Microstructural evolution during electromigration in eutectic SnAg solder bumps

Abstract

Microstructural changes induced, by electromigration were studied in eutectic SnAg solder bumps jointed to under-bump metallization (UBM) of Ti/Cr-Cu/Cu and pad metallization of Cu/Ni/Au. Intermetallic compounds (IMCs) and phase transformations were observed during a current stress of 1 X 104 A/cm(2) at 150 degrees C. On the cathode/ substrate side, some of the (Cu-y,Ni1-y)(6)Sn-5 transformed into (Ni-x,Cu1-x)(3)Sn-4 due to depletion of Cu atoms caused by the electron flow. It is found that both the cathode/ chip and anode/chip ends could be failure sites. On the cathode/chip side, the UBM dissolved after current stressing for 22 h, and failure may occur due to depletion of solder. On the anode/chip side, a large amount of (Cu-y,Ni1-y)(6)Sn-5 or (Ni-x,Cu1-x)(3)Sn-4 IMCs grew at the low-current-density area due to the migration of Ni and Cu atoms from the substrate side, which may be responsible for the electromigration failure at this end.