Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 260 of 707

Table IV. Defects in Tin-Nickel Plated Alloy and Their Probable Causes Defect Cause Cloudy/whitish deposit Dark deposit Gray deposit Burnt deposit Poor coverage Poor solderability Low nickel, high tin, low temperature, organic contamination. In low current density areas—due to copper >50 ppm, contamination. In high current density areas—due to low tin, low free fluoride, iron >750 ppm and chromium >50 ppm contamination. Low tin content, low temperature, and pH imbalance. Excessive iron contamination. Deposit passivated and/or aged before soldering. mitted to prevent oxidation of stannous tin. Continuous filtration is necessary to remove suspended as well as organic matter. Effects of Process Variables in Chloride-Fluoride Bath The factors exerting influence on the tin-nickel deposit are the following: • Very little change in the composition of the deposit with minor variations in tin (stannous, Sn2+ ) and nickel contents. • Profound effect in the composition of the deposit with minor changes in the free fluoride content of the deposit. Since fluoride makes a com- plex with stannous tin, an increase in fluoride content will lower the tin content (percentage) in the deposit and vice versa and accordingly the deposit could be grayish (due to the lack of tin) or whitish (due to the excess of tin) respectively. Effect of Contaminants in Chloride-Fluoride Bath • The presence of grease, oil, and organic contaminants will give rise to seri- ous pitting in the tin-nickel deposit. • The presence of lead over 25 ppm is detrimental, accordingly plating over leaded substrates should be avoided. • Metallic impurities, such as copper, antimony, iron, zinc, cadmium etc., over 200 ppm are also detrimental, and these must be removed by dum- mying the solution. Chloride-Fluoride Bath Troubleshooting Guidelines Defects in tin-nickel plated alloy are shown in Table IV along with their probable causes. Nonfluoride (Pyrophosphate) Bath Where environmental problems exist because of the toxic and corrosive nature of Table V. Solution Composition and Operating Conditions for Pyrophosphate Bath Tin (stannous) chloride (hydrated) Nickel chloride (hydrated) Potassium pyrophosphate (hydrated) Glycine Temperature pH Cathode current density 50° 28.2 g/L 31.3 g/L 192.2 g/L 20 g/L C (122° 7.5-8.5 5-15 F) 259

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