Degradation detecting of solder joints by time domain reflectometry technology

Solder joints on the Printed-Circuit board (PCB) serve as mechanical and electrical connections. Their function maybe simple, but the reliability of the solder joints may affect the entire device or system. They are usually the weak links when the electronic products are exposed to a variety of load conditions from their operation and storage, such as temperature cycling, vibration, shock and humidity. One of the most important parameters of solder joints is electrical stability. The resistance or impedance will rise as the fracture spreads through the solder joints. The reliability of solder joints is often determined through continuous DC resistance measurement, but it not very sensitive. In recent years, Time Domain Reflectometry (TDR) technique has been found to be a better alternative that are required for monitoring the degradation of interconnects. 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. A physical crack initiated at the surface of the solder joints results in the increase of RF impedance. Thus, the crack length should be correlative with RF impedance or Reflection Coefficient, and the failure time of the solder joint could be estimated in view of electronic theory and the theory of fracture. A mathematic model was proposed to predict the relationship between the time domain reflection coefficient of solder joint and the crack length in it. Impedance-controlled circuit boards on which surface mount components were soldered have been developed. Partially and entirely cracked solder joints were produced for test in advance. The RF impedance was observed to increase in response to partial cracking of the solder joint, while the DC re- istance did not indicate any anomalies. The technique of RF impedance analysis shows potential as a prognostic tool that can provide advanced warning of impending failures of solder joint.