Citric Acid → Nickel indium bismuth sulfate

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The nickel/indium/bismuth sulfate and inckel/indium/bismuth sulfamate plating baths were prepared similarly. Preparation of one liter of the sulfate and sulfamate formulations is outlined below. Citric acid (57 grams) is dissolved in 300 ml of deionized water heated to 45° C. Nickel carbonate (45 grams) is then dissolved in the solution. The pH of the bath at this point should be about 4.0. Nickel chloride (15 or 75 grams/liter of nickel chloride hexahydrate for the sulfate and sulfamate plating baths, respectively) and nickel sulfate (300 grams/liter of nickel sulfate hexahydrate or 350 grams/liter of nickel sulfamate hexahydrate) are then added. The solution is brought up to a volume of 1L by addition of deionized water and the pH of the bath is adjusted to 3.2 either with nickel carbonate or sulfuric/sulfamic acid. Indium metal is then added electrochemically. Indium metal (either strips or shot in an anode basket) is dissolved anodically at about 5 amps/dm2. The cathode current density should be greater than about 50 amps/dm2. Current is applied until the indium reaches the desired concentration, e.g. 2.5 grams/liter for the sulfate formulation and 5 grams/liter for the sulfamate bath. Bismuth in solution form is then added. Bismuth is added as an HCl solution prepared by dissolving sodium bismuthate in 20% HCl, precipitating a white solid by the addition of water, filtering and redissolving the solid in 20% HCl. The bismuth concentration is varied from 0.1-3.0 g/L metal in the sulfate system and from 0.2-2.0 g/L metal in the sulfamate system. The base formulation for the sulfate based Ni/In/Bismuth alloy electroplating bath is set forth the following Table I.

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