Effect of electromigration on interfacial reaction in Ni/Sn3.0Ag0.5Cu/Au/Pd/Ni-P flip chip solder joints

The effect of electromigration (EM) on the interfacial reaction in the Ni/Sn3.0Ag0.5Cu/Au/Pd/Ni-P flip chip solder joint is investigated under a current density of 1.0× 104 A/cm2 at 150℃. The (Cu,Ni)6Sn5 intermetallic compounds (IMCs) form at both solder/Ni and solder/Ni-P interfaces in the as-reflowed state. During aging at 150℃, the (Cu,Ni)6Sn5 interfacial IMCs grow thicker and transform into (Ni,Cu)3Sn4 type after 200 h at solder/Ni interface and 600 h at solder/Ni-P interface, respectively. During EM, the current direction plays an important role in Ni-P layer consumption. When electrons flow from Ni-P to Ni, EM enhances the consumption of Ni-P, i.e., the Ni-P s completely consumed and transforms into Ni2SnP after EM for 600 h. There is no Cu-Sn-Ni ternary IMC at the solder/Ni-P interface (cathode). Crack forms at the Ni2SnP/Cu interface due to the weak bonding force between Ni2SnP and Cu. When electrons flow from Ni to Ni-P, no obvious consumption of Ni-P is observed during EM; the current crowding effect induces a rapid and localized dissolution of Ni UBM and Cu pad at the chip side (cathode). The dissolved Ni and Cu atoms are driven along the flowing direction of electrons and form a large number of IMC particles in the solder matrix. During EM, the (Au,Pd,Ni)Sn4 phase prefers to be redistributed only at the anode interface, regardless of the direction of electron flow.