Low-Frequency, High-Temperature Low Cycle Fatigue of 60Sn-40Pb Solder

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Abstract
Solder thermal fatigue is a critical problem for large surface mounted leadless chip carriers. This fatigue results from the thermal mismatch between the Al2O3 chip carrier and the circuit board. This study was aimed at this problem, particularly the influence of time-dependent effects.Isothermal tests were run on 0.15 to 0.23 mm (0.006 to 0.009 in.) solder layers tested in simple shear. It has been found, as expected, that solder exhibits Coffin-Manson low-cycle fatigue (LCF) behavior. The present study details the influence of cycling frequency on the fatigue life. Results were obtained on 60Sn-40Pb tested at 35 and 150°C. In agreement with previous studies made on Pb, it is shown that, at both temperatures, the fatigue life decreases as the frequency decreases, especially below 10−3 Hz. The fatigue life can be described by Coffin's frequency modified LCF law; i.e., (Nfνκ−1)αΔγp=θThe data were also analyzed by means of strain range partitioning (SRP). It was found that this approach does not describe the data. A time-modified SRP approach, i.e., the time to failure tf is given by 1tf=fcctcc+νfppNpp was found to describe the data. The primary difference between this and the standard SRP approach is that with the time-modified SRP we assume that at low frequencies the time to failure tcc is a constant, not the number of cycles to failure Ncc. The correlation between this approach and the frequency-modified fatigue law is discussed. There is also a characterization of the different types of fatigue fracture surfaces that are observed.
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