Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 659 of 707

in place with the added support of a threaded top nut. This threaded top nut is needed to provide addi- tional torque support for the top thumb screw when tightening the rack to the work bar. PCB cleaning can be done in a slotted basket. The basket should have 1 /4-in. spacing between slots, be fab- ricated out of stainless steel, or be plastisol- or Halar- coated steel. Fig. 6 Close up view of printed circuit board (PCB) tip showing only the stainless steel contact and plastisol trimmed away for a square fit for the PCB (two-point contact). Halar is a highly protective coating with high tem- perature characteristics, this coating is much more expensive than plastisol. In the case of very flexible contacts, it is necessary to design racks for each individual operation depending on parameters that are specific to each PCB opera- tion. Some PCB racks have been designed with adjustable spine or cross members to accommodate dif- ferent size boards in each production process. PCB racks after a period of time will accumulate plating buildup in the contact area and will need to be repaired to continue to be productive. The PCB racks are repaired by stripping off the metal buildup and plasti- sol, repairing and cleaning the contact area for corrosion, and fixing the spine. The rack is then plastisol coated and trimmed to customer specifications. The contact area on a PCB rack is a slot with a contact point between the thumb screw and contact point. This slot is a specified width and the outside of the slot can be V-shaped to help with the racking of the board. Electroless Nickel Plating racks designed for electroless nickel can be as simple as using a strand of copper wire to hold the piece to be finished. Stainless steel contacts can also be utilized. Electropolishing racks can be grouped in the category of a rack that needs pos- itive contact, usually a titanium tip, because of the need for chemical resistance. Copper spines are still used and the rack is plastisol coated. Racks should hold the work so that gas pockets will be eliminated. Electropolishing 658

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