Electromigration of eutectic SnPb and SnAg3.8Cu0.7 flip chip solder bumps and under-bump metallization

Abstract

The electromigration damage in flip chip solder bumps of eutectic SnPb and SnAg3.8Cu0.7 was studied after current stressing at 120 °C with 1.5 A. The diameter of the bumps was about 200 μm. The under-bump metallization (UBM) on the chip side was electroless Ni and on the board side was electroplated Cu. Surface marker motion was used to measure the atomic flux driven by electromigration and to calculate the effective charge number, Z*, of the solder. For eutectic SnPb, Z* is about 36–100 after 39.5 h of electromigration, but for SnAg3.8Cu0.7 the marker movement was too small to measure Z* even after 200 h of current stressing. While the effect of electromigration in SnAg3.8Cu0.7 is much smaller than that in eutectic SnPb, hillocks of intermetallic compounds at the anode have been observed in the former. An extensive growth of Ni–Cu–Sn intermetallic compounds in the matrix of both solder bumps was found. A localized dissolution of electroless Ni UBM was observed under current stressing, yet no dissolution of the Cu UBM by electromigration was found.