Atomic migration on Cu in Sn-58Bi solder from the interaction between electromigration and thermomigration

Abstract

This paper mainly studied the atomic migration in molten Sn-58Bi solder joints induced by the thermo-electric coupling effect. Individual electromigration (EM), individual thermomigration (TM) and the coupling EM with TM were conducted to confirm the prominent diffusing species and its migrating behaviors in the solder, respectively. During EM, step loading with different current densities was designed to verify that there existed a transition on the prominent diffusing species when Sn-58Bi solder melted. Cu atoms from the cathode were driven towards the anode to produce thicker Cu6Sn5 intermetallic compound (IMC) layer at the anode interface under current stressing. During TM, it was found that the main migrating species was also Cu atoms which were driven towards the cold end to form thicker Cu6Sn5 IMC layer at the cold interface under temperature gradient. Finally, thermo-electric coupling was realized by superimposing TM on EM with current density of 1.0 × 104 A cm−2. TM played the assisting or counteracting effect on the Cu migration occurred in EM when the anode was connected with cold end or hot end. With anode on hot end, TM played the reverse effect on the migration of Cu atoms, which prevented Cu atoms to be accumulated at the anode/hot interface. With anode on cold end, TM played the accumulative effect on the migration of Cu atoms, which led to a thicker IMC layer at the anode/cold interface.