Effects of PCB design variations on bend and ATC performance of lead-free solder joints

Abstract

Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to high stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risks in solder joints failures. Memory module type lead-free BGA packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long term reliability of memory module lead-free BGAs. It is believed that the PCB design variations cause dynamic and long term failure discrepancies in the fields. In this study, different pad and trace designs were introduced to evaluate the effects of PCB design variations on the bend and Accelerate Thermal Cycling (ATC) performance of lead-free solder joints. Pad designs with NSMD (Non-solder mask defined), SMD (Solder mask defined), and a unique web design were assembled and tested. Different solder alloys including SAC305, SAC105, and SnPb solders, have been evaluated in this study. Different PCB materials have also been evaluated in the test. Four point monotonic bend tests were performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. SMD pad is shown to have the best bend performance among all other types of designs in this study. In addition, this design also shows improvement in mitigation of PCB pad cratering with lead-free solders. Wide trace width seems to degrade the strength and is not preferred. Same as its superior shock resistance comparing with SAC305, SAC105 solder alloy shows better bend performance. There is no significant improvement of bend performance with Web Design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29% respectively. However, the bend performance of eutectic SnPb was actually improved after aging. ATC tests were performed to investigate the effects of design variations on the long term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.