Realistic service conditions of electronic assemblies tend to involve significant variations in loading. However, reliability prediction of solder joints relies on conducting accelerated tests with fixed amplitudes and extrapolating results to service conditions. This can be very misleading without proper constitutive relations and without understanding the interacting effects of cycling parameters under realistic service conditions. In this work, the effects of amplitude variations and strain rate on solder joint fatigue life are studied. Individual SnAgCu solder joints with two different Ag contents (SAC305 and SAC105) were tested in low cycling shear fatigue under single and varying stress amplitudes with different strain rates. Work accumulation and the evolution of solder deformation properties during cycling were measured. The results showed that cycling with a higher strain rate at fixed stress causes less damage per cycle and thus longer fatigue life. Alternating between high and low stress at a fixed strain rate leads to damage acceleration and rapid failure. In addition, alternating between low stress at a high strain rate and high stress at a low strain rate leads to ongoing increases in the rate of damage at the mild amplitude and thus relatively rapid failure. In comparing SAC305 with SAC105, SAC305 is more fatigue resistant than SAC105 in single and varying amplitude cycling. However, the effect of strain rate on both alloys is almost the same.