Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 614 of 707

is applicable only to solid strip. Spot Plating (Step and Repeat) Small, well-defined, round or Fig. 5. Spot plating cell. Step-and-repeat system is hydraulic, clamshell, or air driven. rectangular plated spots are produced by spot plating using rubber masks backed by a rigid plastic as depicted in Figure 5. The technique is used primari- ly for plating the tips of semi- conductor lead frames and can also be employed for plating tiny discrete spots on strip materials before they are formed into contacts or components. Using specially designed, form- fitting plating cells, the proce- dure could be employed to spot plate preformed components and contacts. To obtain the spots in pre- cise locations, prepunched guideholes are used to position the strip. In most spot plating equipment, the strip is intermittently advanced through the plating cells by a step-and-repeat system. The plating masks or plating tools, precision- molded silicon rubber, are closed on the strip via a clamshell mechanism during the plating cycle. Typically, from about five to 20 spots are plated at the same time. Step-and-repeat selective plating permits the very precise location of well- defined and discrete plated spots. The main disadvantages are lowered produc- tion rates and the requirement for particular masks for each application. Brush Plating An absorbent pad, such as one made of felt, is used to conduct plating solution to only that area of the strip where plating is required. The pad, which may be in the form of a roller or a narrow belt, shown respectively in Figs. 6 and 7, is backed by an insoluble metal anode. The pad is saturated as it turns through the plating solu- tion when the roller technique is used, or by wicking in the case of the belt arrange- ment. Brush plating is an excellent method for limiting the plating to the raised con- tacting surfaces of electrical contacts and can be employed to plate only one or two sides of rectangularly shaped connector pins. It is also very effective for tab plating printed circuits and can be used for such applications as plating the contacting pads of membrane-type electrical switches. Other Methods Several other ways for selectively plating continuous strip materials have been developed, but are not in general use. These include: Slotted wheels with the solution jetted through the slots (or holes) in the wheel. Using continuously applied photoresists, inks, or paints. Controlled jet plating with the electrolyte directed in a small steam at the area to be plated. 613

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