Metal Finishing Guide Book

2013

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Table II. Alkaline Electroless NickelPhosphorus Bath Nickel sulfate 30 g/L Sodium hypophosphite 30 g/L Sodium pyrophosphate Triethanolamine pH 60 g/L 100 ml/L 10.0 Temperature 30-35°C (86-95°F) the solution, thus allowing solution stability. The stabilizer(s) acts as a catalytic inhibitor, retarding potential spontaneous decomposition of the electroless bath. Few stabilizers are used in excess of 10 ppm, because an electroless bath has a maximum tolerance to a given stabilizer. The complexing agent(s) and stabilizer(s) determine the composition and brightness of the deposit. Excessive use of stabilization material(s) can result in a depletion of plating rate and bath life including poor metallurgical deposit properties. Trace impurities and organic contamination (i.e., degreasing solvents, oil residues, mold releases) in the plating bath will affect deposit properties and appearance. Foreign inorganic ions (i.e., heavy metals) can have an equal effect. Improper balance and control will cause deposit roughness, porosity, changes in final color, foreign inclusions, and poor adhesion. ELECTROLESS NICKEL The most widely used engineering form of electroless plating is, by far, electroless nickel. Electroless nickel offers unique deposit properties including uniformity of deposit in deep recesses, bores, and blind holes. Most commercial deposition is done with an acid phosphorus bath owing to its unique physical characteristics, including excellent corrosion, wear and abrasion resistance, ductility, lubricity, solderability, electrical properties, and high hardness. Electroless nickel baths may consist of four types: 1. Alkaline, nickel-phosphorus. 2. Acid, nickel-phosphorus. a) 1-4% P (low phosphorus) b) 5-9% P (medium phosphorus) c) 10-13% P (high phosphorus) 3. Alkaline, nickel-boron. 4. Acid, nickel-boron. The chemical reducing agent most commonly used is sodium hypophosphite (NaH2PO2); others include sodium borohydride (NaBH4), or an aminoborane such as n-dimethylamine borane (DMAB) [(CH3)2NHBH3]. Typical reactions for a hypophosphite reduced bath are as follows: H2PO2— + H2O H+ + HPO32— + 2H Ni2+ + 2H Ni + 2H+ H2PO2— + H H2O + OH— + P (1) (2) (3) 403

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