Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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to process a large variety of shapes, sizes, and types of materials is a hoped-for fea- ture of any continuous plating equipment; however, the trade-off between a practical machine and a very complex one, which is difficult to impossible to oper- ate and maintain, must be recognized when the equipment is being designed. Continuous strip plating is performed with the strip materials conveyed through the equipment horizontally or vertically. In the latter case, conveying and handling the strip is more complicated but this technique is usually preferred because: 1. Cleaning, rinsing, and activation processes are improved. 2. Plating thickness will be much more uniform per side and also side to side. 3. Higher solution agitation rates are possible. 4. Drag-out is reduced. 5. Anodes can be more strategically placed, adjusted, and maintained. 6. Selective plating can be done by controlled depth immersion. The general design of a processing station is comprised of two sections, an upper process cell and a lower reservoir tank. The solution is pumped from the reservoir into the upper cell where, when reaching the desired level, it flows over weirs or downspouts back to the reservoir. Heating and filtering of the solutions are generally performed in the reservoir. As the material being plated leaves each process cell, air knives and/or soft rub- ber wipers are required to remove drag-out. Spray rinses follow each operation, with air and mechanical wipers again employed to prevent diluting the subsequent solutions. (An important point to remember is that the materials being plated, due to their configurations and the speed at which they are run, will carry a con- siderable amount of solution with them as they exit each station. Rapid depletion of the solutions, or contamination of those next in line, must be avoided.) At each processing station, where the strip must be made cathodic or anodic, one or more of several types of contacting is used. Examples of these are: 1. Through the stainless steel drive or guide sprockets. 2. Copper alloy contact rolls. 3. Copper or nickel wiping strips. 4. Soft stainless steel or copper alloy brushes. The type and design of the contacts used are important and depend on the style of parts being plated. A firm contact force is necessary to prevent interruption of current, but it must not be so firm that the materials are mechanically damaged. Contact rolls and brushes must be kept clean and smooth to avoid arcing and burning. (Sometimes they are nickel and gold plated to improve their function.) It is necessary to position the contacts as closely as possible to the process cell to prevent overheating and drying, or oxidizing the materials being plated; how- ever, not so close that they might be contaminated by the solutions. When pos- sible, contacting is performed in the rinse stations where the water helps keep the contacts clean and cool. SELECTIVE PLATING SYSTEMS The selective plating of strip materials for electronics applications is an impor- tant process capability for both economic and technical reasons. Being able to deposit a precious metal only where it's actually required for its functional attributes will obviously result in an electrical component that will be far less costly than if it 610

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