Comparative study between Au/Pd/Cu and Au/Pd(P)/Cu films in soldering applications

A comparative study between Au/Pd/Cu and Au/Pd(2-5 wt% P)/Cu films in soldering applications was carried out by means of field-emission scanning electron microscopy (FE-SEM), field-emission electron probe X-ray microanalysis (FE-EPMA), ultraviolet/visible/near infrared (UV/VIS/NIR) spectrophotometry, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and high-speed ball shear (HSBS) test. We investigated the interfacial microstructure via FE-SEM and FE-EPMA, the mechanical properties of solder joints via HSBS test, and the light properties and the microstructure of the exterior surfaces of solder bumps via UV/VIS/NIR spectrophotometry, XPS, and TEM. The interfacial microstructures/mechanical properties (with Cu pads) were approximately independent of the P content in the Pd(P) films. However, the discoloration in the solder bumps resulting from the formation of a SnO2 nanolayer (or Au/Pd/Cu) could be greatly improved by replacing it with a Sn-O-P phase through a minor addition of P into the Pd film. This suggested that the Au/Pd(P)/Cu pads enabled an appropriate solderability. The results of the above investigations indicated that the dissolved P from the Pd(P) film during soldering played a significant role in determining the light properties of solder joints, even though the Pd(P) film was less than one micron thick and its P content was quite limited (2-5 wt%).