Isothermal shear fatigue mechanism of lead free solder joints

This paper presents the results of isothermal shear fatigue tests conducted on Sn-Ag-Cu lead-free and Pb-Sn solder alloys in fully assembled PBGA configurations. Our study, aimed to better understand the fatigue behaviors of solder alloys in the package assembly, reveals that the shear fatigue test is indeed capable of measuring the fatigue properties of the solder itself and may lead to material parameters necessary for the prediction of the solder joints fatigue reliability in general. The cyclic shear fatigue test conducted under various fatigue conditions yields the result suggesting the close linkage between the fatigue in PBGA and the fatigue property of solder alloy. This is a result of the existence of consistency in crack propagation path in solder joint, that is fixed to the chip-side of solder matrix (not interface), irrespective of test conditions that includes temperature, strain range, and frequency. As a result, fatigue failure of Pb-free and Pb-Sn solder can be fitted to the Coffin-Manson fatigue model with a reasonable consistency. The resulting numerical constants, namely ductility coefficient and ductility exponent, are consistent with what is expected from general consideration of metallic fatigue system. However, their temperature dependence shows a significant deviation from expectation, probably due to an addition of thermal strain to the shear strain and its variation with temperature.