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Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).

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The creep behavior of air-cooled and liquid nitrogen-quenched soldered joints of 60/40 Sn-Pb at 65°C has been studied. The stress exponent, n, in the power law, γ˙ (steady state strain rate) ∝σn (applied stress), changes from a value of about 6 to values of 2 to 3, as γ˙ decreases below 10−4 in/in per second. This result, combined with the authors’ previous stepped load creep test results, indicates a transition of the creep deformation mechanism from conventional dislocation climb to superplastic grain boundary sliding. The superplastic creep of the soldered joints is ascribed to their non-lamellar microstructure due to their fast cooling rate. During creep deformation, recrystallization of the soldered joints occurs, causing softening.