Abstract
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.
Highlights
Predicting the reliability of solder interconnections, whether subjected to thermal mechanical fatigue (TMF), mechanical shock, or vibration, is depending to a greater extent upon computation modeling techniques
The limiting factor is having access to the time-dependent and time-independent mechanical properties of the materials that comprise the joint structure and, in particular, the solder. These properties are essential towards developing a unified creep-plasticity (UCP) constitutive equation that predicts deformation in the computational model [1,2]
In the study reported here, the creep behavior was evaluated for a Pb-free solder comprised of tin (Sn), silver (Ag), and bismuth (Bi)
Summary
Predicting the reliability of solder interconnections, whether subjected to thermal mechanical fatigue (TMF), mechanical shock, or vibration, is depending to a greater extent upon computation modeling techniques. The limiting factor is having access to the time-dependent (creep) and time-independent (stress-strain) mechanical properties of the materials that comprise the joint structure and, in particular, the solder. These properties are essential towards developing a unified creep-plasticity (UCP) constitutive equation that predicts deformation in the computational model [1,2]. In the study reported here, the creep behavior was evaluated for a Pb-free solder comprised of tin (Sn), silver (Ag), and bismuth (Bi). Later variants of this material have included Cu additions, this effort examines the commercially-available, ternary alloy, 91.84Sn-3.33Ag-4.83Bi (wt.%, abbreviated Sn-Ag-Bi) [3]
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