Detection of solder joint failure precursors on tin-lead and lead-free assemblies using RF impedance analysis

During the lifetime of electronic products, interconnects are susceptible to failures by mechanisms such as fatigue, creep, corrosion, and mechanical over-stress. Regardless of the failure mechanisms, interconnect degradation often starts from the surface and propagates inward. DC resistance, which has been used by the electronics industry to monitor the reliability of board level interconnects, does not offer an adequate means to predict an impending failure. However, RF impedance does respond to the early stages of interconnect degradation due to the skin effect, and thus can provide a failure precursor for an interconnect. In this study, we compared early changes in RF impedance to changes in DC resistance to identify solder joint failure precursors under creep testing conditions. We report the effect of different solder alloys on RF impedance. The test vehicle consisted of an impedance-controlled circuit board, a surface-mount low-pass filter, and two solder joints providing both mechanical and electrical connection between them. The solder alloys under investigation were eutectic tin-lead (Sn- 37Pb) and SAC305 (Sn-3.0Ag-0.5Cu). Constant mechanical load was directly applied to the filter at an elevated temperature in order to generate creep failures of the solder joints. During solder joint degradation, RF impedance and DC resistance were simultaneously monitored in order to allow a direct comparison between their respective sensitivities in detecting failure precursors before the solder joint showed a DC open circuit. The test results showed that regardless of solder alloy RF impedance failure precursors were detectable prior to changes in DC resistance during solder joint degradation. This demonstrates that RF impedance can serve as a nondestructive and real-time degradation indicator of interconnects and can predict impending failures.