Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 612 of 707

had been plated overall. Of almost equal importance to the electronics engineer is the need to preclude plat- ing from certain zones, or to utilize the characteristics of two, or even three, types of plating on a single contact—something made possible by selective plating. There are a number of selective plating techniques employed, each with its advantages, but none with- out at least a few operational and/or maintenance problems. Controlled Depth or Tip Plating The simplest method of selective plating is to only partially immerse the material being plated into the plating solution (Fig. 1). The height of the deposit is set with adjustable solution overflow weirs and/or downspouts and by the position of the guide sprockets. Plastic or glass shields can be used to improve plating thickness uniformity; however, it is impossible to avoid plating the entire submerged surface and this is the main drawback of the technique. Another unavoidable problem is the "loss" of some precious metals because of the meniscus at the surface of the solution. This zone will range in width from approximately 0.045 to 0.065 in. and over a period of time a significant amount of precious metal (i.e., gold) will be plated in that area. Fig. 1. Controlled depth plating cell. Notes: (1) Heating and filtering are usually performed in the reservoir tank. (2) Level of solution in process cell controlled by weirs at slots or by downspouts. (3) Cells of the same design are used for cleaning and activation and solutions maintained so that the strip is entirely submerged. Moving Belts Fig. 2. Moving belt mask. Plating solution touches the strip only in the slotted area. Stripes may be on one or both sides; spots can also be plated via this technique. Belts made of soft rubber bonded to an insoluble metal backing strip prevent the plating solutions from reaching the surface except for nar- row bands or spots on the materials being plated (Fig. 2). The plating solution is jetted from a plating cell through slots or holes in the masking belts firmly pressed against and turning with the strip. Solid materials and preformed components can be plated with one or more stripes on one or both sides. If properly designed and built, the moving belt can be expected to hold a stripe width tol- erance of at least -0.010 in. Two individual belts can also be used to produce a stripe, with stripe location and width adjusted with 611

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