Prognostics of Interconnect Degradation using RF Impedance Monitoring and Sequential Probability Ratio Test

For electronic products, interconnect failures may occur due to mechanisms such as fatigue, creep, corrosion, and mechanical over-stress. Regardless of the failure mechanism, interconnect degradation often starts at a 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. This paper presents a prognostic technique to detect interconnect failure precursors using RF impedance monitoring and the sequential probability ratio test (SPRT) under thermomechanical loading conditions. Creep tests were conducted with a test vehicle including an impedance-controlled circuit board, a surface-mount low-pass filter, and two solder joints providing both mechanical and electrical connections between them. Constant mechanical loading was directly applied to the filter at an elevated temperature in order to generate creep failures of the solder joints. During solder joint degradation, the RF impedance provided detectable failure precursors by the sequential probability ratio test, while the DC resistance remained constant with no precursors prior to the generation of an open circuit. Failure analysis of a partially degraded solder joint revealed that the change in RF impedance resulted from a partial crack that initiated at the surface of the solder joint and propagated only part of the way across the solder joint. These test results indicate that the combination of RF impedance and SPRT can provide a non-destructive and real-time means to detect solder joint degradation.