Evolution of the tension/compression and shear cyclic stress-strain behavior of lead-free solder subjected to isothermal aging

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during elevated temperature aging were unexpectedly large and universally detrimental to reliability. Solder joints in electronic assemblies are typically subjected to thermal cycling, either in actual application or in accelerated life testing used for qualification. Mismatches in the thermal expansion coefficients of the assembly materials leads to the solder joints being subjected to cyclic (positive/negative) mechanical strains and stresses. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure. While the effects of aging on solder constitutive behavior (stress-strain and creep) have been recently examined in some detail, there have been no prior studies on the effects of aging on solder failure and fatigue behavior.In our current work, we have examined effects of several parameters (aging temperature and time, testing temperature, strain/stress limits, and solder alloy composition) on the cyclic stress-strain behavior of SAC lead free solders. Both uniaxial specimens subjected to cyclic tension/compression and Iosipescu lap shear samples subjected to cyclic positive/negative shear have been studied. Samples were subjected to mechanical cycling under both strain control (constant positive and negative strain limits) and stress control (constant positive and negative stress limits). The hysteresis loop size (area) was calculated from the measured cyclic stress-strain curves for a given solder alloy and temperature. This area represents the energy dissipated per cycle, which is correlated to the damage accumulation in the specimen. In this paper, we report on our findings on the effects of isothermal aging on the cyclic stress-strain behavior of SAC lead free solders. Prior to cyclic testing, the specimens were aged (preconditioned) at 125
^o
C for various durations (up to one year). From the recorded cyclic stress-strain curves, we have been able to characterize and empirically model the evolution of the solder hysteresis loops with aging. Similar to solder stress-strain and creep behavior, there is a strong effect of aging on the hysteresis loop size (and thus the rate of damage accumulation) in the solder specimens. The observed degradations in the fatigue/failure behavior of the lead free solders are highly accelerated for lower silver content alloys, and in this paper we concentrate on presenting the results for SAC105 (Sn-1.0Ag-0.5Cu) lead free solder.