Diffusion and Electromigration of Impurities in Lead-Based Solders

Studies of diffusion and electromigration of Ni in Pb-Sn-Ni ternary alloys and of Cu and Ni in Pb-In alloys, the materials of solders used in VLSI industry, have been carried out by the standard sequential-sectioning technique and steady state method, respectively. From a simple trapping approach, it was found that the binding energy of Ni with Ni and with In are of the order of 0.17 eV and 1.1 eV, respectively. The good agreement of the theory with the experiments implied that one Sn atom trapped only one atom of Ni. The measurement of chemical diffusivity of Ni in Pb proved that the thermodynamic factor had a great influence on the Z* value of Ni in Pb-Ni alloys. The thermodynamic factor of Sn in Pb was determined to be about 1 at 0% Sn and 1.6 at 8.4% Sn. The electromigration measurements indicated that the addition of In (1% In) had almost no influence on Z* value of Cu in Pb, but cuased a large increase (4 times) on that of Ni in Pb at lower temperatures (200(DEGREES)C). A Monte Carlo simulation of the diffusion of silver in lead -indium alloys ruled out a local enhancement mechanism in this system.