Many types of electronic products are now operating at higher 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 the 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 an interconnect may change RF impedance and adversely affect the performance of high speed electronic circuits. Traditional methods used to monitor interconnect reliability are based on a measurement of DC resistance. But, 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. During the test, the RF impedance provided detectable failure precursors by the sequential probability ratio test (SPRT), while the DC resistance remained constant with no precursors. Failure analysis revealed that the change in RF impedance resulted from a physical crack that initiated at the surface of the solder joint and propagated only part of the way across the solder joint. These test results indicate that the combination of RF impedance and the SPRT can serve as a non-destructive early indicator of solder joint degradation.
Index Terms: Impedance, Interconnection, Prognostics, Reliability, SPRT, Time domain analysis
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