Thermomigration and electromigration in Sn58Bi ball grid array solder joints

In the present study, individual effect of thermomigration (TM) and combined effects of TM and electromigration (EM) in Sn58Bi ball grid array (BGA) solder joints were investigated using a particular designed daisy chain supplied with 2.5 A direct current (DC) at 110 °C. Driven by the electric current, Bi atoms migrated towards the anode side and formed a Bi-rich layer therein. With a thermal gradient, Bi atoms tended to accumulated at the low temperature side. When the effects of TM and EM were in same direction, TM assisted EM in the migration of Bi, otherwise it counteracted the effect of EM. The effect of electron charge swirling were detected when the electric current passed by the Cu trace on the top of the solder bump instead of entering into it. For the joint without current passing by or passing through, only TM induced the migration of the Bi atoms.