The effect of cooling rate on the thermo-mechanical properties of micro-alloyed solders

In this paper, the effect of reflow cooling rate on the thermomechanical properties of a traditional SAC (Sn96.5/Ag3/Cu0.5) and of a micro-alloyed SAC (mSAC) solder (Sn98.9/Ag0.3/Cu0.7/Bi0.1/Sb0.01) was investigated. An FR4 based testboard was designed for the investigation; and preformed solder balls (0 600 μm) were soldered onto it with different cooling rates (1, 1.8, 2.8 K/s). After the soldering, the samples were subjected to Thermal Shock (TS) test (-40 to 140 °C, 30 min. cycle time) up to 2500 cycles. The thermomechanical properties of the samples were investigated by measuring the shear strength of the solder bumps, and by analysing the intermetallic layer growth on aged samples as well as on as-reflowed samples. Based on the shear strength measurement, the characteristic life of the samples was calculated. It was found that the characteristic life of the micro-alloyed solder is only slightly longer at faster cooling rates. On the contrary, the characteristic life of the micro-alloyed solder is considerably longer than that of the traditional alloy at the slowest cooling rate. Furthermore, the growth of Cu 6 Sn 5 intermetallic layer was faster in the case of the traditional alloy because of the larger amount of grain boundaries; however, the growth of Cu 3 Sn was larger at the micro-alloyed solder because of the higher number of valleys between the grains of the Cu 6 Sn 5 layer.