Decrease in fatigue life of Sn - 3.8 wt-%Ag - 1.2 wt-%Cu alloy solder joints due to thermal cycling

Copper plates joined with a thin solder layer (60 μm thick) of Sn - 3.8 wt-%Ag - 1.2 wt-% Cu alloy were subjected to heat treatments: a thermal cycling of a temperature range between 321 K and 381 K (Δ T = 60 K) and an isothermal heating at 357 K, and then subjected to a fatigue test at 6 MPa stress amplitude. Solder joints made with a thin solder layer of Sn - Pb eutectic alloy were also examined for comparison. After heat treatments, the η phase developed and dispersed at the bonding interface of the solder joints with increasing numbers of thermal cycling and with increasing time of isothermal heating. Small voids also appeared in the η phase after heat treatments. Fine cracks appeared in the η phase after thermal cycling for 2000 cycles and higher, but no cracks were observed after isothermal heating. There was no large difference in fatigue lifetime after thermal cycling between Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints. The fatigue lifetime of Sn - Ag - Cu alloy solder joints and Sn - Pb eutectic alloy solder joints was 2 - 3 × 105 with no thermal cycling and was greatly reduced to 0.1 - 0.6 × 10⁵ after 8000 thermal cycles. The fatigue lifetime was also decreased to 0.6 - 1.0 × 10⁵ after isothermal heating for 16 000 min, but the decrease in fatigue lifetime was gradual compared to that after thermal cycling. The decrease in fatigue lifetime after smaller numbers of thermal cycles is explained by coarsening of the η phase, and the large decrease in fatigue lifetime after a large number of thermal cycles is explained by the appearance of cracks in the η phase during thermal cycling.