The solderability of a nickel-palladium-gold (Ni-Pd-Au) finish on a Cu substrate was evaluated for the Pb-free solder, 95.5Sn-3.9Ag-0.6 Cu (wt.%, abbreviated Sn-Ag-Cu) and the eutectic 63Sn-37 Pb (Sn-Pb) alloy. The solder temperature was 245ºC. The flux was a rosin-based mildly activated (RMA) solution. The Ni-Pd-Au finish was tested in the as-fabricated condition as well as after exposure to one of the following accelerated storage (shelf life) regiments: (1) 33.6, 67.2, or 336 hours in the Battelle Class 2 flowing gas environment or (2) 5, 16, or 24 hours of steam aging (88ºC, 90%RH). The solderability metrics were the contact angle, theta C, and the wetting rate, as determined by the meniscometer/wetting balance technique. Auger electron spectroscopy (AES) surface and depth profile analyses were used to assess surface contamination and layer interdiffusion, respectively, as a function of the accelerated test conditions. The Ni-Pd-Au finish significantly improved the solderability of both the Sn-Ag-Cu and Sn-Pb solders by reducing the contact angles when compared to bare Cu. The Battelle Class 2 exposures caused only slight increases of contact angle values for either solder alloy. Exposure to the three steam aging times caused no significant change to the contact angles. The wetting rate of the Sn-Ag-Cu solder decreased only slightly with Battelle Class 2 exposure time. However, the wetting rate of the Sn-Pb solder exhibited a significant drop-off between 33.6 and 67.2 hours, to values similar to those of the Pb-free solder. The steam aging environment did not cause a significant change to the wetting rates of either solder. Exposure to the Battelle Class 2 or steam aging condition brought the low, as-fabricated surface C concentrations to levels commensurate with longterm storage in an air environment. Copper and Fe contaminants were observed on the surface, but at concentrations that were too low to impact solderability. A portion of the Ni layer diffused into the Pd layer to a concentration of 4 at.%. However, Ni diffusion was halted from reaching the Au layer to any appreciable degree by the Au-Pd interface reaction layer. In terms of printed wiring assembly applications, this Ni-Pd-Au finish is particularly well suited for enhancing the solderability of the Pb-free Sn-Ag-Cu alloy. The improved solderability is relatively insensitive to storage time as predicted by either the Battelle Class 2 or steam aging environments, implying that this Ni-Pd-Au finish can provide a relatively wide process window for the Pb-free solders. That process window is only slightly more limited for the Sn-Pb baseline solder.
Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
Key words: Ni-Pd-Au, Storage Environments, Solderability, Pb-free