Small amount of rare earth element La were added into the traditional Sn60-Pb40 solder alloy in order to improve the reliability of solder joints. The results of aging tests of solder joints showed that, the growth of interface Cu6Sn5 intermetallic compound was depressed by La addition. Furthermore, the thermal fatigue life of solder joints was improved by 3 times. Thermodynamic analysis indicated that La had higher affinity with Sn in the Sn-Pb-La system, and then the driving force for Cu6Sn5formation was reduced since the activity of Sn was lowered.
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