Effect of La on the Sn-rich halo formation in Sn60-Pb40 alloy

Thermodynamic models and eutectic growth theories have been used to investi gate the effect of La on the Sn-rich halo formation in Sn60-Pb40 solder all oy. SEM observation showed that, by adding a small amount of the rare earth element La, the formation of the coarse Sn-rich halo surrounding the prima ry Pb-rich phase was depressed, and a homogeneous microstructure was obtain ed. Thermodynamic analysis showed that La has a higher affinity to Sn in th e Sn-Pb system. This will lead to a variation of the Sn-rich phase contact angle at the three-phase junction of the solid/liquid interface during the eutectic solidification process. As a result, the eutectic can directly gro w from the primary Pb-rich phase and the formation of the Sn-rich halo is d epressed. (C) 2001 Elsevier Science B.V. All rights reserved.