Fluoborate Baths

Lead fluoborate baths are prepared by adding the required amount of lead fluoborate concentrate and fluoboric acid to water followed by peptone as the preferred addition agent.

Until methane sulfonic acid (MSA) baths became widely used in the past few years, fluoroborate baths were the most important bath for lead plating. Good lead deposits up to 1.5 mm (60 mils) in thickness can be achieved with a fluoroborate bath of the following composition:

Hydrofluoric acid (50% HF)

480 g/L (64 oz/gal)

Boric acid, H3BO3

212 g/L (28 oz/gal)

Glue

0.2 g/L (0.03 oz/gal)

A bath of half the above concentration is suitable for thinner deposits at low current densities, but the lead concentration should be kept high if smooth deposits and good throwing power are required (Ref 1). More detailed information on fluoroborate formulations and performance for lead plating is covered in Ref 2, 3, 4, and 5.

Many different types of glue and gelatin additives are available, but no one type is manufactured specifically for lead plating. Depending on the method of manufacture, each can exhibit different levels of solubility and impurities that may be of concern to the plater.

Glue and gelatin addition agents must be swelled and dissolved in water by the plater just prior to addition to the bath. The resultant colloidal solution has a limited shelf-life and is prone to bacterial degradation on standing. Glue and hydroquinone are relatively expensive. Often, it is a by-product of an industrial process and can contain organic and inorganic impurities detrimental to the lead plating process. No grade is manufactured and sold specifically for lead plating.

Concentrates of lead fluoborate and fluoboric acid contain free boric acid to ensure bath stability. An anode bag filled with boric acid in each corner of the plating tank is recommended to maintain a stable level of boric acid in the bath solution. The concentration of boric acid in the bath is not critical and can vary from 1 g/L (0.13 oz/gal) to saturation. The water used in the bath preparation must be low in sulfate and chloride, as these lead salts are insoluble.

Table 1 provides the compositions and operating conditions of high-speed and high-throwing-power fluoborate plating baths. The high-speed bath is useful for plating of wire and strip where high current densities are used. The high-throwing-power formulation is used in applications such as barrel plating of small parts or where thickness distribution on intricate or irregularly shaped parts is important. The high-throwing-power bath should be operated at a lower current density because of the lower lead content of the bath.

Table 1 Compositions and operating conditions of lead fluoborate baths

Anode composition, pure lead; anode/cathode ratio, 2:1

Table 1 Compositions and operating conditions of lead fluoborate baths

Anode composition, pure lead; anode/cathode ratio, 2:1

Bath

Bath composition

Temperature

Cathode current density(a)

Lead

Fluorobic acid (min)

Peptone solution, vol%

Free boric acid

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

°C

°F

A/dm2

A/ft2

High-speed

225

30

100

13.4

1.7

1 to saturation

0.13 to saturation

20-41

68105

5

50

High-throwing-

15

2

400

54

1.7

24-71

75-

1

10

(a) Values given are minimums. Current density should be increased as high as possible without burning the deposit; this is influenced by the degree of agitation.

Fluoborate baths rank among the most highly conductive plating electrolytes and thus require low voltage for the amperage used.

Maintenance and Control. The very high solubility of lead fluoborate in solution with fluoboric acid and water accounts for its almost universal use for lead plating. In the high-speed bath formulation of Table 1, neither the lead nor acid content is critical, and the bath can be operated over a wide range of lead and acid concentrations.

The high-throwing-power bath formulation of Table 1 must be operated fairly close to the guidelines given. Lowering the lead concentration improves the throwing power characteristics; however, a reduction in lead concentration must be followed by a corresponding decrease in the cathode current density. On the other hand, an increase in lead content above the optimum permits the use of higher current densities, with a corresponding decrease in throwing power.

Sludge may form in the fluoborate bath as a result of the use of impure lead anodes that contain bismuth or antimony or as a result of the drag-in of sulfates. Fluoborate baths should be constantly filtered through dynel or polypropylene filter media to remove any sludge that may form. Anodes must be bagged in dynel or polypropylene cloth. Absence of gas bubbles at the cathode or anode while plating indicates all electric energy is theoretically being used to transfer lead from the anode to the workpiece; in other words, the process is operating at 100% anode and cathode efficiency. The plating bath concentration therefore remains unchanged except for changes due to evaporation and dilution from placing wet parts in the bath in combination with dragout when the parts are removed from the bath.

Methods are available for analyzing lead and fluoboric acid concentrations. Additive concentration can be adequately evaluated through the use of the Hull cell. Low concentration of additive results in loss of throwing power, coarse-grained deposits, and treeing. (Treeing is the formation of irregular projections on a cathode during electrodeposition, especially at edges and other high-current-density areas).

References cited in this section

1. H. Silman, G. Isserlis, and A.F. Averill, Protective and Decorative Coatings for Metals, Finishing Publications Ltd., 1978, p 443-448

2. S. Hirsch, Tin-Lead, Lead and Tin Plating, Metal Finishing Guidebook and Directory, Elsevier Science, 1992, p 262-278

3. F.A. Lowenheim, Modern Electroplating, 2nd ed., John Wiley & Sons, 1963, p 242-249

4. A. Graham, Electroplating Engineering Handbook, 3rd ed., Van Nostrand Reinhold, 1971, p 238, 246, 266

5. The Canning Handbook, 23rd ed., Canning, 1982, p 742-746 Fluosilicate Baths

Fluosilicic acid is formed by the action of hydrofluoric acid on silicon dioxide. The lead fluosilicate (PbSiF6) electrolyte is formed when fluosilicic acid is treated with litharge. No great excess of silicic acid can be held in solution; therefore, the fluosilicate solution is less stable than the fluoborate solution. Table 2 lists compositions and operating conditions for two lead fluosilicate baths.

Table 2 Compositions and operating conditions of lead fluosilicate baths

Temperature, 35-41 °C (95-105 °F); cathode current density, 0.5-8 A/dm2 (5-80 A/ft2); anode current density, 0.5-3 A/dm2 (5-30 A/ft2); anode composition, pure lead

Table 2 Compositions and operating conditions of lead fluosilicate baths

Temperature, 35-41 °C (95-105 °F); cathode current density, 0.5-8 A/dm2 (5-80 A/ft2); anode current density, 0.5-3 A/dm2 (5-30 A/ft2); anode composition, pure lead

Bath

Lead

Animal glue

Peptone equivalent

Total fluosilicate

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

g/L

oz/gal

1

10

1.3

0.19

0.025

5

0.67

150

20

2

1SÖ

24

5.6

0.75

150

20.1

140

1S.75

Although at low current densities it is possible to secure smooth deposits of lead from the fluosilicate bath without additive agents, higher current densities are likely to produce treeing, especially in heavy deposits. Therefore, an additive agent, such as peptone glue or other colloidal materials or reducing agents, is always used. The use of excess glue in lead plating baths, however, may result in dark deposits. Maintenance and control procedures for the fluosilicate baths are similar to those described for the fluoborate baths.

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