Identification of interconnect failure mechanisms using RF impedance analysis

This paper presents RF impedance analysis as a non-destructive indicator of interconnect failure mechanisms. Physical changes on a printed circuit board can be detected using the TDR reflection coefficient as a measure of RF impedance. The test circuit in this study consisted of an impedance-controlled circuit board, a surface mount low-pass filter, and two solder joints providing both mechanical and electrical connections between them. A cyclic mechanical load was applied to generate either solder pad separation failures or cracking of the solder joints. In the case of pad separation, the test results showed that the TDR reflection coefficient gradually decreased as the pad lifted off the circuit board, which was distinguishable from the TDR response due to solder joint cracking. These results are explained using a simple theoretical analysis of the effect of each failure mechanism on impedance. This demonstrates that RF impedance analysis provides for the determination of failure mechanisms as well as early detection of both pad separation and solder joint degradation. The detection sensitivity for solder joint cracking using this technique is also discussed.