Comparing and benchmarking fatigue behaviours of various sac solders under thermo-mechanical loading

While the fatigue behaviours (including fatigue life predictions) of lead-free solder joints have been extensively researched in the last 15 years, these are not adequately compared and benchmarked for different lead-free solders that are being used. As more and more fatigue properties of lead-free solders are becoming available, it is also critical to know how fatigue behaviours differ under different mathematical models. This paper addresses the challenges and presents a comparative study of fatigue behaviours of various mainstream lead-free Sn-Ag-Cu (SAC) solders and benchmarked those with lead-based eutectic solder. Creep-induced fatigue and fatigue life of lead-based eutectic Sn63Pb37 and four lead-free SAC solder alloys: SAC305, SAC387, SAC396 and SAC405 are analysed through simulation studies. The Anand model is used to simulate the inelastic deformation behaviour of the solder joints under accelerated thermal cycling (ATC). It unifies the creep and rate-independent plastic behaviour and it is used to predict the complex stress-strain relationship of solders under different temperatures and strain rates, which are required in the prediction of fatigue life using the fatigue life models such as Engelmaier, Coffin-Mason and Solomon as the basis of our comparison. The ATC was carried out using temperature range from −40°C to 150°C. The fatigue damage propagation is determined with finite element (FE) simulation, which allows virtual prototyping in the design process of electronics devices. The simulation was carried out on a BGA (36 balls, 6 × 6 matrix) mounted onto Cu padded substrate. Results are analysed for plastic strain, Von mises stress, strain energy density, and stress-strain hysteresis loop. The simulation results show that the fatigue behaviours of lead-based eutectic Sn63Pb37 solder is comparable to those of lead-free SAC solders. Among the four SAC solders, SAC387 consistently produced higher plastic strain, strain energy and stress than the other solders. The fatigue life’s estimation of the solder joint was investigated using Engelmaier, Coffin-Manson, and Solomon models. Results obtained show that SAC405 has the highest fatigue life (25.7, 21.1 and 19.2 years) followed by SAC396 (18.7, 20.3 and 17.9 years) and SAC305 (15.2, 13.6 and 16.2 years) solder alloys respectively. Predicting the fatigue life of these solder joints averts problems in electronics design for reliability and quality, which if not taken care of, may result in lost revenue. Predictive fatigue analysis can also considerably reduce premature failure, and modern analysis technique such as one used in this research is progressively helping to provide comprehensive product life expectancy data.

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