Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 621 of 707

and suppress the effects of contaminants. The design of the reservoir should also be such that additions of chemicals can be easily made. Water can be added automati- cally to maintain the required volumes. There are sophisticated techniques available for continuously analyzing plat- ing solutions. Further, it is possible that these analyses may be used to direct auto- matic additions of chemicals to the baths. Obviously, the equipment required for these functions will significantly increase the cost of a plating line; however, this increase could be quickly justified by the prevention of a few major quality problems and/or enabling closer gold plating thickness control. Process control in continuous plating encompasses equipment as well as solution maintenance. Spray rinses need to be checked frequently. The air knives, or air wipes, have to be looked at regularly, as well as any mechanical devices used to avoid or reduce drag-out. In the processing cells, anodes and solution levels need to be looked at frequently as do the electrical contacts, and all of the ancillary and supporting equipment must be inspected and maintained. OTHER CONSIDERATIONS DC Current and Rectification Although high current densities are used in continuous strip plating, the total number of amperes required is rather low because of the small surface area present in each plating station. Therefore, in choosing a power source, there are three main features that should be included: 1. Ripple, which for precious metals is recommended to be held to <1%. 2. Precise current control for the entire range of current supplied. 3. The ability to maintain a constant current density. Contacts The contacts used to supply current to the strip must be designed to be effective and reliable and to require little attention because even very brief periods of inter- mittent contact will result in the production of many feet of defective material. Parts Counters Parts counters should not be considered an expendable option. One is needed on both ends of the line to preclude arguments with the base material supplier and the receiver of the plated parts. Liquid Level Controllers Liquid levels should be automatically maintained in the reservoir tanks to assure that operating variables are kept within the required control limits. Tank Heaters Quartz, Teflon, stainless steel, and tungsten heating elements are most often used. Low level safety switches must be included to avoid the possibility of fires. INSPECTION As in any plating for electronics operation, the finished product will be subjected to a number of tests to assure good plating quality. Typically these tests will include: 1. Adherence: twist, bend, bake, and/or tape. 2. Appearance: visual and microscopic to 20[times]. 3. Bondability: a use test for lead frames. 620

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