Troubleshooting Electroplating Installations: Nickel Sulfamate Plating Systems by N.V
Total Page：16
File Type：pdf, Size：1020Kb
Technical Article Troubleshooting Electroplating Installations: Nickel Sulfamate Plating Systems by N.V. Mandich * & D. W. Baudrand The properties and behavior of nickel electrodeposits High-speed nickel electroforming in general, and sulfa- plated from a sulfamate solution are determined by mate baths in particular, have become popular as micro- many variables, including operating conditions, impu- electroforming techniques for the manufacture of micro- rities in plating solutions composition, additives and systems technologies. A knowledge of the effects of impu- others. Changes in operating variables such as solu- rities, and alloying metals such as cobalt, are important to tion temperature, current density, agitation or cur- the successful use of nickel sulfamate plating solutions. rent form can change hardness, internal stress and crystalline structure. Changes in process chemistry Solution Composition & such as nickel concentration, boric acid, chlorides and Operating Conditions brightener system can produce minor or remarkable changes in the deposit. Some impurities have a large Addition agents and impurities have a profound effect on infl uence on the deposit; others have little infl uence. deposit characteristics as is to be expected. Changes in Characteristics of electrodeposits from nickel-sulfa- solution composition, operating temperature, current den- mate plating solutions resulting from these changes sity, solution agitation, can also alter the deposit charac- are discussed here. The mechanism of anodic reac- teristics. Table 1 presents a typical solution composition tions resulting in the decomposition of sulfamate ions and operating conditions while Table 2 gives the mechan- is detailed. Testing and troubleshooting information is ical properties of sulfamate nickel deposits from various 1 also provided, along with an up-to-date bibliography. common bath formulations. It should be noted that the compositions shown in Table 2 are used for special purposes. In the concentrated sulfa- Introduction mate solution, extremely high rates of deposition are possi- In the last 50 years, nickel sulfamate solutions have been ble with current densities up to 40 A/dm2 (372 A/ft2) with- used extensively in electroforming and in electroplating out high agitation. For a given plating rate in this bath, the for engineering applications. The properties of the depos- internal stress decreases rapidly with increasing tempera- its or the operating factors that make those baths attractive ture. are low residual stress, high deposition rates and good It has been reported that when operating above 40 A/dm2 ductility. In these respects, sulfamate baths are generally (372 A/ft2), 300 g/L (40 oz/gal) baths can produce brittle regarded as being superior to the conventional Watts bath. deposits.2 To avoid this, a more concentrated bath (675 For decorative deposits, however, both bright and dull, the g/L; 90 oz/gal) is used, with a higher temperature of 71°C Watts bath has continued to be favored, because of the (160°F) and an increased anode area to minimize anode enormous background of practical operating experience sludge formation. with it. The characteristics and properties of sulfamate nickel Function of Bath Ingredients deposits that are infl uenced by plating conditions and impurities are numerous. They include ductility, hardness, Nickel. Nickel sulfamate is the source of the nickel metal. tensile strength, intrinsic stress, deposit structure, porosity, Concentrations of 90 to 135 g/L (12 to 18 oz/gal) of nickel smoothness, density, specifi c heat, coeffi cient of expan- metal are used in most cases for high-speed plating, when sion, thermal conductivity, specifi c resistivity and modulus coupled with very high solution agitation. Current densi- 2 2 of elasticity. ties of 43 to 430 A/dm (400 to 4000 A/ft ) have been used. High metal content is also used to improve throwing power at very low current densities of 0.1 to 0.4 A/dm2 (0.9 to 3.7 A/ft2). High metal content does not appreciably change the characteristics or properties of the deposit. Low metal con- tent coupled with moderate to high current densities will Nuts & Bolts: cause deposition of basic nickel salts (i.e., “burning”). What This Paper Means to You This paper covers sulfamate nickel plating from top to bottom and offers a troubleshooting guide to operating the process. The *Corresponding Author: information here is indispensable to those performing electro- Dr. N. V. Mandich forming or heavy nickel plating. HBM Electrochemical & Engineering Company 2800 Bernice Road, Lansing, IL 60438 E-mail: [email protected] 68 Plating & Surface Finishing • September 2002 Traditionally, electroplaters and electroformers Table 1 prefer faster deposition rates and higher cathode current densities of at least 4 A/dm2 (37 A/ft2). Typical Solution Composition & Operating Conditions Since the anode/cathode ratio is seldom larger than one-to-one, in order to plate fast, the chloride concentration is raised to 8 to 10 g/L (1.0 to 1.3 Nickel, as metal...................................................................... 