Filtration

Two types of filtration are used for electroless nickel systems, cartridge filters and filter bags. Both require the use of an external circulation pump, and both should be capable of removing particles larger than 5 pm (0.2 mil) in size. Wound cartridge filters are supported in CPVC or polypropylene chambers located outside of the tank. The installation cost of these filters is high, however, and replacement of the cartridges is a large maintenance cost. Also the added back pressure of the filter can significantly reduce the flow of the pump and often its life.

Woven polypropylene bags are now being used to filter electroless nickel solutions. These bags are mounted above the plating tank itself, allowing the solution to flow through the bag by gravity. Filter bags are relatively inexpensive and result in only a minimum restriction on the discharge of the pump. When bags become soiled or begin to plate out, the change is obvious to the operator, and the bags can be quickly and easily replaced. Filter bags with stainless steel support rings rather than plated steel rings should be used. Plated rings can introduce cadmium or zinc into the bath and slow or stop deposition. A filter assembly is shown in Fig. 21.

Filter cartridges and bags should be washed using hot water prior to use for electroless nickel. Antistatic agents often found in these filter media can be harmful to the plating solution.

For extremely critical applications such as memory disks, filtration should be through a 1 pM filter cartridge followed by a 0.2 pM cartridge using flow rates sufficient to turn over the volume of plating solution 10 to 20 times per hour. Filter discharge is best done through a sparger to distribute the solution uniformly in the tank, and not impinge on the parts being plated.

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