Effect of Solder Joint Degradation on RF Impedance

The trend for many types of electronic products is toward higher operating frequencies or digital bit rates. At high frequencies, signal propagation is concentrated at the surface of interconnects, a phenomenon known as the skin effect. Degradation of interconnects, such as cracking of solder joints due to fatigue or shock loading, also usually initiates at the surface and propagates inward. Therefore, even a small crack at the surface of a solder joint may change RF impedance and adversely affects the performance of high speed electronic circuits. Traditional methods used to monitor interconnect reliability are based on measurement of DC resistance. More accurate and sensitive alternatives are required for monitoring the reliability of current and future electronic products. RF impedance analysis offers an improved means of sensing interconnect degradation. 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