Metal Finishing Guide Book

2013

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ALKALINE CYANIDE BATHS (GROUP 1, CLASS D) Table VI lists typical alkaline cyanide baths that are still used. Note that for matte deposits, the higher the temperature the better the deposit and the higher the speed of plating; however, temperatures over 150°F result in a rapid breakdown of the free cyanide and a buildup of cyanide breakdown impurities. The alkaline cyanide baths are particularly sensitive to organic impurities, both those introduced by drag-in and by the absence of general cleanliness, as well as those caused by cyanide breakdown. To maintain a deposit that has a good appearance and is structurally sound it is necessary to carbon treat and filter the solution periodically. The grade of carbon used must be pure enough not to introduce more impurities than it removes. Constant filtration through a filter packed with carbon is accepted practice but is not as efficient in removing impurities as a batch treatment. If the solution is quite contaminated before treatment it is important to save the used carbon and the used filter cartridge for refining to recover any gold lost in the treatment. The best method to carbon treat a solution is as follows: (1) Heat the solution to 150 to 160°F. (2) Transfer the hot solution to an auxiliary tank. (3) Add 1/8 to 1/4 oz carbon per gallon of solution. (4) Mix for no longer than 20 to 30 min. (5) Filter the solution by decantation back into the original tank. No general rule can be given for the frequency of carbon treatment. This will depend on general cleanliness and housekeeping as well as the work being processed; however, it will vary from once every two weeks to once every two months. The room temperature bright bath will require much less carbon treatment than the hot cyanide bath. NEUTRAL CYANIDE SOLUTIONS (GROUP 2, CLASS D) The neutral cyanide baths are primarily used by the semiconductor industry. Considerable care must be exercised to prevent contamination of the solution because even a few parts per million of undesirable inorganics can cause the deposit to fail in compression or die bonding. Typical nonproprietary solutions are listed in Table VII. Pulse plating may be used to advantage with the high-speed formulation. Most effective is a 10% duty cycle. Proprietary baths add grain refiners that decrease porosity, increase the maximum allowable current density, decrease grain size, and generally improve the appearance of the deposit. ACID CYANIDE PLATING SOLUTIONS (GROUP 3, CLASS E) Table VIII lists typical acid cyanide plating solutions. Note that pulse plating can be applied with the high-speed bath, but is not widely used. 291

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