Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 256 of 707

Table XII. Operating Conditions for Tin Reflowing Plating thickness, in. Immersion time, sec. Temperature of oil bath, OF Acid neutralization value, mg KOH/g of oil Table XIII. Troubleshooting for Tin Reflowing Problem Etching Cause Acid neutralization value too high Dewetting Streaky Tin coating too thick Part not cleaned well before plating Acid neutralization value too low Not fully bright Tin coating too thin Spangling Molten tin comes in contact with water in quench Optimum 0.00025 6 490 2.7 Range 0.0001-0.0003 2-10 480-510 2.3-6.6 Remedy Remove part of oil bath and replace with new oil. Plate for shorter time. Check cleaning cycle. Add fatty acid flux. Plate for longer period of time. Increase in quench. Lower parts more slowly. of peroxide are necessary, make sure the hydroxide concentration is not too high and/or the anode current density is not too low. A 10% solution of acetic acid is used to lower the free hydroxide content of the bath; one gallon of 10% acetic acid will neutralize 9.25 ounces of sodium hydrox- ide or 12.75 ounces of potassium hydroxide. Acetic acid must be added slowly, with constant stirring, so that the stannic acid formed will redissolve. Filtration of stannate baths is very difficult. Sludge removal should be done when parts being plated are rough from occluded dirt. Sludge usually consists of hydrated tin oxide and carbonates. Allow the bath to cool and settle overnight, decant the clear solution to another tank, and shovel out the remaining sludge. Refer to Table XI for troubleshooting. REFLOWING TIN DEPOSITS Tin can be plated either matte or bright. Although the characteristics of each are comparable, bright tin plating has its proponents and is quite successful, commercially. Converting the matte finish of tin plate to a bright one can be accomplished by means of a process called reflowing, flow melting, or flow brightening. The tin coat- ing is heated momentarily to a temperature slightly above its melting point of 450O F, and then quickly quenched to produce the bright finish. Among the methods used in heating the plated material are induction, con- duction, radiant heating, and immersion. The most commonly used is immersion in hot oil or fat for a short time. Typical operating conditions are given in Table XII. The oil is usually a long chain fatty acid ester of glycerin such as tallow, palm oil, or partially hydrogenated oil, which has a sufficiently high flash point for the temperature used. It should also have some free fatty acid to serve as a flux. It is best to reflow as soon as possible after plating. The heating to the molten stage should be completed within 2 to 10 seconds before removing and quench- ing. For best results, the time to melt the tin coating should be precisely calculated. Unsatisfactory results, in the form of dewetting or balling-up, can occur if the parts 255

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