Acid Plating Baths

Electrodeposition of copper from acid baths is used extensively for electroforming, electrorefining, and decorative electroplating. Acid copper plating baths contain copper in the bivalent form and are more tolerant of ionic impurities than alkaline baths. They also have less macro throwing power and poorer metal distribution. Acid baths have excellent micro throwing power, resulting in the ability to fill or level scratches, grooves, or other substrate conditions, and additionally they are effective in sealing porous substrates. In most instances the smooth deposits produced by these solutions reduce or eliminate the need for mechanical smoothing for various substrates. A cyanide, noncyanide copper, or nickel strike must be applied to steel or zinc-alloy die castings before they are plated in acid copper solutions. Acid copper solutions cannot be used directly over substrates that are attacked by the high acidity or those where the copper forms an immersion deposit. Immersion deposits usually have poor adhesion to the substrate. Concentration limits and operating conditions of acid copper plating baths are given in Table 4.

Table 4 Compositions and operating conditions of acid copper plating baths

Constituent or condition

Copper sulfate bath

Copper fluoborate bath

General

Printed circuit through-hole

Low copper

High copper

Bath composition, g/L (oz/gal)

Copper sulfate, CuSO4 ■ 5H2O

200-240 (27-32)

60-110 (8-15)

Sulfuric acid, H2SO4

45-75 (6-10)

180-260 (24-35)

Copper fluoborate, Cu(BF4)2

225 (30)

450 (60)

Fluoboric acid, HBF4

To pH

40 (5)

Bath analysis, g/L (oz/gal)

Copper

50-60 (7-8)

15-28 (2-4)

8 (1)

16 (2)

Sulfuric acid

45-75 (6-10)

180-260 (24-35)

Specific gravity at 25 °C (77 °F)

1.17-1.18

1.35-1.37

Operating conditions

Temperature, °C (°F)

20-50 (68-120)

20-40 (68-105)

20-70 (68-160)

20-70 (68-160)

Current density, A/dm2 (A/ft2)

2.0-10.0 (20-100)

0.1-6.0 (1-6)

7.0-13.0 (70-130)

12-35 (120-350)

Cathode efficiency, %

95-100

95-100

95-100

95-100

Voltage, V

6

6

6

6-12

pH

0.8-1.7

<0.6

Anodes

Copper(a)

Copper(a)

Copper(b)

Copper(b)

(a) Phosphorized copper (0.02-0.08% P) is recommended.

(a) Phosphorized copper (0.02-0.08% P) is recommended.

(b) High-purity, oxygen-free, nonphosphorized copper is recommended.

The copper sulfate bath is the most frequently used of the acid copper electrolytes and has its primary use in electroforming. In this application, the advantages of acid copper lie in its strength and ductility. Acid copper sulfate is used to plate thick deposits over 150 ^m (6 mils) on large nickel-plated rolls; it is then engraved to electroform textile printing screens. It is also used extensively for the application of copper as an undercoating for bright nickel-chromium plating, especially for automotive components. Plates and rolls have been plated with acid copper sulfate for graphic arts and rotogravure printing where thicknesses of 500 ^m (20 mils) or more are not uncommon. Bright acid copper sulfate baths are used extensively as an underlayer in decorative plating of the plastic trim found on automobiles, appliances, and various housewares. By altering the composition of the copper sulfate bath, it can be used in through-hole plating of printed circuit boards where a deposit ratio of 1 to 1 in the hole-to-board surface is desired. In some applications, acid copper sulfate solutions are used to plate over electroless deposited copper or nickel. With additives, the bath produces a bright deposit with good leveling characteristics or a semibright deposit that is easily buffed. Where copper is used as an undercoating, deposit thicknesses will generally range up to about 50 ^m (2 mils).

The copper fluoborate bath produces high-speed plating and dense deposits up to any required thickness, usually 500 /'m (20 mils). This bath is simple to prepare, stable, and easy to control. Operating efficiency approaches 100%. Deposits are smooth and attractive. Deposits from the low-copper bath operated at 49 °C (120 °F) are soft and are easily buffed to a high luster. The addition of molasses to either the high copper or the low copper bath operated at 49 °C (120 °F) results in deposits that are harder and stronger. Good smoothness of coatings up to 500 ^m (20 mils) thick can be obtained without addition agents. For greater thicknesses, addition agents must be used to avoid excessive porosity.

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