Publications

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A study was performed to validate a first-principles model for whisker and hillock formation based on the cyclic dynamic recrystallization (DRX) mechanism in conjunction with long-range diffusion. The test specimens were evaporated Sn films on Si having thicknesses of 0.25 μm, 0.50 μm, 1.0 μm, 2.0 μm, and 4.9 μm. Air annealing was performed at 35°C, 60°C, 100°C, 120°C, or 150°C over a time duration of 9 days. The stresses, anelastic strains, and strain rates in the Sn films were predicted by a computational model based upon the constitutive properties of 95.5Sn-3.9Ag-0.6Cu (wt.%) as a surrogate for pure Sn. The cyclic DRX mechanism and, in particular, whether long whiskers or hillocks were formed, was validated by comparing the empirical data against the three hierarchal requirements: (1) DRX to occur at all: εc = A Do mZn, (2) DRX to be cyclic: Do < 2Dr, and (3) Grain boundary pinning (thin films): h versus d. Continuous DRX took place in the 2.0-μm and 4.9-μm films that resulted in short stubby whiskers. Depleted zones, which resulted solely from a tensile stress-driven diffusion mechanism, confirmed the pervasiveness of long-range diffusion so that it did not control whisker or hillock formation other than a small loss of activity by reduced thermal activation at lower temperatures. A first-principles DRX model paves the way to develop like mitigation strategies against long whisker growth.

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Compression creep tests were performed on the ternary 91.84Sn-3.33Ag-4.83Bi (wt.%, abbreviated Sn-Ag-Bi) Pb-free alloy. The test temperatures were: -25 °C, 25 °C, 75 °C, 125 °C, and 160 °C (± 0.5 °C). Four loads were used at the two lowest temperatures and five at the higher temperatures. The specimens were tested in the as-fabricated condition or after having been subjected to one of two air aging conditions: 24 hours at either 125 °C or 150 °C. The strain-time curves exhibited frequent occurrences of negative creep and small-scale fluctuations, particularly at the slower strain rates, that were indicative of dynamic recrystallization (DRX) activity. The source of tertiary creep behavior at faster strain rates was likely to also be DRX rather than a damage accumulation mechanism. Overall, the strain-time curves did not display a consistent trend that could be directly attributed to the aging condition. The sinh law equation satisfactorily represented the minimum strain rate as a function of stress and temperature so as to investigate the deformation rate kinetics: dε/dtmin = Asinhn (ασ) exp (-ΔH/RT). The values of α, n, and ΔH were in the following ranges (±95% confidence interval): α, 0.010-0.015 (±0.005 1/MPa); n, 2.2-3.1 (±0.5); and ΔH, 54-66 (±8 kJ/mol). The rate kinetics analysis indicated that short-circuit diffusion was a contributing mechanism to dislocation motion during creep. The rate kinetics analysis also determined that a minimum creep rate trend could not be developed between the as-fabricated versus aged conditions. This study showed that the elevated temperature aging treatments introduced multiple changes to the Sn-Ag-Bi microstructure that did not result in a simple loss ("softening") of its mechanical strength. © 2012 by Sandia Corporation.

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Thick film conductors provide the circuitry for hybrid microcircuit (HMC) assemblies. The integrity of solder joints made to those conductors is a function of the solid-state interface reactions that occur under long-term service environments. A study was performed, which examined the mechanical strength of 63Sn-37Pb (wt.%, Sn-Pb) solder joints made to the thick film conductor, 76Au-21Pt-3Pd (Au-Pt-Pd), on 96% Al2O3 substrates. The Au-Pt-Pd layer was 18±3 μm thick. Isothermal aging accelerated the solder/thick film interface reaction, which resulted in the growth of an intermetallic compound (IMC) layer and consumption of the thick film layer. The aging temperatures were 70°C, 100°C, and 135°C. The aging times were 5-5000 hours. The sheppard's hook pull test was used to assess the strength of the Sn-Pb solder joints at two displacement rates: 10 mm/min and 100 mm/min. A measurable loss of joint strength was observed after aging, which did not generate a great deal of IMC layer growth. The aging effects occurred at the thick film/Al2O3 interface as concluded by other authors. However, the present investigation showed those strength losses to be reversible after more extended aging times at elevated temperature. The strength and failure modes were sensitive to displacement rate when IMC layer development was minimal. Extensive growth of the IMC layer was accompanied by the formation of a Pb-rich layer ahead of it, which was responsible for a gradual decrease in the pull strength. In this case, pull strength and failure mode were less sensitive to displacement rate. The solder joints maintained a nominal level of pull strength, even after nearly all of the thick film conductor had been consumed by IMC layer formation. Copyright © 2010 MS&T'10®.