An improved method of selective plating of metals and possibly other materials involves the use of directed high-intensity acoustic beams. The beams, typically in the ultrasonic frequency range, can be generated by fixed- focus transducers (see figure) or by phased arrays of transducers excited, variously, by continuous waves, tone bursts, or single pulses. The nonlinear effects produced by these beams are used to alter plating processes in ways that are advantageous.
One of the nonlinear effects is acoustic streaming, which can contribute to selective plating of an object immersed in a plating solution by providing fresh plating solution to the portion of the object at or near the focus of a beam. The combination of acoustic streaming and acoustic- radiation pressure is effective in removing debris and bubbles, which, if allowed to remain, can contaminate the plating material and/or inhibit the plating process. Acoustic streaming can also be used to reduce concentrations and gradients of concentrations of gases (especially hydrogen) in order to prevent the formation of bubbles. Acoustic streaming can be utilized further to counteract effects of localized electric fields and of gradients of concentration of the plating solution that can give rise to undesired components of spatial nonuniformity in the plating process.
Another nonlinear effect is heating of the plating solution in the focal region. The local increase in temperature causes a local increase in the rates of chemical reactions and thus in the rate of deposition of plating material.
As an alternative to the immersion form of selective plating, acoustic streaming can be utilized to create a fountain of plating solution, which strikes a selected small area of a part suspended over a pool of plating solution. Plating occurs only on the area in contact with the plating solution. Whether the immersion or the fountain version of the method is used, the spatial selectivity afforded by the method reduces the need for masking materials, masking processes, and masking devices.
The maskless-plating capability afforded by this method is most applicable to plating applications in which small amounts of excess plating in the areas outside the acoustic-beam focal regions are tolerated or in which plating processes can be reversed to remove this excess plating. An example of such an application is that of a circuit board coated over its entire surface with a thin layer of gold to increase its resistance to corrosion and enhance its solderability. Typically, there is a need for thicker gold plating in specific locations on such a circuit board — especially at connector contact areas or push-button contact points, where there is a need to maintain reliable electrical contacts in the presence of physical wear. The present method makes it possible to plate metal onto the board with spatially varying thickness in a single operation, without masking.
This work was done by Richard C. Oeftering of Glenn Research Center and Charles Denofrio of Alchemitron Corp. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17041.