Thermomechanical behavior of ball grid array solder joints under thermal and vibration loading: Testing and modeling

This dissertation provides the first true insight of the thermomechanical behavior of Sn/Pb solder joints in actual BGA electronic packages under various thermal and vibration cycling. The nonlinear interaction effect of thermal and vibration loading on creep fatigue of solder joints was experimentally observed. Special attentions were also paid to the effect of temperature profiles. A high sensitivity laser Moiré interferometry technique was developed and applied to fatigue analysis of BGA solder joints.; Extensive thermal cycling, vibration, shock, and concurrent thermal and vibration cycling tests, with different loading combinations and temperature profiles, were conducted on BGA electronic packages. The inelastic deformations were recorded in the form of whole field contour maps during thermal and vibration cycling using Moiré interferometry. The inelastic strain accumulations were correlated to fatigue life prediction. It is found that creep and damage mechanism slows down if there is vibration or temperature mini cycles in thermal cycling. The interaction of thermal and vibration loading is nonlinear in nature, and the linear superposition rule-Miner's rule is not correct for BGA solder joints under combined thermal and vibration cycling. Test results also showed that the Coffin-Manson equation and Darveaux' theory are not accurate in predicting thermal fatigue life of solder joints because they do not account for microstructural evolution, and do not capture the intrinsic damage mechanisms in an evolving material as solder alloys.; A damage mechanics based unified thermal viscoplasticity constitutive model with isotropic hardening and kinematic hardening for Pb/Sn solder alloy was developed. This model is capable of predicting a broad range of deformation mechanisms in monotonic, cyclic, and creep regimes. The effect of microstructure is taken into account by incorporating grain size in the flow rule. Temperature effects are incorporated into the material properties. The internal damage variable provides direct reflection of fatigue life. This model was verified against various tensile and shear test data for a wide range of temperatures and strain rates. Implemented into commercial finite element software, this model was effectively applied to the thermal fatigue analysis of BGA packages.