Thermal Fatigue of SnPb and SAC Resistor Joints: Analysis of Stress-Strain as a Function of Cycle Parameters

Accelerated thermal cycling (ATC) has been widely used in the microelectronics industry for reliability assessment. The relative effects of thermal cycling parameters (temperature range, dwell time, and ramp rate) and the failure mechanisms they induce have been the subject of many studies; however, uncertainty remains, particularly regarding the role of a very high ramp rate such as encountered in a thermal shock chamber. In the present research, thermomechanical finite-element (FE) models were used to analyze the stress/strain response of a resistor test vehicle during ATC testing under three accelerated test conditions: two ramp rates (14 degC/min and 95 degC/min) and two temperature ranges (DeltaT=0degC-100degC and -40degC-125degC), denoted 14-100 (ramp rate-temperature range), 95-100, and 95-165. The temperature gradients through the thickness of the assemblies were measured during the ATC test with the high ramp rate and were used to create an FE model that included transient stresses and strains. The effect of the transient temperature gradients during thermal shock was found to be negligible in these resistor joints. The FE models were then used to simulate the above three ATC test conditions with either SnPb or Pb-free (SAC) solders and were compared with previously published thermal fatigue lives for this resistor test vehicle. For both SnPb and SAC resistors, the maximum total solder strains (sum of elastic, plastic, and creep) and strain energy dissipation per cycle predicted by the FE models in the 95-165 test condition were much greater than those in either the 14-100 or 95-100 test conditions, which produced almost identical strains and energy dissipation. In all cases, the strain energy density dissipation per cycle due to creep was much larger than that due to plastic deformation. The trend of these results was in accordance with the ATC tests, which showed that the thermal cycling lives decreased in the same order for both the SAC and SnPb solders;- - i.e., the fatigue life decreased as the predicted total strain increased