High Temperature Vibration Fatigue Life Prediction and High Strain Rate Material Characterization of Lead-Free Solders

Abstract

Current trends in the automotive industry warrant a variety of electronics for improved control, safety, efficiency and entertainment. Many of these electronic systems like engine control units, variable valve sensor, crankshaft-camshaft sensors are located under-hood. Electronics installed in under-hood applications are subjected simultaneously to mechanical vibrations and thermal loads. Typical failure modes caused by vibration induced high cycle fatigue include solder fatigue, copper trace or lead fracture. The solder interconnects accrue damage much faster when vibrated at elevated temperatures. Industry migration to lead-free solders has resulted in a proliferation of a wide variety of solder alloy compositions. Presently, the literature on mechanical behavior of lead-free alloys under simultaneous harsh environment of high-temperature vibration is sparse. In this research, a test vehicle representative of an engine control unit has been used. The reduction in stiffness of the PCB with temperature has been demonstrated by measuring the shift in natural frequencies with temperature. The test vehicle has been tested to failure by subjecting it to two elevated temperatures and harmonic vibrations at the corresponding first natural frequency. PCB deflection has been shown to increase with increase in temperature. The full field strain has been extracted using high speed cameras operating at 100,000 fps in conjunction with digital image correlation. Material properties of the PCB at elevated test temperatures have been measured using Dynamic Mechanical Analyzer (DMA).

FE simulation using global-local finite element models is then correlated with the system characteristics such as modal shapes, natural frequencies and displacement amplitudes for every temperature. The solder level stresses have been extracted from the sub-models. Stress amplitude versus cycles to failure curves are obtained at all the three test temperatures. Temperature dependent terms have been added to the Basquin power law to predict the high cycle fatigue life of SAC305 solder at elevated temperatures. In the second part of the thesis, the effect of aging and high strain rates on a newly developed high-temperature, high-performance lead-free substitute called Innolot by InnoRelTM targeting the automotive electronics segment has been studied. Innolot contains Nickel (Ni), Antimony (Sb) and Bismuth (Bi) in small proportions in addition to Sn, Ag and Cu. Recent studies have highlighted the detrimental effects of isothermal aging on the material properties of SAC alloys. This phenomenon has posed a severe design challenge across the industry and remains a road-block in the migration to Pb-free. This research includes the high strain rate material characterization of Innolot as the alloy ages at an elevated temperature of 500C. The strain rates chosen are in the range of 1-100 per-second which are typical at second level interconnects subjected to drop-shock and vibration environments. The strain rates and elevated aging temperature have been chosen also to facilitate comparison with tests conducted on SAC105 and SAC305 alloys at CAVE3 by other researchers. Ramberg-Osgood non-linear model parameters have been determined to curve-fit through the experimental data. The parameters have been implemented in Abaqus FE model to obtain full-field stresses which correlates with contours obtained experimentally by Digital Image Correlation (DIC).