Fatigue investigation of lap shear solder joints using resistance spectroscopy

The electrical resistance of lap shear (60%Sn-40%Pb) solder joints has been measured while the specimens were undergoing cyclic fatigue testing. The resistance measuring system had a noise level of less than one nano-ohm. An FFT analyzer was used to transform the time varying resistance to the frequency domain. Measurements were made of the average resistance (zero frequency amplitude), amplitude of the resistance change at the mechanical drive frequency, and the amplitudes of the resistance change at the second and third harmonics of the drive frequency. The average resistance and resistance amplitude at the drive frequency exhibited a systematic behavior as the specimens were cycled toward failure. Initially the average resistance increased, followed by a much stronger decrease, and lastly increased markedly as the specimen fractured. The decreasing resistance portion of this signature which occurs prior to mechanical failure (i.e. crack propagation) can be used to detect failure in solder joints much sooner than present techniques