Reducing Agents

A number of different reducing agents have been used in preparing electroless nickel baths, including sodium hypophosphite, aminoboranes, sodium borohydride, and hydrazine.

Sodium Hypophosphite Baths. The majority of electroless nickel used commercially is deposited from solutions reduced with sodium hypophosphite. The principal advantages of these solutions over those reduced with boron compounds or hydrazine include lower cost, greater ease of control, and better corrosion resistance of the deposit.

Several mechanisms have been proposed for the chemical reactions that occur in hypophosphite-reduced electroless nickel plating solutions. The most widely accepted mechanism is illustrated by the following equations:

Heat

In the presence of a catalytic surface and sufficient energy, hypophosphite ions are oxidized to orthophosphite. A portion of the hydrogen given off is absorbed onto the catalytic surface (Eq 1). Nickel at the surface of the catalyst is then reduced by the absorbed active hydrogen (Eq 2). Simultaneously, some of the absorbed hydrogen reduces a small amount of the hypophosphite at the catalytic surface to water, hydroxyl ion, and phosphorus (Eq 3). Most of the hypophosphite present is catalytically oxidized to orthophosphite and gaseous hydrogen (Eq 4) independently of the deposition of nickel and phosphorus, causing the low efficiency of electroless nickel solutions. Usually 5 kg (10 lb) of sodium hypophosphite is required to reduce 1 kg (2 lb) of nickel, for an average efficiency of 37% (Ref 2, 3).

Early electroless nickel formulations were ammoniacal and operated at high pH. Later, acid solutions were found to have several advantages over alkaline solutions. Among these are higher plating rate, better stability, greater ease of control, and improved deposit corrosion resistance. Accordingly, most hypophosphite reduced electroless nickel solutions are operated between 4 and 5.5 pH. Compositions for alkaline and acid plating solutions are listed in Table 1 (Ref 2, 3, 4, 5).

Table 1 Hypophosphite-reduced electroless nickel plating solutions

Constituent or condition

Alkaline

Acid

Bath 1

Bath 2

Bath 3

Bath 4

Bath 5

Bath 6

Composition

Nickel chloride, g/L (oz/gal)

45 (6)

30 (4)

30 (4)

Nickel sulfate, g/L (oz/gal)

21 (2.8)

34 (4.5)

45 (6)

Sodium hypophosphite, g/L (oz/gal)

11 (15)

10 (1.3)

10 (1.3)

24 (3.2)

35 (4.7)

10 (1.3)

Ammonium chloride, g/L (oz/gal)

50 (6.7)

50 (6.7)

Sodium citrate, g/L (oz/gal)

100 (13.3)

Ammonium citrate, g/L (oz/gal)

65 (8.6)

Ammonium hydroxide

To pH

To pH

Lactic acid, g/L (oz/gal)

28 (3.7)

Malic acid, g/L (oz/gal)

35 (4.7)

Amino-acetic acid, g/L (oz/gal)

40 (5.3)

Sodium hydroxyacetate, g/L (oz/gal)

10 (1.3)

Propionic acid, g/L (oz/gal)

2.2 (0.3)

Acetic acid, g/L (oz/gal)

10 (1.3)

Succinic acid, g/L (oz/gal)

10 (1.3)

Lead, ppm

1

Thiourea, ppm

1

Operating conditions

pH

8.5-10

8-10

4-6

4.3-4.6

4.5-5.5

4.5-5.5

Temperature, °C (°F)

90-95 (195-205)

90-95 (195-205)

88-95 (190-205)

88-95 (190-205)

88-95 (190-205)

88-95 (190-205)

Plating rate, ^m/h (mil/h)

10 (0.4)

8 (0.3)

10 (0.4)

25 (1)

25 (1)

25 (1)

Aminoborane Baths. The use of aminoboranes in commercial electroless nickel plating solutions has been limited to two compounds: N-dimethylamine borane (DMAB)-(CH3)2 NHBH3, and H-diethylamine borane (DEAB)--(C2H5)2 NHBH3. DEAB is used primarily in European facilities, whereas DMAB is used principally in the United States. DMAB is readily soluble in aqueous systems. DEAB must be mixed with a short chain aliphatic alcohol, such as ethanol, before it can be dissolved in the plating solution.

Aminoborane-reduced electroless nickel solutions have been formulated over wide pH ranges, although they are usually operated between 6 and 9 pH. Operating temperatures for these baths range from 50 to 80 °C (120 to 180 °F), but they can be used at temperatures as low as 30 °C (90 °F). Accordingly, aminoborane baths are very useful for plating plastics and nonmetals, which is their primary application. The rate of deposition varies with pH and temperature, but is usually 7 to 12 pm/h (0.3 to 0.5 mil/h). The boron content of the deposit from these baths varies between 0.4 and 5%. Compositions and operating conditions for aminoborane baths are listed in Table 2 (Ref 2, 5, 6).

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