Time- and temperature-dependent wetting behavior of eutectic SnPb on Cu leadframes plated with Pd/Ni and Au/Pd/Ni thin films

We have studied the wetting behaviors of eutectic SnPb solder on Cu leadframes plated with Pd/Ni and Au/Pd/Ni thin films. For the Pd/Ni, we used two thicknesses of Pd of 760 and 2500 Angstrom to study the thicknesses effect. For the Au/Pd/Ni, we used either immersion Au (50 Angstrom) or acid Au (50 Angstrom) flashed on 1000 Angstrom Pd to study the effect and method of adding Au to the Pd surface. In addition, we investigated the wetting on a PdSn3 compound surface made by plating Sn or eutectic SnPb on Pd/Ni samples and then stripping the Sn or SnPb layer off. In all these samples, we found the wetting behavior to be time and temperature dependent. No stable wetting angles could be observed; the diameter of the solder caps increased with wetting time. Yet rather surprisingly, the diameter decreased with increasing wetting temperature from 200 degrees C to 240 degrees C. The largest cap diameter was observed at the lower temperature of 200 degrees C for Pd/Ni and stripped Pd/Ni, but at the intermediate temperature of 220 degrees C for Au/Pd/Ni. The smallest cap diameter was obtained at the higher temperature of 240 degrees C for all substrates. In examining the interfaces, we found the reaction products to be a ternary Pd-Ni-Sn compound and Ni3Sn4 for all substrates. The ternary compound grains were broken off from the interface, and scattered into the molten solder as soon as the Ni3Sn4 was formed. The latter forms a rather uniform layer consisting of small scallop-type grains on the unreacted Ni. We found that the growth rate of Ni,Sn, is much slower, by a factor of about 10, than that of Cu6Sn5 in the reaction between Cu and eutectic SnPb. (C) 1998 American Institute of Physics. [S0021-8979(98)06814-5]