ICSR (Soldering and Reliability) Conference Proceedings

The Study of Dissolution of Bi Precipitates in Sn Using A Novel In Situ Heating Technique in the SEM

Authors: Peter Banh, André M. Delhaise, Doug D. Perovic
Company: University of Toronto and Celestica Inc.
Date Published: 6/5/2018   Conference: ICSR (Soldering and Reliability)

Abstract: Lead-free alloys such as SAC305 (Sn-3.0Ag-0.5Cu) have been shown in studies to have mechanical properties that degrade over time as a result of its microstructure coarsening. Further studies have shown that the inclusion of Bi in lead-free alloys can stabilize the mechanical properties if the alloy is aged above its solvus. This stabilization is due to the Bi particles dissolving into the Sn matrix when heated above its solvus, and then precipitating out uniformly when cooled. This creates a uniform and homogenous microstructure. Due to these characteristics, it has been proposed that with Sn-rich Bi-containing alloys, a thermal treatment can be used to improve the long-term reliability of solder joints in electronics assembly. This necessitates the need to further understand the kinetics of dissolution.

This paper details a study to understand the kinetics of Bi particle dissolution using a novel in situ heating technique in the scanning electron microscope (SEM). Three Sn-Bi samples with 3% Bi, 6% Bi, and 9% (all wt%) Bi were heated at 90°C in the SEM to determine the effects of Bi concentration on dissolution. Three Sn-Bi samples with 6 wt% Bi were subjected to different aging conditions (1 day, 3 days, and 7 days, all at 100°C) to determine the effects of aging on dissolution. Another four Sn-Bi samples with 6 wt% Bi were subjected to different aging conditions (as-cast, 1 day, 3 days, and 7 days, all at 100°C) but heated in the SEM at 65°C to determine the effects of temperature on Bi dissolution. SEM images of the samples were taken periodically, and the area of the Bi over time was analyzed.

It is proposed that the area of Bi precipitates in the field of view when heated could be modeled with either one exponential term or two exponential terms. Each of these terms are hypothesized to be a mode of dissolution. The parameters used in the modes are influenced by the aging condition, concentration, and the in situ heating temperature. When modeled by one exponential term, the dissolution is dominated by a dissolution mode in an unsaturated solid solution. When modeled by two exponential terms, the dissolution is dominated by an unsaturated dissolution mode initially, but transitions into a saturated dissolution mode after.

Using the proposed models, it was found that increasing the concentration of Bi slows the dissolution rate, and changes the dissolution behaviour from one mode to two modes. Increasing the time the sample spent aging increases the dissolution rate and changes the dissolution behaviour from two modes to one. Lastly, it was found that a lower in situ heating temperature will slow down the dissolution rate significantly.

Key Words: 

in situ heating, dissolution, aging, SEM, Bismuth

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