Effective suppression of electromigration-induced Cu dissolution by using Ag as a barrier layer in lead-free solder joints

Dissolution of Cu metallization induced by electromigration is a serious reliability issue in flip-chip packaging. The passage of electron currents greatly increases the consumption of Cu layer while large amounts of Cu6Sn5 phase form in the solder matrix. In this study we proposed a new approach of using Ag as a barrier layer to retard the electromigration-induced Cu dissolution. Two different interfacial structures Sn/Cu/Ag/Cu and Sn/Ni/Ag/Cu were designed to perform electromigration tests where Cu or Ni could prevent the dissolution of the 1.5 μm Ag layer during reflow soldering. After aging the outer Cu or Ni layers form Cu6Sn5 and (Cu,Ni)6Sn5 layers which are dispersed in the solder matrix by electromigration. Surprisingly thin Ag3Sn phase remained layer-structured. Even after prolonged current stressing, inner Cu substrate was not significantly consumed. Therefore electromigration force could not fully drive Cu atoms across the Ag3Sn layer. These results have clearly demonstrated that Ag served as a very effective barrier layer in the current-induced Cu dissolution.

► A new approach of using Ag as a barrier layer to retard the electromigration-induced Cu dissolution.
► Two kinds of interfacial structures, Sn/Cu/Ag/Cu and Sn/Ni/Ag/Cu, were designed to perform electromigration tests.
► Under current stressing, thin Ag3Sn phase remained layer-structured.
► Electromigration force cannot fully drive Cu atoms across the Ag3Sn layer.
► The resistance to electromigration damage in Sn-based IMC phases: Ag3Sn > (Cu,Ni)6Sn5 > Ni3Sn4 > Cu6Sn5.