Effect of electromigration on the Cu-Ni cross-interaction in line-type Cu/Sn/Ni interconnect

The line-type Cu/Sn/Ni interconnects were used to determine the effect of electromigration (EM) on the Cu-Ni cross-interaction under the current density of 1.0×10⁴ A/cm² at 150 °C for 100 h and 200 h. For the purpose of comparison, the line-type Cu/Sn/Ni interconnects were also aged at 150 °C for 100 h and 200 h. After soldering, Ni₃Sn₄ and Cu₆Sn₅ IMCs formed at the Sn/Ni and Sn/Cu interfaces, respectively. No cross-interaction occurred during soldering. The Cu concentration in the IMCs at the Sn/Ni interface increased with the increasing aging time, and the original Ni₃Sn₄ IMC at the Sn/Ni interface transformed into (Cu_0.56Ni_0.44)₆Sn₅ IMC after being aged at 150 °C for 200 h; while the IMC at the Sn/Cu interface remained Cu₆Sn₅ even after being aged at 150 °C for 200 h. A thermal-electric finite element simulation showed that no thermomigration (TM) effect occurred. During EM, the direction of electric current played an important role. When Cu atoms were under downwind diffusion and Ni atoms were under upwind diffusion (electrons flowed from Cu side to Ni side), more Cu atoms were driven to the opposite side than that of the aging case. The original interfacial Ni₃Sn₄ IMC at the Sn/Ni interface (anode side) had already transformed into (Cu_0.53Ni_0.47)₆Sn₅ after EM at 150 °C for 100 h, which was faster than the aging case. After EM for 200 h, (Cu_0.60Ni_0.40)₆Sn₅ IMC layer formed at the Sn/Ni interface. When Cu atoms were under upwind diffusion and Ni atoms were under downwind diffusion (electrons flowed from Ni side to the Cu side), Cu atom diffusion was retarded, while Ni atom diffusion was enhanced. More Ni atoms were dissolved into the Sn. The interface betwe- - en Ni and Sn became uneven after EM for 100 h and became rougher after EM for 200 h. Large Ni₃Sn₄ particles precipitated in the Sn near the Sn/Ni interface (cathode side). The size of Ni₃Sn₄ particles that precipitated in the Sn increased with the increasing EM time. Cu atoms were very sensitive to the direction of electron flow. When Cu atoms were under downwind diffusion, more Cu atoms could diffuse across the solder to the Ni side and alter the interfacial IMC type. When Cu atoms were under upwind diffusion, few Cu atoms could diffuse across the solder to the Ni side and no interfacial IMC transformation occurred at the Ni side. Regardless of electron flow direction, few Ni atoms could diffuse across the solder to the Cu side due to its high diffusion activation energy.