Constitutive Model and Solder Joint Reliability Predictionsfor BGA Packages Subjected to Aging
Type of DegreeDissertation
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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. 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. 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. To determine the effects of aging, uniaxial tensile tests were conducted on SAC305 samples that were aged for various durations (0-360 days) at a temperature of 100 oC. Mechanical tests have been performed using both water quenched (WQ) and reflowed (RF) SAC305 samples (two unique specimen microstructures). For each set of aging conditions, several sets of constant strain rate and temperature tests were conducted on the aged solder samples. Using the measured uniaxial test data, the Anand parameters were calculated for each set of aging conditions, and the effects of aging on the nine Anand model parameters were determined. From the experimental results, the differences between the extracted Anand model parameters of water quenched and reflowed samples were high for samples with no prior aging. For both the water quenched and reflowed specimens, significant degradation of the mechanical properties was observed with aging. After long aging times, the water quenched and reflowed SAC305 materials were found to exhibit similar mechanical properties, and thus their Anand parameters converged and became nearly identical. Moreover, an investigation on the Anand constitutive model and its application to SAC solders of various silver contents (i.e. SACN05, with N = 1, 2, 3, 4) has been performed. For each alloy, both water quenched (WQ) and reflowed (RF) solidification profiles were utilized to establish two unique specimen microstructures, and the same reflow profile was used for all four of the SAC alloys so that the results could be compared and the effects of Ag content could be studied systematically. The nine Anand parameters were determined for each unique solder alloy and microstructure from a set of stress strain tests performed at several strain rates and temperatures. As expected, the mechanical properties (modulus and strength) increase with the percentage of Ag content, and these changes strongly affect the Anand parameters. The sensitivity of the mechanical properties and Anand parameters to silver content is higher at lower silver percentages (1-2%). After deriving the Anand parameters for each alloy and microstructure, the stress-strain curves have been calculated for various conditions, and excellent agreement was found between the predicted results and experimental stress-strain curves. In addition, tensile testing was performed on reflowed SACN05 specimens subjected to 180 days of aging at 100 oC. After this severe level of aging, any further changes in the mechanical response and properties from subsequent aging will be rather small. The developed 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. In the simulations, BGA packages were subjected to isothermal aging followed by thermal cycling accelerated life testing. The Anand model parameters were chosen based on the prior aging conditions. The model predictions were correlated with solder joint reliability test data for the same components. The experimental test vehicle incorporated several sizes (5, 10, 15, 19 mm) of BGA daisy chain components with 0.4 and 0.8 mm solder joint pitches (SAC305). PCB test boards with 3 different surface finishes (ImAg, ENIG and ENEPIG) were utilized. Before thermal cycling began, the assembled test boards were divided up into test groups that were subjected to several sets of aging conditions (preconditioning) including 0, 180, and 360 days aging at T = 125 oC. After aging, the assemblies were subjected to thermal cycling (-40 to +125 oC) until failure occurred. The failure data for each test group were fit with the two parameter Weibull model, and the failure plots have demonstrated that the thermal cycling reliabilities of pre-aged assemblies were significantly less than those of analogous non-aged assemblies with degradations of up to 53% for one year of prior aging. The coefficients in the aging aware crack growth model were selected to reflect the board surface finish and SAC solder combination. With this approach, good correlation was obtained between the new FEA-based 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.