Methane Sulfonic Acid Baths

Methane sulfonic acid (MSA) baths consist essentially of MSA-lead concentrate mixed with MSA to arrive at a total acid concentration of 300 mL/L. The overall system is stable and is considered to be a strong acid. Compositions and operating conditions for two MSA baths are given in Table 4.

Table 4 Compositions and operating conditions of lead methane sulfonic acid (MSA) baths

Temperature, 45 °C (110 °F); anode composition, pure lead; anode/cathode ratio, 1:1

Bank

Lead

MSA, mL/L

Additive, vol%

Cathode current density

g/L

oz/gal

A/dm2

A/ft2

Rack/barrel

30

4

300

4

0.5-5

5-50

High-current

100

13.3

300

4

0.5-20

5-200

The materials used to formulate MSA baths are highly soluble liquids. The baths listed in Table 4 are metal concentrations and, as such, are sensitive to current density. A lead concentration of 30 g/L (4 oz/gal) supports a maximum current density of 5 A/dm2 (50 A/ft2); an increase in the lead concentration to 100 g/L (13.3 oz/gal) allows a corresponding increase in the maximum current density to 20 A/dm2 (200 A/ft2). The use of a proprietary additive (4% of bath composition) is required to produce the smooth, fine-grained deposits usually provided by colloidal agents in fluoborate systems.

The principal advantage of MSA baths, in addition to their overall chemical stability, is the absence of the fluoride and borate ions present in other lead plating baths. These ions are heavily regulated or prohibited in many states because of their deleterious effects on fruit-bearing trees when released to the environment. An additional advantage of MSA baths is that when they are applied to 60Pb-40Sn solder alloys, these eutectic alloys can be plated over an extremely broad range of current densities. MSA baths are easily operated and controlled, but they are more expensive to make up.

Maintenance and Control. The MSA system is extremely stable and requires little or no maintenance other than control of the metal, acid, and additive concentrations within relatively broad ranges. Of these, it is of greatest importance to control the acid concentration in actual production situations. Additive concentration is evaluated using the Hull cell; metal and acid concentrations can be evaluated through simple titrations. Deionized water must be used for rinsing the part prior to immersion in the plating bath because MSA is sensitive to chloride ions in the makeup water.

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