Impact of Interfacial Roughness on Tensile vs. Shear Creep Rupture of Solder Joints

Brittle intermetallic compounds (IMCs) that grow at the interface between the substrate metallization and the solder during the creation of solder joints play an important role in the joint's strength and reliability. Substrate roughness is a key parameter which can possibly influence the morphology and strength of interfacial IMCs and of the solder joint. Therefore, in this study, creep rupture life of SAC 305 lead free solder interconnects on smooth and rough substrates was experimentally investigated under tensile vs shear loading modes. A constant load creep test was conducted at high temperature (125°C) and the test was monitored at regular intervals. The tests were performed using custom tensile and shear test fixtures. Modified Iosipescu specimens with two levels of interface surface roughness (Rrms: 1.1 μm and 0.3 μm) were fabricated using SAC305 solder alloy. Scanning Electron Microscopy (SEM) was used to perform failure analysis and observe failure modes in the specimens.Preliminary creep rupture results clearly indicate that, specimens with rough interfaces were found to be less durable than those with smooth interfaces, in tensile loading. In contrast, interfacial roughness was not seen to affect creep rupture durability in shear loading. The observed failure modes were a mix of (i) rupture near the interfacial IMC layers; and (ii) rupture within the bulk of the solder materials. The relative contribution of these two failure modes was found to be sensitive to interfacial roughness under tensile loading (with rougher surface producing more of the interfacial fracture mode). In contrast, the relative contribution of these two failure modes was almost independent of substrate interfacial roughness under shear loading (with the majority of damage being within the bulk of the solder material). The goal of this study is to draw attention to the role of the surface roughness of the substrate metallization surface on the solder joint strength, when printed wiring assemblies are subjected to multiaxial stresses under long term loads during their operational life.