Use of the Skin Effect for Detection of Interconnect Degradation

Measurements based on DC resistance have traditionally been used to monitor the reliability of electronic products. Unfortunately, DC resistance is not useful for detecting intermediate stages between a short and an open, such as a partially degraded interconnect. Under cyclic loading conditions, interconnect degradation is caused by fatigue cracking, which often initiates at the surface where the strain range is maximized. At high operating frequencies, signal propagation is concentrated at the circumferential region of an interconnect due to the skin effect. Therefore, RF impedance analysis offers a more sensitive means of detecting interconnect degradation than DC resistance. The skin effect also has implications for the reliability of electronics used in applications such as radar and telecommunications. The use of higher frequencies will make these circuits increasingly susceptible to performance degradation as a result of small cracks or deformation that would go unnoticed in lower frequency applications. This study demonstrates applications of the skin effect to detect interconnect degradation. Mechanical fatigue tests have been conducted with an impedance-controlled circuit board on which a surface mount component was soldered. During solder joint degradation, simultaneous measurements were performed of DC resistance and the time domain reflectometry (TDR) reflection coefficient as a measure of RF impedance. Two TDR reflection coefficients with different frequency ranges were monitored to evaluate the effect of frequency range on the sensitivity of RF impedance to mechanical degradation. The TDR reflection coefficients were consistently observed to increase in response to early stages of solder joint cracking, while the DC resistance remained constant until the solder joint was completely separated. The TDR reflection coefficient measured over a higher frequency range responded earlier than one with a lower frequency range. This demonstrates that as signal frequencies increase, smaller cracks are capable of producing detectable amounts of signal integrity degradation.