75 g/L (10 oz/gal) oz/gal) to avoid anode polarization, the precursor Boric acid ................................................................................. 45 g/L (6 oz/gal) of sulfamate decomposition. In turn, one needs to Chloride................................................................................ 2.2 g/L (0.3 oz/gal) add stress reducers to counter the infl uence of the Wetting agent ............................................................................... 0.1 - 0 3 vol% higher chlorides. True high-speed plating achiev- Brightener / stress reducer..................................................................... Optional able at the levels obtained with a Watts bath is Temperature .................................................................................. 49°C (120°F) often impractical with sulfamate solutions. Cathode current density .............................. 0.5 - 21.0 A/dm2 (4.6 - 195.0 A/ft2) Cathode current effi ciency ..................99.0 - 99.9% (for S-depolarized anodes) Infl uence of Processing Variables Specifi c gravity ..................................................................................1.23 - 1.28 pH................................................................................................................... 4.0 pH. The acidity of the bath can also affect deposit stress. A solution with 76.5 g/L (10.2 oz/gal) of nickel yields the lowest stress at pH 3.8 to 4.8. A solution with 107 g/L (14.3 oz/gal) of nickel Boric acid. Its concentration should vary with operating temper- yields its lowest stress at pH 2.9 to 3.8. The stress increases rapidly ature from 30 g/L (4.0 oz/gal) at ambient temperature to over 45 at pH values above 5.0 because of the codeposition of basic nickel g/L (6.0 oz/gal) at 52°C (125°F). Low boric acid can cause “orange salts. peel” type of pitting. High boric acid will tend to salt-out of solu- Hardness is affected by pH as with other nickel plating solutions. tion at lower temperatures. Once crystallized (salted out), it is diffi - As pH rises, it is fairly constant up to a critical value of about 5.0, cult to redissolve. About 30 g/L (4.0 oz/gal) boric acid will remain above which it rises sharply. A pH below 3.5 reduces the cathode in solution at 20°C (68°F). The lowest practical operating tempera- effi ciency, slows the plating rate slightly, but does not change the ture for current densities over 16 A/dm2 (149 A/ft2) is 32°C (90°F). deposit hardness. High current densities and/or low temperatures can cause famil- For rack plating, the standard pH is 4.0. At this value, iron con- iar “burning.” Lower nickel contents (below 75 g/L; 10 oz/gal) taminants cannot remain in solution. Iron precipitates as ferric require higher amounts of boric acid. hydroxide and is picked up in the fi lter. Deposit characteristics Chlorides/bromides. Chlorides or bromides are used in small such as ductility remain good. Above pH 5.0, hardness increases amounts and are useful in promoting optimum anode corrosion. and ductility decreases through codeposition of small amounts of Poor anode corrosion results in compressive stress, brightness, loss basic nickel salts. For low current density purifi cation (dummying), of ductility, increased hardness and increased porosity. The addi- a low pH (3.0 to 3.5) favors the removal of metallic impurities. tion of small amounts of chlorides to the plating solution slightly Barrel plating must be done at a pH between 3.2 and 3.5 to pre- increases the tensile stress of the deposit, a trade-off that serves the vent laminated deposits arising from the making-and-breaking of purpose. For most applications, the increased stress is too small to electrical contact as the barrel rotates. Low pH also results in faster cause any problem such as distortion of an electroformed product. accumulation of impurities. Others have found that for low or moderate current densities, chlo- Temperature. The infl uence of temperature on hardness is a ride is not necessary if sulfur-depolarized anodes are used with suf- parabolic function. As it is increased or decreased from 39°C fi cient anode area. (102°F), the hardness of the deposit is increased, the increase Table 2 Mechanical Properties of 127 µm (5 mil) or Thicker Sulfamate Nickel Deposits1 Hardness Elongation Type of Bath Tensile Strength Vickers (% in 2 in.) Internal Stress Electroforming 483 MPa (70,000 lb/in.2) 200 20 27.6 MPa (4,000 lb/in. 43°C (110°F), pH 4 Sulfamate (conventional) 621 MPa (90,000 lb/in.2) 200 25 27.6 MPa (4,000 lb/in.2) 60°C (140°F), pH 4 5.4 A/dm2 (50 A/ft2) Sulfamate (concentrated) 483 MPa (70,000 lb/in.2) 170 25 27.6 MPa (4,000 lb/in.2) 60°C (140°F), pH 4 22 A/dm2 (200 A/ft2) Sulfamate (hard) 896 MPa (130,000 lb/in.2) 470 10 -34.5 MPa (-5,000 lb/in.2) 54°C (130°F), pH 5 (compressive) 6.4 A/dm2 (60 A/ft2) Plating & Surface Finishing • September 2002 69 being more rapid at lower temperatures.3,4 Hardness is not appreciably affected between 35 and 50°C (95 and 126°F).