The present invention relates to electroless nickel plating, more particularly to the electroless nickel plating of surfaces such as copper or fused tungsten, and still more particularly to the activation of, e.g., copper or fused tungsten surfaces for receipt of electroless nickel.
In many applications of the fabrication of printed circuits having conductive copper circuity paths and areas, as well as in applications of fused tungsten circuitry on ceramic substrates, electroless nickel is desired to be plated over the copper or over the fused tungsten. In printed circuits, for example, electroless nickel plating over copper is often used to form a diffusion barrier for a subsequently-applied gold plating, while in tungsten circuitry electroless nickel plating is used, e.g., to provide more readily solderable surfaces.
A wide variety of electroless nickel plating baths are available, comprised generally of aqueous solutions containing a source of nickel ions, a reducing agent for the nickel and a complexing agent, capable of operating over predetermined ranges of pH, with the most common such baths being based upon hypophosphite reducing agents. In order to electrolessly plate nickel from such baths onto copper surfaces (e.g., in printed circuit boards), it is necessary to first activate the copper surfaces. The most common means for effecting this activation is by contact of the copper surfaces with species catalytic to deposition, typically colloidal sols or solutions of palladium-tin (see, e.g., the one-step catalyst compositions of U.S. Pat. Nos. 3,011,920 and 3,532,518, requiring an acceleration step, and the one-step catalyst compositions of U.S. Pat. No. 4,863,758, which do not require acceleration). This same method is used to activate tungsten circuitry surfaces for subsequent electroless nickel plating.
For copper surfaces, it is also known to activate them by application of a prestrike layer of nickel from an electroless bath utilizing a boron compound, e.g., dimethyl amine borane or borohydride, as the reducing agent, which prestrike layer then serves to catalyze electroless nickel deposition from a more conventional bath (e.g., hypophosphite-reduced).
The palladium-based activators for electroless nickel plating of copper or fused tungsten surfaces are relatively expensive and must be maintained at carefully controlled concentrations in order to be effective. More problematic is the fact that for printed circuits having copper surfaces which are to be activated for electroless plating in this manner, the procedure involves immersion of the entire circuit board, including insulating areas, into the palladium-containing sol or solution. Although post-rinsing procedures are employed, the catalytic species may nevertheless become entrapped into insulating areas, and will there catalyze electroless nickel plating when the board is immersed in an electroless nickel bath. The resultant unwanted areas or paths of metal on the insulating areas can lead to undesirable cross-talk or shorting between conductive circuitry on the board. This same problem occurs with tungsten-ceramic packages, i.e., the palladium-based activator can become entrapped or adhered to insulating ceramic surfaces and promote undesired metallization thereof.
The same problem occurs with the prestrike method of activation for copper surfaces, i.e., the very activity of the borane or borohydride-reduced electroless nickel bath which enables it to deposit a strike layer of nickel over the copper also can result in undesired deposition of nickel on insulating areas of the board. Also in common with the palladium-based activators, these nickel strike baths are relatively expensive and their composition must be carefully controlled to avoid spontaneous plating of all metal in the solution.