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Traditional finite element based predictions for solder joint reliability during thermal cycling accelerated life testing are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle model) that do not evolve with material aging. Thus, there will be significant errors in the calculations with lead free SAC alloys that illustrate dramatic aging phenomena. In this study, we have developed a revised set of Anand viscoplastic stress-strain relations for solder that include material parameters that evolve with the thermal history of the solder material. The effects of aging on the nine Anand model parameters have been examined by performing stress-strain tests on SAC305 samples that were aged for various durations (0–6 months) at temperature of 100 C. The stress-strain data were measured at three strain rates (.001, .0001, and .00001 1/sec) and five temperatures (25, 50, 75, 100, and 125 C). The mechanical tests have been performed using both water quenched (WQ) and reflowed (RF) samples (two unique specimen microstructures). In the case of the water quenched samples, there is rapid microstructural transitioning during the brief time that occurs between placing molten solder into the glass tubes and immersing the tubes in water bath. On the other hand, the reflowed samples are first cooled by water quenching, and then sent through a reflow oven to re-melt the solder in the tubes and subject them to a desired temperature profile matching that used in PCB assembly. As expected, the observed mechanical properties of water quenched samples were better (higher in magnitude) than the corresponding mechanical properties of the reflowed samples. Although the differences in elastic modulus between the water quenched and reflowed samples are small, significant differences are present for the yield and ultimate tensile stresses (for each aging condition). For both the water quenched and reflowed specimens, significant degradation of the mechanical properties has been observed with aging. Using the measured stress-strain and creep data, mathematical expressions have been developed for the evolution of the Anand model parameter with aging time. Our results show that 2 of the 9 constants remain essentially constant during aging, while the other 7 show large changes (30–70%) with up to 6 months of aging. The revised Anand constitutive equations for solder with aging effects have also been incorporated into commercial finite element codes (ANSYS and ABAQUS).

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Isothermal aging causes detrimental changes in the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies. These material changes also degrade the reliability of solder joints in assemblies subjected to aging prior to field use. In the current work, we have extended our previous research on the effects of aging on lead free solder material behavior to explore the effects of prior aging on solder joint (board level) reliability in actual assemblies. Our overall objective was to develop new reliability prediction procedures that incorporate aging effects, and then to validate the new approaches through correlation with thermal cycling accelerated life testing experimental data for pre-aged assemblies.Traditional finite element based predictions for solder joint reliability during thermal cycling accelerated life testing are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle model) that do not evolve with material aging. Thus, there will be significant errors in the calculations with lead free SAC alloys that illustrate dramatic aging phenomena. This work has implemented a theoretical framework for correcting this limitation and including aging effects in the reliability modeling. The developed approach involved the use of: (1) a revised set off Anand viscoplastic stress-strain relations for solder that included material parameters that evolve with the thermal history of the solder material, and (2) a revised solder joint failure criterion that included aging effects. The effects of aging on the nine Anand model parameters were determined experimentally for SAC305 lead free solder as a function of aging temperature and aging time. The revised Anand constitutive equations for solder with aging effects were then incorporated into standard finite element codes. The applied aging-aware failure criterion was based on the Morrow-Darveaux (dissipated energy based, DeltaW) approach, with both the fatigue criterion for crack initiation and the crack growth law incorporating material constants that depend on the prior aging of the solder material. Fatigue data were measured for SAC solder using uniaxial and shear test specimens that were aged for various durations and temperatures prior to cycling.The developed reliability modeling procedure has been applied to a family of assembled PBGA components. In the simulations, the packages were subjected to isothermal aging followed by thermal cycling accelerated life testing. The model were selected to reflect the board surface finish and SAC solder combination. With this approach, good correlation was obtained between the new reliability modeling procedure that includes aging and the entire set of measured solder joint reliability data that includes multiple component sizes, prior aging conditions, and board surface finishes.

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