Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 363 of 707

Table II. Alkaline Electroless Nickel- Phosphorus Bath Nickel sulfate Sodium hypophosphite Sodium pyrophosphate Triethanolamine pH Temperature 30 g/L 30 g/L 60 g/L 100 ml/L 10.0 30-35° C (86-95° F) trol pH and maintain control over the "free" metal salt ions available to the solu- tion, 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 maxi- mum tolerance to a given stabilizer. The complexing agent(s) and stabilizer(s) determine the composition and brightness of the deposit. Excessive use of sta- bilization 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, electro- less nickel. Electroless nickel offers unique deposit properties including unifor- mity of deposit in deep recesses, bores, and blind holes. Most commercial depo- sition is done with an acid phosphorus bath owing to its unique physical characteristics, including excellent corrosion, wear and abrasion resistance, duc- tility, 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. phite (NaH2PO2 rane such as n-dimethylamine borane (DMAB) [(CH3 for a hypophosphite reduced bath are as follows: H2PO2— + H2O H+ Ni2+ H2PO2— 362 + HPO3 2— + 2H Ni + 2H+ + H H2O + OH— The chemical reducing agent most commonly used is sodium hypophos- ); others include sodium borohydride (NaBH4 )2 NHBH3 + 2H + P (1) (2) (3) ), or an aminobo- ]. Typical reactions

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