Effect of isothermal aging on properties of In-48Sn and In-Sn-8Cu alloys

Using flexible polymeric substrates in organic electronic devices is the development trend because of its advantages such as, lighter, easy to roll and offer a lower cost. However, they are easy to transform and change dimensions at high temperatures. This is a big challenge for some available low-temperature bonding processes and solders. Therefore, developed solders with low melting temperature, high ductility and satisfying mechanical properties are necessary.The tin-indium eutectic (In-48Sn) alloy is a promising candidate for low-temperature soldering applications especially for flexible electronic devices owing to its low melting temperature (118 °C), good wettability, and high ductility. However, In-48Sn alloy has a low mechanical strength and low creep resistance, which limits its applicability as a low-temperature solder. In this study, we investigated effect of 8.0 mass % Cu addition on thermal behavior, microstructure, and mechanical properties of In-48Sn eutectic alloy before and after isothermal aging from 7-42 days at 60 °C. The results revealed that Cu-addition was refined microstructure of the new In-Sn-8.0Cu alloy, which lead to enhance tensile strength of In-Sn-8Cu alloy (17.0 MPa) compared to that of In-48Sn alloy (11.0 MPa). Moreover, the η-Cu6(In, Sn)5 and τ-Cu(In, Sn)2 intermetallic compounds (IMCs) were formed in the new In-Sn-8Cu alloy, which also supported for the improvement of tensile strength of new alloy.After 7 days aging, tensile strength of In-48Sn alloy was reduced from 11.0 MPa to 9.5 MPa while that of In-Sn-8Cu alloy was decreased from 17.0 MPa to 15.5 MPa. After 42 days aging, In-48Sn alloy exhibited a significant decrease in tensile strength (approximately 22%), while In-Sn-8.0Cu alloy exhibited a lower reduction percentage (approximately 15%). This indicated that 8.0 mass % Cu addition not only improved mechanical properties of In-48Sn alloy as reflow condition but also enhanced thermal stability of this alloy. This indicated that In-Sn-8.0Cu has better thermal stability than In-48Sn.Additionally, the In-48Sn and In-Sn-8.0Cu alloys exhibited a similar semi-ductile fracture mode with planar stress fractures on inclined planes and some cleavages with sharp edges were observed on the surface. Owing to the large grain, the dimples size on the In-48Sn fracture surface was larger than that of In-Sn-8.0Cu fracture surface.