Experimental characterization of material degradation of solder joint under fatigue loading

Thermal fatigue damage is a progressive process of material degradation. The objective of this study is to experimentally quantify the material degradation of solder joint in electronic BGA package under thermal fatigue loading. Elastic modulus degradation under thermal cycling, which is considered as a physically detectable quantity of material degradation, was measured by nano-indenter. It was compared with tendency of inelastic strain accumulation of solder joint in BGA package under thermal cycling, which was measured by Moire interferometry. Fatigue damage evolution of solder joint with traditional load-drop criterion was also investigated by strain-stress hysteresis loops from strain-controlled cyclic shear testing of thin layer solder joint. Load-drop tendency from isothermal shear testing was compared with elastic modulus degradation of solder joint under thermal cycling. Following conventional Coffin-Manson approach, S-N curve was obtained from isothermal fatigue testing with load-drop criterion. In this study, thermal fatigue life prediction of solder joint in electronic BGA package was conventionally made based on this Coffin-Mason S-N curve and measured inelastic strain accumulation of solder joint under thermal cycling, and compared with measured elastic modulus degradation tendency. The experimental investigation results will be used for further thermal fatigue damage evolution modeling using thermodynamic continuum damage mechanics theory. The primary contribution of this project is to observe material degradation of BGA solder joints at actual micron scale under thermal fatigue loading, and compare the results with isothermal fatigue response of solder alloys. All previous research on the subject has been done on bulk scale specimens.