Thermomigration and electromigration in Sn58Bi solder joints

Abstract

Thermomigration (TM) in Sn58Bi solder was detected in Cu/Sn58Bi/Ni solder joints supplied with direct current. Thermal electric finite-element simulation showed that a thermal gradient of 527 °C/cm existed in a solder joint when a current density of 5×103 A/cm2 was applied to the structure at 50 °C. Depending on the direction of the current, TM was found to assist or counteract electromigration (EM) on the diffusion of Bi atoms. The atomic fluxes of Bi induced by EM and TM were estimated separately. EM-enhanced cross interaction between Cu and Ni across the solder joint was also detected, which led to the intermetallic compound (IMC) at Ni side or the Cu side of the joint that electron flow was from the Cu side to the Ni side is thinner than that of corresponding IMC in the joint stressed with current in opposite direction. For the joint as reflowed, the IMC at the Ni side was (Cu,Ni)6Sn5 instead of Ni3Sn4 and the IMC at the Cu side was Cu6Sn5 without Ni atoms contained. When Ni wire set as anode, the IMC at the Ni side was still (Cu,Ni)6Sn5 even after the joint was stressed for 384 h. However, it turned into (Ni,Cu)3Sn4 if Ni wire set as cathode. Regardless of the directions of the electric current, the IMC at the Cu side was Cu6Sn5 at all time. For the joint that Cu wire set as anode, the growth of Ni3Sn4 at the cathode side was enhanced by EM and TM, while the growth of Cu6Sn5 at the anode side was retarded.