Very long-term aging of 52In–48Sn (at.%) solder joints on Cu-plated stainless steel substrates
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Long-term metallurgical aging was studied in thermal switches comprised of 52In–48Sn (at.%) alloy solder plugs contained in Cu-plated stainless steel cylinders. These switches are locking devices designed so that, if overheated, a “fusible” alloy melts and allows the activation of a spring-loaded mechanism. The soldered assemblies studied ranged in age from about 24 to 28 years old at the time of this analysis. A concern has been the buildup of intermetallic compound (IMC) within the solder or at the solder/substrate interface, which could raise the switch operating temperature. In this work, the melting temperature of the aged solder alloy was slightly lower (116.3 ± 0.3 °C) than the expected value, 118.4 °C (245 °F), based on differential scanning calorimetry (DSC). The slight decrease in melting temperature range was caused by the diffusion of a small amount of Cu into the solder during processing and possibly during long-term service. The interfacial IMC layer was primarily Cu2In3Sn. The IMC thickness agreed with that predicted by growth kinetics determined in a previous study, assuming aging temperatures in the vicinity of room temperature. Differences in the IMC phase chemistries were found between earlier research, which employed bulk Cu substrates, and the present analyses with thin electroplated Cu substrates. Evidence was found for depletion of the thin Cu plating layer over time, as well as incorporation of Fe and Ni from the stainless steel into the IMC layer.
KeywordsDifferential Scanning Calorimetry Solder Alloy Bulk Solder Differential Scanning Calorimetry Experiment Plating Layer
The authors would like to thank Dr. Mike Dugger for careful review of the manuscript. Sincere thanks also to Alice Kilgo for metallography and to Bonnie McKenzie for SEM analysis. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.
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