MSA Plating Solutions for Tin Lead

In the electronics industry, MSA solutions are replacing fluoborate solutions for tin-lead plating of contacts on integrated circuits, surface-mount devices, radio-frequency components, and similar devices. The tin-lead MSA solution is well-established worldwide for rack, vibratory bowl, barrel, reel-to-reel, and especially high-speed cut-strip plating. Rack plating of components is being replaced where possible by semiautomated cut-strip lines.

Advantages. The MSA process is preferred over fluoborate solution for several reasons. First, it produces a better-quality, more uniform finish. For a typical specification of a coating thickness of 7 to 20 pm (300 to 800 pin.) with a composition of 80% Sn + 20% Pb ± 10%, it can maintain 6-sigma reliability (fewer than 3.4 rejects per million). MSA solutions are faster and have higher throwing power than fluoborate solutions, and they are able to produce a finer grain size. A recently developed, patented process is able to produce a semibright solderable finish. Because of low levels of occluded codeposited organic substances (<500 ppm C), coatings are suitable for soldering as-plated or after standard thermal excursions. Coatings produced from MSA solutions have excellent storage life and pass the bake and steam age solderability requirements of MIL-883, Method 2003.7. They also pass MIL-38510 requirements for surface finish on electronic components.

Second, the MSA process is environmentally more acceptable. MSA is less corrosive than fluoborate solutions, and because there is no boron or fluorine in the solution, it is more acceptable for wastewater treatment and water reuse. The electrolyte is safer to handle, and the MSA activator (10 to 20 vol%) can be recycled. The latter consideration offsets the higher initial cost and higher operating cost of MSA.

Third, the MSA process is easily automated. Cut strips of electronic components are loaded into high-speed plating equipment by magazine or cassette for easy handling. Deposition rates are two to five times higher than for fluoborate solutions, ranging from 5.4 to 21.5 A/dm2 (50 to 200 A/ft2). Table 1 lists deposition rates for a high-speed solution. There are high-speed fluoborate baths operating at from 500 to 1000 A/ft2.

Table 1 Time required to plate 2.54 pm (100 pin) of 80Sn-from high-speed methane sulfonic acid (MSA) solution

Total metal content

Time required at indicated current density, min

g/L

oz/gal

10.8 A/dm2 (100 A/ft2)

13.5 A/dm2 (125 A/ft2)

16.1 A/dm2 (150 A/ft2)

18.8 A/dm2 (175 A/ft2)

21.5 A/dm2 (200 A/ft2)

20

2.7

0.78

0.72

0.65

0.59

0.56

30

4.0

0.70

0.64

0.56

0.52

0.47

40

5.3

0.62

0.63

0.44

0.40

0.37

50

6.7

0.55

0.47

0.40

0.36

0.33

60

8.0

0.54

0.46

0.38

0.34

0.30

70

9.3

0.53

0.46

0.37

0.33

0.28

80

i0.7

0.52

0.45

0.36

0.32

0.27

90

i2.0

0.5i

0.43

0.35

0.31

0.26

i00

i3.4

0.50

0.42

0.34

0.29

Source: Ref i

Experimental modified MSA tin-lead plating solutions are being evaluated for semiautomatic plating of leaded-glass sealed integrated-circuit packages, with excellent results.

A minimum of 2% Pb in a tin deposit is reported to prevent whiskering, eliminating the need for reflow as required by MIL-38510 for integrated-circuit devices. The electrolyte will accommodate any tin-lead composition.

Automation. Commercially available automated process lines for electronic devices are loaded manually or from a magazine or cassette. The process operations include deflashing, deoxidation, activation, tin-lead plating, neutralization (if necessary), final rinse, hot deionized water rinse, hot air dry, and automatic reloading into the magazine or cassette. Rinses are pressure spray, with air knives to remove the maximum water for recovery and reuse.

At the time of this writing, equipment for electronic components is limited to strips 23 cm (9 in.) long and 6.4 cm (2.5 in.) wide. A contact rail 3 mm (0.12 in.) wide for light strips and 5 mm (0.20 in.) wide for heavy strips is necessary for gripping the top edge of the strip.

Substrates. The MSA process is generally used for electronic solderable leads on integrated circuits, surface-mount devices, radio-frequency components, and similar devices. The materials used are typically copper or alloy 42 (42Ni-58Fe). Silicon and zirconium copper alloys require special, proprietary preparations for plating.

Properties of the Coated Surface. The deposit usually has a smooth matte finish. Some newly developed solutions give a semibright finish with dense deposits and excellent solderability.

Health and Safety Considerations. Local exhaust is required for fumes from the electrolyte. This is generally provided in the commercial equipment. Chemical goggles, a face shield, rubber gloves, and an acid-resistant apron should be worn when handling the electrolyte. Ordinarily, exhaust fans eliminate the problem of lead fumes in the air (Ref 2), and there should be no problem with meeting Occupational Safety and Health Administration requirements OSHA Standard 1910.1025.

Parameters. Any desired tin-lead alloy composition can be plated from an MSA solution. The composition of the deposit depends on the amount of stannous tin and lead in the solution, the type and amount of addition agent, the current density, and the tin-lead content of the anodes (usually 85Sn-15Pb for high-speed plating). Solution temperature and degree of agitation affect composition, especially in high-speed solutions.

Multivariable fractional factorial experiments have shown that the 80Sn-20Pb (80/20) coating has a slightly higher solderability quality than 90/10 or 60/40 compositions, and much better than rack 100% Sn.

Solution Components. Concentrated solutions of stannous and lead nonfluoborates, MSA, and additives are available commercially, so alloy plating solutions are made by mixing and diluting concentrates. Additives are wetting agents, antioxidants, and fungicides.

80Sn-20Pb high-speed MSA solution (Ref 3) is used for high-speed automated plating of leads for electronic components, including dual-inline-pin and surface-mount integrated circuits, radio-frequency components, and similar devices where highly consistent coating thickness, surface finish, and solderability are required, along with high production rates. The solution composition is:

• Mixed nonionic surfactant: 5 to 10 g/L

The operating conditions are:

• Current density: 10.8±5.4 A/dm2 (100±50 A/ft2) with less than 1% current ripple on the plating rectifiers

• Agitation: very rapid, mechanical pump and hydraulic pressure

Filtration: continuous through polypropylene or acrylic filter cartridges

80Sn-20Pb MSA solution (Ref 4) is used for barrel plating of electronic components and applications requiring higher throwing power than the high-speed solution. The solution composition is:

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