Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 637 of 707

between 50 and 10,000 A. These values will vary depending on whether still- or barrel-plating methods are employed, the type of finish required, and the size of the parts being plated. Direct current plating power supplies are relatively straightforward. The incoming AC is converted to DC by means of the main power transformer and either a primary thyristor/secondary diode or secondary thyristor rectification sys- tem. In modern systems, the output voltage and current are controlled by the phase angle of the thyristors. Most rectifiers today are equipped with both auto- matic voltage control (AVC) and automatic current control (ACC) as standard equipment. In many cases, a variable ramp system is also provided to regulate automatically the rate at which the output is increased from minimum to the desired level. The ripple component of the output at full-rated power is nominally 5% rms of nameplate rating. This will increase as the thyristor's phase angles are changed to reduce the output. If particular processes demand continuous use of a system phased back, either a properly sized unit should be utilized, or a ripple filter should be installed to bring the ripple component to an acceptable level. Cooling can be by a number of different methods. Forced air and direct water are the most common. Forced air is acceptable when the surrounding environ- ment is relatively clean and free of contaminants. In a forced-air system, air is drawn in through a series of filtered openings in the rectifier enclosure, forced past the internal power-supply components, and exited through an opening, typically in the top of the supply. Air that contains corrosive materials can cause acceler- ated deterioration inside the power supply, resulting in reduced life and efficiency. If a plating rectifier is situated in an aggressive atmosphere, direct water cooling should be considered. Direct water-cooling systems pass water through a series of cooling passages in the main power transformer and semiconductor heat sinks. Water-cooled systems are more compact than air-cooled designs, and multiple rectifier systems can be placed closer to each other than air-cooled power supplies; however, water-cooled systems are sensitive to contamination and minerals in the supply water, and in these cases, the power supplies may require periodic maintenance to clean the water passages and filters. Direct current plating deposits metal utilizing a continuous application of ener- gy, pulse-plating systems provide the opportunity to modulate the voltage or cur- rent to achieve different results. The application of gold, silver, and copper with pulse plating results in finer grain structures, higher surface densities, and low- er electrical resistance. Additionally, plating times can be reduced by up to 50%. These characteristics make pulse plating attractive, if not mandatory, in the electronics industry. From an industrial standpoint, pulse plating has found a number of important applications. For example, when used in chromium plating, pulse plating will result in a harder, more wear-resistant surface. In a nickel plating application, using pulse plating may eliminate the need to add organic compounds to control stress and will result in a brighter finish with better thickness control and reduced plating times. Many plating profiles are available, including standard pulse, superimposed pulse, duplex pulse, pulsed pulse, and pulse on pulse. These waveforms can be obtained from a unipolar power supply. Other variations, possible when using a bipolar pulsing rectifier, include pulse reverse, pulse reverse with off time, pulsed Pulse Plating 636

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