SMTA International Conference Proceedings

Assessment of the Behavior of High Reliability Solder Alloys in Accelerated Thermal Testing

Authors: Luke Wentlent, Ph.D., Jim Wilcox, Ph.D., Fei Dong, Ph.D.
Company: Universal Instruments Corp. and State University of New York at Binghamton
Date Published: 9/22/2019   Conference: SMTA International

Abstract: Within a number of different industries there is a significant demand for Pb-free solder alloys that exhibit a superior performance in harsh environments. Accordingly, a number of different solder alloys have been developed for these high temperature and/or high delta T types of service conditions. Unfortunately, there is often only limited mechanical testing performed on these solders resulting in a significant deficiency with respect to knowledge of the mechanisms and properties driving the mechanical performance. In an effort to address this critical knowledge gap, a study was conducted to examine the microstructural evolution of two different high reliability solder alloys in both accelerated thermal cycling and accelerated thermal shock testing. Resistor test samples were subjected to several intervals of cycling, after which the microstructure was characterized using optical and scanning electron microscopes. Additionally, a number of samples were mechanically sheared at the same intervals to observe how the changes in the microstructure might be driving the mechanical properties. The solder alloys studied were common commercially available formulations marketed as high reliability options. The compositions were Sn-3.8%Ag-0.7%Cu-3.2%Bi-3.0%Sb or Sn-3.8%Ag-0.7%Cu-0.5%Bi-3.5%Sb-2.5%In, as well as a SAC 305 control.

Results showed that there was a significantly different evolution in the microstructure between all three of the alloys studied. These differences were exacerbated within the high temperature accelerated thermal shock test. Additionally, each alloy exhibited unique intermetallic precipitates that formed within the Sn matrix. Results showed that both alloys displayed failure behavior different from that observed with the SAC 305 control samples with little to no recrystallization of the ß-Sn grain. This deviation in behavior was linked to the behavior and influence of the elements being alloyed into the system.

Key Words: 

Pb-free, high reliability, microstructure, solder, harsh environments, shear testing

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