Multiscale modeling of the interfacial fracture behavior in the Sn-Cu ₆ Sn ₅-Cu system

With the increasing focus on developing environmentally-benign electronic packages, Pb-free alloys have received a great deal of attention. Mishandling of packages, during manufacture, assembly, or by the user may cause failure of solder joint. Very few reports to-date in the literature have delved into the microstructural mechanisms for fracture in Pb-free solder joints that usually comprising many interfaces between Cu and intermetallic (IMC) layers, as well as IMC and Sn. Such an understanding is necessary for designers to make informed decisions when designing new robust and environmentally-benign electronics packages. In this paper, we have established a cohesive law for IMC and Sn interfaces based on the modified embedded atom methods. By using the continuum homogeneous approach, the interfacial stress/separation relation and the cohesive energy is obtained. The theoretical analysis predicts that the interface has the largest cohesive energy compared with pure Sn and IMC, which explains the reason why the facture always occurs at the pure Sn instead of the interface. The experiments have been conducted to validate this analysis. This study provides a constitutive relation of IMC/Sn interfaces that can be used to other fracture behavior of Pb-free solder joints, such as under mechanical vibration conditions.