Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

Keywords: Shear strength Microstructure Single-shear lap joint Intermetallic compound (IMC) Aging a b s t r a c t The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 Â 10 À3 mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu 6 Sn 5 + Cu 3-Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 Â 10 À17 and 3.72 Â 10 À17 m 2 /s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/ IMC interface. Solder joints play a crucial part to provide the necessary electrical and mechanical interconnection in an electronic assembly. Furthermore , solder joints must be capable of withstanding the high thermal and mechanical loadings which frequently occur during handing and system use at higher temperatures [1]. Commonly, metallurgical reaction between liquid solder and Cu pad during the soldering process will occur and result in a layer of Cu–Sn intermetallic compounds (IMC) at the solder/Cu interface. Furthermore , the rapid formation and thickening of Cu–Sn IMC at the sol-der/Cu interface during reflowing process and/or high temperature service attracted serious concerns about the reliability of these solders [2–5]. Some electronic products which are referred to as high-performance electronic products are so essential to modern society that their operational integrity must be maintained, for instance, the servers, storage and storage array systems, network infrastructure equipment and telecommunication equipment for network management. For these products, the introduction of new and unpro-ven materials could pose a significant reliability risk although numerous Sn-based …