Detection of solder joint degradation using RF impedance analysis

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 affect the performance of high speed electronic assemblies. Traditional DC resistance measurements are not appropriate for detecting such a small fault. 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 use of RF impedance changes as an early indicator of physical degradation of solder joints, due to the skin effect, and compares this to DC resistance measurements. Mechanical shear tests at an elevated temperature 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 earlier than the DC resistance. These results were qualitatively repeatable over multiple trials.