Effect of electromigration on intermetallic compound formation in Pb-free solder-Cu interfaces

The polarity effect of electromigration on intermetallic compound (IMC) formation at the cathode and the anode in solder lines has been investigated. The lines were formed by flowing molten lead-free solder SnAg_3.8Cu_0.7 into V-grooves etched on [001] silicon wafers, and two copper wires were used as electrodes. The V-groove solder line samples, with width of about 100 μm and length of 600-800 μm, were used to study the changes in thickness and morphology of IMC forming at the cathode and the anode in the SnAg_3.8Cu_0.7/Cu system under different current density and temperature settings. The current densities were in the range of 10³ to 10⁴ A/cm² and the temperature settings were 150°C and 180°C. We found that the same IMCs of Cu₆Sn₅ and Cu₃Sn formed at solder/Cu interfaces with or without the passage of electric current. IMC growth has been enhanced by electric current at the anode and inhibited at the cathode, compared with samples without applied current. With a high current density of 10⁴ A/cm², IMC growth at the anode obeys the parabolic growth rule, which was not predicted previously. We observed that the IMC forming after the initial reflow had a scallop-type morphology, and it transformed into layer-type morphology in current stressing. This is similar to the result of solid state aging without passage of current. However, how it transforms from scallop-type into layer-type morphology is different from the cathode to the anode, which is also different from the way it goes in solid state aging. This is because of the polarity effect of electric current and the existence of local current crowding due to the scallop-type morphology of the IMC.