The plating of metallized patterns on electrically insulating substrates is known as attested to by the existence of numerous polymer metallization techniques. See, for example, U.S. Pat. Nos. 4,374,709; 4,560,445; 4,576,685. In spite of these examples, however, the metal-polymer interfacial chemistry is not well understood. The surfaces of two identically prepared samples of a polymer will be different if stored in different conditions of temperature and humidity. The surface condition will also relate to the molding method, stress, local temperature and humidity during the molding process, as well as the inherent porosity of the polymer. Consequently, the polymer-metal adhesion can vary on seemingly identically prepared polymer products. A machined flat surface, plated immediately after machining, presents an ideal surface for reproducible metallization. However, typically, the molded polymer part on which it is desired to plate will have a curved molded surface making machining impractical. Additionally, such parts are often stored for varying lengths of time prior to plating. It is also known to pre-treat materials, particularly plastic materials, to enhance their receptability of the plating materials. See, for example, U.S. Pat. Nos. 4,748,056; 4,775,449; and published EP Patent Application No. 0 272 420 (application no. 87116339.0). Surface treatment immediately before metallization is essential in order to render all surfaces chemically equivalent and to achieve reproducible results. Pretreatments do work to a greater or lesser degree. For example, fine sand blasting prior to, and/or instead of, chromic acid etching, gives somewhat more uniform results; but, the surface quality of the subsequent copper plates is likely to mirror the "rough" blasted surface. In addition, the sand blasting procedure is difficult to standardize. Pretreatment by air plasma (glow discharge), is also a useful replacement for etching, but, again, reproducibility of plating quality is not assured by plasma treatment alone.
It would, therefore, be an advance in the art to provide a pretreatment that would allow high quality, reproducible plating on polymers.
It is believed that this can be accomplished by providing the polymer parts to be plated with, as nearly as possible, identical pristine surfaces, similar to that of the aforementioned machined polymer surface. The time necessary for electroless plating has been found to vary from sample to sample. In some cases, areas will plate rapidly and begin to blister before other areas begin to plate. It is known that polymers retain molding induced stresses and that the surfaces may not be identical either physically or chemically. This variability may explain the differing plating results.