the work accumulation and fatigue life were systematically addressed. The correlation between different loading scenarios and the accumulated work to failure was also discussed. The results showed that the accumulated work until the development of a major crack is constant regardless of the load amplitude. After that the accumulated work to failure is lower for larger load amplitudes. For some reason, a larger fraction of the work appears to be dissipating as heat at lower load amplitude, but only during crack growth. On the other hand, the strain rate affects the fraction of the work going to heat even before the development of a major crack.