Stopoff Media for Selective Plating

During plating, part surfaces that are not to be plated may need to be protected from the solution by stopoff media, such as lacquers, foils, tapes, waxes, and machined reusable fixtures. Stopoff media must adhere well to the metal surface, not become soft at the temperature of the solution or brittle at room temperatures, be resistant to solutions used for cleaning, etching, and plating, and be easy to remove after plating.

Lacquers used to prevent surfaces from being plated can be easily applied by brushing, spraying, or dipping. After plating, the lacquer can be stripped off or dissolved in an appropriate solvent.

Lead sheet, foil, and wire not only provide a positive stopoff but also act as thieves to aid in current distribution. Lead can be pounded into holes, keyways, or slots and trimmed with a sharp knife.

Tapes of several kinds are used as stopoff media. They vary from adhesive tapes backed with lead foil to tapes made of vinyl and other plastics. Lead foil tapes combine a specially compounded lead foil with a highly pressure-sensitive adhesive to provide a quick and convenient stopoff for short runs. The lead backing is useful as a thief in areas of high current density, or it can be lacquered when used in areas of low current density for equalizing current distribution. It is soft enough to conform to various configurations. Vinyl and other plastic tapes are soft and pliable and have extruded edges for providing a lead-free seal on almost any contour.

Sheet Materials. For large production runs, it is convenient to make stop-off forms that can be reused many times. Plastic sheet, generally 0.1 to 0.15 mm (0.004 to 0.006 in.) thick, is excellent for masking simple plates, cylinders, or other configurations. Steel sheet is sometimes substituted for plastic if it is desirable to equalize current distribution. To prevent plating or corrosion of the base metal, the plastic or steel stopoff must adhere firmly to the area being masked. Snug-fitting cylinders can be made to fit inside or outside diameters. To mask areas that are flat or of irregular shape, lacquer may be used to glue the stop-off material to the part; the lacquer may be removed with a thinner after plating.

Waxes. Several waxes designed for use as stopoffs are obtainable commercially. The use of a dip tank, thermostatically controlled to maintain the temperature at 15 to 20 °C (30 to 40 °F) above the melting point of the wax, makes the use of these materials comparatively fast and simple. The portion of the part to be plated can be covered with masking tape to prevent wax from adhering, or, if desired, the whole part can be coated and the wax stripped with a knife from areas to be plated. The wax mixture should have a melting point low enough to allow removal of the bulk with boiling water. Because they evolve poisonous fumes when heated, waxes containing chlorinated naphthalene must be used with exhaust equipment. High-melting-point mineral and vegetable waxes are not dangerous to use.

Decorative Chromium Plating

Revised by: James H. Lindsay, General Motors; Donald L. Snyder, Atotech USA Inc.

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