Table 5 Composition of anode and fluoborate solution for deposits up to 50 Sn

Plated 3.2 A/dm2(30 A/ft2); compositions of all baths contains a minimum of 100 g/L (13.3 oz/gal) of free HBF4, 25 g/L. (3.3 oz/gal) of free H3BO3, and 5.0 g/L (0.7 oz/gal) of peptone

Composition of deposit and anode, %

Composition of bath

Stannous tin

Lead

Tin

Lead

g/L

oz/gal

g/L

oz/gal

5

95

4

0.5

85

11.3

7

93

6

0.8

88

11.8

10

90

8.5

1.1

90

12.0

15

85

13

1.7

80

10.7

25

75

22

2.9

65

8.7

40

60

35

4.8

44

5.8

50

50

45

6.0

35

4.7

Table 5 is based on an operating density of 3.2 A/dm2 (30 A/ft2). Higher or lower current densities may result in deposition of alloys of compositions differing from those given in the table. It is then necessary to make compensating corrections in solution composition. Deposition rates of tin-lead coatings can be controlled by current density. Table 6 shows that as the current density of a fluoborate solution is increased, the rate of 60Sn-40Pb deposition also increases.

Table 6 Rate of 60Sn-40Pb deposition from the fluoborate solution

100% cathode efficiency

Table 6 Rate of 60Sn-40Pb deposition from the fluoborate solution

100% cathode efficiency

Current density

Time in bath, min, at thickness of:

A/dm2

A/ft2

25^m (0.0001 in.)

7.5 ^m (0.0003 in.)

12.5^m (0.0005 in.)

25 ^m (0.001 in.)

1.0

10

4.5

13.5

22.5

45

1.5

15

3.0

9.0

15.0

30

2.0

20

2.3

6.8

11.3

22.5

2.5

25

1.8

5.4

9.0

18

3.0

30

1.5

4.5

7.5

15

Temperature. Tin-lead fluoborate solutions operate efficiently in a temperature range of 18 to 38 °C (65 to 100 °F). Upper temperatures slightly increase tin in deposits, and lower temperatures can decrease tin.

Current densities below the specified amount for a particular solution formula can decrease the tin content of deposits. Higher current densities can increase tin content.

Agitation is an important factor in tin-lead plating. Optimum conditions exist when mild agitation is used. Use of a still bath results in nonuniform deposits because of local exhaustion of the solution at the cathode surface. Vigorous agitation may increase the stannic tin content of a solution, resulting in a decrease of tin in deposits. Cathode rod agitation or circulation through an outside pump provides suitable agitation for a tin-lead plating solution. Air agitation should not be used because it can oxidize stannous tin.

Boric acid is added to maintain solution stability. Approximately 25 g/L (3.4 oz/gal) of boric acid has been found desirable, but its concentration is not critical. An anode bag filled with boric acid may be hung in a corner of the tank to maintain the required concentration. Excess boric acid prevents fluoride precipitates, which can deplete lead from the solution. This is based on the following formula:

0 0

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