Computational Parametric Study on the Strain Hardening Effect of Lead-free Solder Joints in Board Level Mechanical Drop Tests

Many lead-free solder alloys have been proposed as alternatives to the conventional eutectic Sn-37wt% Pb (SnPb). This replacement induces the drop/impact reliability issues of portable products. Under the drop/impact loading, the main failure mode is interfacial brittle failure between lead-free solder joints and intermetallic compound (IMC) layer. The identification of failure mechanism in the solder joint under drop test becomes a very critical issue. In this paper, a finite element analysis of BGA assembly with lead-free solder joints is conducted to investigate IMC interfacial stresses under repetitive mechanical drops. For the two drop conditions of concern, the dominant strain rates for the solder joints are studied. The results indicate that the stress on the PCB side is larger than that on the package side in both the solder and the IMC. After multiple drops, the stress in the IMC gradually increases due to the strain hardening effect of the solder. During the repetitive drop tests, the increasing stress will eventually exceed the strength of IMC and leads to the brittle fracture of solder joint. The findings in the present study result in a better understanding in the failure mechanism of solder joint under repetitive mechanical drop tests.