The influence of the Pb-free solder alloy composition and processing parameters on thermal fatigue performance of a ceramic chip resistor

This paper presents the results of a thermal fatigue study of a 2512 ceramic chip resistor assembled with various Pb free solders including SnCu, SAC105, SAC205, SAC305, and SAC405. The test matrix also includes some limited evaluations with other variables such as cooling rate (solidification rate), thermal preconditioning and nitrogen (inert) reflow atmosphere. The matrix also includes a SnPb eutectic control cells. The resistor test vehicle provides an expedient and self-consistent method for evaluating the relative fatigue performance of the various alloys. A study of the as-assembled solder joints was conducted to characterize the microstructure of the solder joints with varying silver content. Thermal fatigue was evaluated using an accelerated temperature cycle of 0/100degC with dwell times of 10 and 60 minutes. The test results show a direct relationship between characteristic fatigue life and Ag content, with the higher Ag content alloys outperforming those with the lowest Ag content. As might be anticipated, there also was a consistent inverse relationship between fatigue life and dwell time for the Pb free solders. The failure analysis and microstructural evolution is characterized with optical metallography and scanning electron microscopy and the fatigue reliability of the Pb free solders is discussed in terms of the microstructures.