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Pb-free Solders, Creep, and Body Temperature
Keywords: Pb-free solders, solder creep, SAC 305
More and more implantable medical devices contain electronic components which have solder joints to maintain electrical connectivity. As in the case of consumer electronics, medical devices too continue to face the demand to transition to Pb-free solders. Current data that are available in the open literature are predominantly for operating temperatures of 25oC, under uniaxial tension. In reality, very few solder joints are subjected to pure uniaxial tension; they are commonly subjected to tensile and shear loading simultaneously. In the case of implantables the service temperature can be expected to be 37oC – body temperature – rather than 25oC. At these temperatures solder joints are operating under “hot” conditions where atomic mobility is relatively fast and solder joints can be expected to undergo creep, i.e., plastic deformation under constant load conditions. In order to address this issue and learn more about deformation under actual service conditions, an experimental investigation was conducted to characterize and measure the contributions of shear and tensile strain components to the overall creep behavior of solder joints. Each copper substrate test vehicle consisted of two halved dog-bone shaped coupons and formed a butt joint at 0°, 30°, 45°, 60°, and 90° angles. Eutectic Pb-63Sn solder and Sn3.0Ag0.5Cu (SAC 305) lead-free solder alloy were used to solder the coupons. The specimens were subjected to 5.2 MPa tensile stress for three days in a thermal chamber at 25oC, 37oC, and 100oC. The experimental results revealed that both tensile creep strains and shear creep strains contribute to the combined creep curves. For both Pb-63Sn and SAC 305 solder joints, the contributions of both tensile strains and shear strains to combined strains were found to be significant. Thus the tensile to shear stress ratio were found to have a significant impact on the creep behavior of solder joints. The creep resistance of the SAC 305 solder joints was better than that of the Pb-63Sn solder joints for all the conditions tested.
Guna Selvaduray, Chair, BCME Department
San Jose State University
San Jose, CA

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