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Tin-bismuth alloys may be an alternative to lead-based solders for low-temperature applications, but very little is known about their manufacturability and reliability. This article presents an overview of these issues. First, experiments to determine the wetting properties of the Sn-Bi solder are presented. The results show that Sn-Bi solders do not wet bare copper well, but that they do wet copper having a hot-dipped Sn-Bi coating. Next, the effects of aging on the microstructure of Sn-Bi solders are described. The results show that during aging, tin is de-pleted from the solder/base metal interface. The two-phase Sn-Bi microstructure coarsens during aging; the rate of coarsening can be slowed by adding 1.0 wt. % Cu to the solder. The aging also affects the shear strength of the solder jOints, where aged joints show an increase in maximum shear stress and ductility at failure.

This study details the steady-state creep properties of Sn-1 wt pct Bi, Sn-2 wt pct Bi, and Sn-5 wt pct Bi as a function of stress and temperature. All data, including previous work on pure Sn, are described by the following empirical equation:describes steady-state creep where at low strain rates there is linear stress dependence and at high strain rates there is an exponential stress dependence. The transition in creep behavior occurs at a critical, breakaway stress, c ϭ E/␣. This stress is compared to the breakaway stresses proposed by Friedel and by Cottrell and Jaswon. There is good agreement at low solute concentrations to the breakaway stress proposed by Friedel, but c is significantly lower than the breakaway stress predicted by Cottrell and Jaswon. Several observations suggest that for Sn-xBi alloys, dislocation climb is the rate-limiting mechanism in the nonlinear region. First, the stress sensitivity of the steady-state strain rate data is similar to that of pure Sn, where dislocation climb is known to be the rate-limiting mechanism. Second, primary creep is observed throughout the tested stress range. Third, incremental additions of Bi decrease the steady-state creep rates, even though Bi has a higher diffusivity in Sn than Sn by self-diffusion.

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3.-Currently with Advanced Micro Devices, 5204 E. Ben White Blvd., M/S 608, Austin, TX 78741.Small bimetallic load-frames with reference assembly stiffness, k′, and fullyconstrained shear strain, γ fc , were used to simulate the thermo-mechanical conditions experienced by eutectic Bi-42wt.%Sn-to-Cu solder joints. Shear stress and strain were induced in the solder joint by a 45-minute, 0 to 100°C temperature cycle and were calculated from the assembly temperature, joint configuration, and measured elastic strain in the load-frame. Early in cycling, a hysteresis loop representing the maximum stress range and minimum strain range was reached. As damage accumulated in the solder, the stress range decreased and the strain range increased. The TMF life of the joints, defined by the load range drop, Φ, as a function of k′ and γ fc , can be determined, defining an effective plastic strain range which allows data for various stiffnesses and thermal expansion mismatches to be summarized on a single Coffin-Mansion plot. The effective plastic strain range also provides an important link to conventional low cycle fatigue (LCF) data taken from an infinitely stiff load-frame.