Stripping of Lead

Table 5 identifies solutions and operating conditions for stripping lead from steel. Method C, at about 16 °C (60 °F), strips 25 ^m (1 mil) of lead in 6 or 7 min with very slight etching of the steel. With Method B, voltage increases suddenly when the lead coating has been removed; at room temperature and 9.3 A/dm2 (92 A/ft2), the voltage may be about 2.7 V during stripping, but increases to 4.6 V when stripping is complete.

Table 5 Solutions and operating conditions for stripping lead from steel

Method A

Sodium hydroxide

100 g/L (13.4 oz/gal)

Sodium metasilicate

75 g/L (10 oz/gal)

Rochelle salt

50 g/L (6.7 oz/gal)

Temperature

82 "C (180 "F)

Anode current density

1.9-3.7 A/dm2 (18.5-37 A/ft2)

Method B

Sodium nitrite

500 g/L (67 oz/gal)

pH

6-10

Temperature

20-82 "C (68-180 "F)

Anode current density

1.9-18.5 A/dm2 (18.5-185 A/ft2)

Method C(a)

Acetic acid (glacial)

10-85 vol%

Hydrogen peroxide (30%)

5 vol%

Method D(a)(b)

Fluoboric acid (48-50%)

4 parts

Hydrogen peroxide (30%)

1 part

Water

2 parts

(a) Formulations should be made up fresh daily.

(b) Alternate method for stripping lead or lead-tin deposits. Work must be removed as soon as the lead is stripped; otherwise, the base metal will be attacked.

With the solutions used in Method A or B, a stain occasionally remains on the steel after stripping. The stain can be removed by immersion for 30 s in the solution used in Method C, leaving the steel completely clean and unetched (unless the nitrate solution of Method B was used at less than about 2 V).

Silver Plating

Alan Blair, AT&T Bell Laboratories

ELECTROPLATED SILVER--which was developed primarily for use on holloware, flatware, and tableware--has proven its usefulness in both decorative and functional applications in both engineering and electrical/electronic applications. Decorative applications of silver plating still predominate; however, silver has been successfully substituted for gold in some functional uses in electronics. Its greatest success has been the virtually complete replacement of gold on metallic leadframes, the devices that support the majority of silicon chips. Here the development of new silicon-to-silver bonding techniques and ultimate encapsulation of the silver allow for the replacement of a much more expensive precious metal without loss of performance. In electrical contact applications, where the long-term integrity of the surface is of paramount importance, silver has been less successful as a gold substitute due to its tendency to form oxides and sulfides on its surface and the resultant rise in contact resistance. Silver has been employed as a bearing surface for many decades. It is particularly useful where the load-bearing surfaces are not well lubricated (e.g, in kerosene fuel pumps on gas turbine engines.)

Solution Formulations. The first patent concerning electroplating was filed in 1840 and reported a process for plating silver from a cyanide solution. To this day, silver is plated almost exclusively with cyanide-based solutions, despite the considerable research effort that has been expended on evaluating less toxic alternatives. A formulation for such a solution is given in Table 1. This type of electrolyte would be used for plating decorative or functional deposits of silver in a conventional way (i.e., on a rack or in a barrel). It is possible to produce fully bright deposits that require no further buffing or polishing. This is achieved by including a brightening agent in the solution formula, (one of several sulfur-bearing organic compounds, or selenium or antimony added as soluble salts). Antimony containing silver deposits are harder than pure silver. A typical antimony content might be 0.1 to 0.2% by weight. However, it should be noted that antimony content will vary with the current density employed during deposition; lower current densities will produce a deposit with higher antimony content.

Table 1 Plating solutions for silver

Component/Parameter

Rack

Barrel

Silver as KAg(CN)2, g/L (oz/gal)

15-40 (2.0-2.5)

5-20 (0.7-2.5)

Potassium cyanide (free), g/L (oz/gal)

12-120 (1.6-16)

25-75 (3.3-10)

Potassium carbonate (min), g/L (oz/gal)

15 (2.0)

15 (2.0)

Temperature, °C (°F)

20-30 (70-85)

15-25 (60-80)

Current density, A/dm2 (A/ft2)

0.5-4.0 (5-40)

0.1-0.7 (1-7.5)

Anodes of pure silver are readily soluble in the excess or "free"cyanide of these solutions. Carbonate is a natural byproduct of atmospheric oxidation of cyanide, but this adds to the solution conductivity, and some carbonate is included when preparing a new solution. Silver metal concentration is normally maintained by anode dissolution, but occasional small additions of the metal salt may be needed. This is facilitated by adding either silver cyanide (80% silver) or potassium silver cyanide (54% silver, sometimes referred to as the double salt). Additions of the former will lower the free cyanide concentration, whereas additions of the double salt will not.

Silver is usually more noble than the metal over which it is being plated, and because of this it has a tendency to form "immersion deposits." These are poorly adherent films of silver that form due to a chemical reaction between the base metal substrate and the silver ions in solution before true electrodeposition can commence. In order to avoid this phenomenon a silver strike should always be used. (A strike is a low-concentration bath operated at high cathode current density.) The following gives a typical silver strike solution formulation.

Component/Parameter

Value

Silver, as KAg(CN)2, g/L (oz/gal)

1.0-2.0 (0.13-0.27)

Potassium cyanide (free), g/L (oz/gal)

80-100 (10-13)

Potassium carbonate (minimum), g/L (oz/gal)

15 (2.0)

Temperature, °C (°F)

15-25 (60-80)

Current density, A/dm2 (A/ft2)

0.5-1.0 (5-10)

Stainless steel anodes should always be used in a silver strike solution to avoid an increase in silver metal concentration.

High-speed, selective plating of leadframes or similar electronic components requires the use of extremely high current densities and short plating times. Typical thicknesses range from 1.5 to 5.0 pm deposited in less than 2 s. Under these conditions, solutions containing free cyanide decompose very rapidly, the cyanide polymerizes and codeposits through electrophoresis, and the deposits cease to provide the desired properties. