Mechanical shock behavior of environmentally-benign Pb-free solders
The mechanical behavior of Pb-free solder alloys is important, since they must maintain mechanical integrity under thermomechanical fatigue, creep, and mechanical shock conditions. Mechanical shock, in particular, has become an increasing concern in the electronics industry, since electronic packages can be subjected to mechanical shock by mishandling during manufacture or by accidental dropping. In this study, the mechanical shock behavior of Sn and Sn-Ag-Cu alloys was systematically analyzed over the strain rate range 10-3 - 30 s-1 in bulk samples, and over 10-3 - 12 s-1 on the single solder joint level. More importantly, the influences of solder microstructure and intermetallic compounds (IMC) on mechanical shock resistance were quantified. A thorough microstructural characterization of Sn-rich alloys was conducted using synchrotron x-ray computed tomography. The three-dimensional morphology and distribution of contiguous phases and precipitates was analyzed. A multiscale approach was utilized to characterize Sn-rich phases on the microscale with x-ray tomography and focused ion beam tomography to characterize nanoscale precipitates. A high strain rate servohydraulic test system was developed in conjunction with a modified tensile specimen geometry and a high speed camera for quantifying deformation. The effect of microstructure and applied strain rate on the local strain and strain rate distributions were quantified using digital image correlation. Necking behavior was analyzed using a novel mirror fixture, and the triaxial stresses associated with necking were corrected using a self-consistent method to obtain the true stress-true strain constitutive behavior. Fracture mechanisms were quantified as a function of strain rate. Finally, the relationship between solder microstructure and intermetallic compound layer thickness with the mechanical shock resistance of Sn-3.8Ag-0.7Cu solder joints was characterized. It was found that at low strain rates the dynamic solder joint strength was controlled by the solder microstructure, while at high strain rates it was controlled by the IMC layer. The influences of solder microstructure and IMC layer thickness were then isolated using extended reflow or isothermal aging treatments. It was found that at large IMC layer thicknesses the trend described above does not hold true. The fracture mechanisms associated with the dynamic solder joint strength regimes were analyzed.