Electromigration and thermomigration in Sn3Ag0.5Cu solder joints

The combined effects of thermomigration (TM) and electromigration (EM) were investigated in Cu/Sn3.0Ag0.5Cu/Ni solder joints by applying a 5×10³A/cm² DC current to the joints. With a thermal gradient, the Cu atoms and Ni atoms were found to migrate towards the lower temperature side while the Sn atoms were found to migrate towards the higher temperature side. For the solder joint stressed with a 5×10³A/cm² DC current, TM was found to play a dominant effect on the migration of Cu atoms in the solder because the Cu atomic flux in the solder which was induced by EM was smaller than that was induced by TM. The Ni atomic flux in the solder that was induced by TM was smaller than that induced by EM, but it could not be ignored because the values were the same order of magnitude. The Sn atomic flux induced by TM could be ignored due to its smaller magnitude than that induced by EM. The Cu₃Sn IMC layer and Cu₆Sn₅ IMC layer at the Cu/solder interface in the joint with Cu as the anode were thicker than corresponding layers at the Cu/solder interface in the joint with Cu as the cathode. The IMC at the Ni/solder interface of joint with Cu as the anode was the Ni₃Sn₄ phase while that in the solder joint with Cu as the anode was the (Cu,Ni)₆Sn₅, and the former was thicker than the latter. EM and TM induced dissolution of the Ni₃Sn₄ IMC into the solder was found to be the main cause of the formation of voids at the Ni₃Sn₄/solder interface from a joint that used Cu as the anode. For the solder joint that used Cu as the anode, the Ni₃Sn₄ IMC layer at the Ni/solder interface was as thick as the Cu-Sn IMC layer at the Cu/solder interface. By contrast, for the solder joint that used Cu as the cathode, the IMC layer at the Ni/solder interface was thinner than that at the Cu/solder interf- - ace.