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Effect of Sn Component Surface Finish on 92.5Pb-5Sn-2.5Ag
Keywords: Solder, Surface Finish, Fatigue
92.5Pb-5Sn-2.5Ag solder, or a close variant, is widely used in high temperature electronics, such as down-hole and well-logging electronics, because of its high melting temperature (296°C) and superior fatigue resistance. However, selection of the right component surface finish is critical for solder joint performance. There are many options for component finish which not only change the composition of the interfacial intermetallic compound, but of the solder joint itself. One popular finish for high temperature electronics is matte Sn. During reflow, Sn, which has a much lower melting temperature (232°C) than 92.5Pb-5Sn-2.5Ag, dissolves into the solder changing the composition of the final joint. The final joint composition is a function of solder volume, Sn thickness, and wetting area. This work investigates the effect of Sn component surface finish on the melting temperature, microstructure, and mechanical behavior of 92.5Pb-5Sn-2.5Ag. 92.5Pb-5Sn-2.5Ag was doped with up to 7% Sn to simulate the final composition of the joint after reflow. Differential Scanning Calorimetry (DSC) was used to measure the change in liquidus temperature with increasing Sn concentration. Microstructure and mechanical tests were carried out on 20 mil solder spheres reflowed on high temperature polyimide test coupons. Solder joints of each composition were cross-sectioned to examine the microstructure and interfacial reactions. The area fraction of β-Sn and Ag3Sn was quantified for each composition using image analysis software. Shear and isothermal fatigue tests of individual solder joints with varying Sn concentrations were conducted at room temperature and 200°C. Joints were also sheared and fatigue tested at 200°C after aging for 1000hrs at 200°C to simulate a down-hole environment. After failure the fracture surfaces were examined to determine the mode of failure. Beyond providing guidance for surface finish selection, this work examines the microstructure and mechanical behavior of 92.5Pb-5Sn-2.5Ag as a function of Sn concentration and temperature. An understanding of the microstructure-mechanical performance relationship will aid in the development of new alloys for high temperature applications.
Harry Schoeller, Research Engineer
Universal Instruments AREA Consortium, Binghamton University
Conklin, NY

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