Effect of La on the Cu–Sn intermetallic compound (IMC) growth and solder joint reliability

By adding a small amount of the rare earth element La (only 0.05 wt.%) into the traditional Sn60–Pb40 solder alloy, the growth of the Cu–Sn intermetallic compound (IMC) at the interface of solder joints has been depressed. Furthermore, the thermal fatigue life of simulated surface mount solder joints was increased by a factor of 3. Thermodynamic analysis showed that, since La has a higher affinity to Sn in the Sn–Pb system, small amounts of La addition will reduce the driving force for Cu–Sn IMC formation in a limited mole fraction range.