Dwell Time, Microstructural Dependencies , and the Interpretation of Thermal Fatigue Test Data SnPb and Pb free Solders

High reliability end users are concerned with the long term attachment performance of SnAgCu (SAC) Pb-free solder joints. Thermally activated solder fatigue is the major wear-out failure for solder in electronic applications. Accelerated temperature cycling (ATC) has been used successfully for years to evaluate the fatigue performance of SnPb solder attachments and as a result, there is a considerable amount of test data for SnPb eutectic solder. Unfortunately, a comparable body of work does not exist for SAC solders, at least partly because the industry has been unable to reach consensus on the test methods needed to evaluate the fatigue performance of the Pb free solders. The lack of a concensus on SAC test parameters is realated to the perception that the microstructure, creep, and fatigue behavior of SAC solders are so different than SnPB solder that the test requirements for SnPB solder are not wholly applicable. This perception is supported by the fact that SAC solder creeps at a much slower rate than SnPb, which explains the well-known dwell time dependence of SAC fatigue lide in ATC tests. Additionally, there are reports that unlike SnPb solder, the fatigue life of SAC degrades with isothermal preconditioning (aging) due to microstructural changes induced by the preconditioning. However, some very recent results indicate that the ATC dwell time and aging has a stronger effecton the performance of SnPb solder than anticipated or reported previously. This paper presents direct comparisons of ACT test data for a number of components using both SnPb and SAC solder attachments. The temperature cycling was conducted using a 0/100 degree C cycle and dwell times times varying from 10 to 60 minutes. Isothermal preconditioning was applied equally to both SAC and SnPb samples to assess the impact on final fatigue life. The characterization of microstructural evolution and the failure analysis was performed using optical metallography and scanning electron microscopy and the fatigue reliability of the SAC and SnPb solders is discussed in terms of the microstructures and testing conditions. Beyond the need for such information to develop a better theoretical understanding of ATC induced degradation of solder joints, these results could have practical implications in terms of defining test parameters and assessing the inherent reliability of SAC solders.