Effect of Solder Joint Degradation on RF Impedance

This study demonstrates the value of RF impedance measurements as an early indicator of physical degradation of solder joints compared to DC resistance measurements. Mechanical fatigue tests have been conducted with an impedance-controlled circuit board on which a surface mount component was soldered. Simultaneous measurements were performed of DC resistance and the time domain reflection coefficient, as a measure of RF impedance, while the solder joints were stressed. The RF impedance was observed to increase in response to cracking of the solder joint while DC resistance remained constant. Failure analysis revealed that the RF impedance increase resulted from a physical crack initiated at the surface of the solder joint, which had propagated only partway across the solder joint.