Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 259 of 707

Table II. Solution Compositions for Chloride-Fluoride Baths Description Stannous chloride (anhydrous) Nickel chloride (hexahydrate) Ammonium bifluoride Ammonium chloride Ammonium hydroxide Proprietary additives Nonproprietary Bath 50 g/L 300 g/L 55 g/L — To adjust pH — Proprietary Bath 50 g/L 250 g/L — 50 g/L To adjust pH 20% by volume As a Diffusion Barrier A layer of tin-nickel over copper prior to solder (tin-lead) plating prevents the dif- fusion of copper into the tin-lead layer. TIN-NICKEL ALLOY PLATING BATHS Tin-nickel alloy is electroplated mainly from chloride-fluoride bath. It can also be plated from nonfluoride (pyrophosphate) baths but the chemistry for the latter remains to be perfected. The chloride-fluoride bath process is described in detail. The Chloride-Fluoride Baths The bath compositions and the operating conditions for the basic nonproprietary and a proprietary bath are provided in Tables II and III. Chloride-Fluoride Bath Preparation The fluoride-compatible PVC tank is two thirds filled with water and heated. The required quantity of nickel chloride is dissolved first. Ammonium fluoride is added next. Stannous chloride is added when both the nickel chloride and stan- nous chloride have been dissolved. Either ammonium hydroxide or hydrofluoric acid may be added to raise or lower the pH respectively. The solution is then fil- tered through particulate and activated carbon filters and also dummied for 4-6 hours to remove suspended solids and organic and metallic contaminants before putting it to use. Anodes for Chloride-Fluoride Bath Both nickel and tin anodes may be used with separate circuits but nickel anodes alone are preferably used. As the tin content of the bath decreases, the deficiency of tin is best compensated with the addition of calculated amounts of stannous chloride salt and the pH adjusted. Bath Agitation and Filtration of Chloride-Fluoride Bath Agitation is not necessary for very thin deposits but cathode rod movement (mechanical) agitation is employed for thicker deposits. Air agitation is not per- Table III. Operating Conditions for Chloride-Fluoride Baths Description Stannous tin (Sn2+) Nickel Total fluorine Free hydrofluoric acid pH Temperature Voltage Cathode current density, A/ft2 Plating rate at 10 A/ft2 Solution agitation 258 Nonproprietary Bath 25-35 g/L 60-80 g/L 30-45 g/L 4-12 g/L 2-2.5 60-70° C (140-160° 1-3 1-30 1 µm (40 µ-inches)/5 minutes Cathode rod movement F) Proprietary Bath 22-30 g/L 42-60 g/L — — 4.2-4.8 50-60° C (120-140° 1-3 1-30 1 µm (40 µ-inches)/5 minutes Cathode rode movement F)

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