Diffusion and intermetallics formation between Pd/Ag metallization and Sn/Pb/Ag solder in surface mount solder joints

Interdiffusion and intermetallics formation between metallization conductor Pd–Ag and solder 62Sn-36Pb-2Ag have been studied. Silver and palladium dot mapping images of solder joints demonstrate that the Pd–Ag metallization layer gradually disappears after 11 days of ageing. It is observed that silver-rich areas exist in the bulk of the 62Sn-36Pb-2Ag solder after ageing but palladium-rich areas are not evident. The diffusion coefficient of the silver in the solder joints for this material system was measured. The activation energy and pre-exponential factor for the silver diffusion were found to be about 0.475 eV and 0.56×10-10 m2 s−1, respectively for the configuration of surface mount thick film solder joints studied. X-ray diffraction results reveal the formation of the intermetallic compounds Ag5Sn, Ag3Sn, Pd3Sn2, Pd2Sn, PdSn2, PdSn4, PdSn, PbPd3, and Pb3Pd5.