Variations In Thermal Cycling Response Of Pb-Free Solder Due To Isothermal Preconditioning
Authors: Joe Smetana, Richard Coyle, Peter Read, Richard Popowich, Debra Fleming, and Thilo Sack Company: Alcatel-Lucent and Celestica Inc. Date Published: 10/16/2011
Abstract: Solder joints age and degrade during service and eventually fail by the recognized wear out mechanism of solder fatigue. In the case of Sn-Ag-Cu (SAC) Pb free solders, it has been suggested that the rate of solder fatigue, unlike that of SnPb eutectic solder, could increase if the joints were subjected to isothermal preconditioning (aging). Presumably, a preconditioned or aged condition could create a long term reliability risk by accelerating microstructural changes that otherwise occur very gradually in service. In this study, printed circuit board test vehicles containing 16 different high strain surface mount (SMT) packages were assembled using SAC solder. Isothermal preconditioning at 125°C was used to accelerate microstructural aging in the solder joints. The test cells included two different preconditioning times, 240 and 1000 hours, and a control set with no thermal preconditioning. All three sets of test vehicles were subjected to accelerated temperature cycling to induce thermal fatigue in the solder joints. Two thermal cycles were used in the study: 0-100°C, which is used often for evaluating high reliability applications and 20-80°C, which is a comparatively mild accelerated thermal cycle condition. A total of 6957 cycles was completed in the 0-100°C testing and a total of 9792 cycles was completed in the 20-80°C thermal cycling over more than two years of elapsed test time. The test data were evaluated using Weibull statistical analysis and the test samples were subjected to destructive physical analysis. Baseline and temperature cycled samples were analyzed to characterize the microstructural evolution and failure mode. The statistical analysis indicates a non-linear effect of the preconditioning time on the Weibull slope for many of the components. This non-linear effect is dramatic with the 0-100°C cycling but practically non-existent with the 20-80°C cycling. The results are discussed in terms of the role of strain on fatigue failure and on microstructural evolution. The practical implications of the findings are discussed and suggestions are made for additional work to address gaps in the data.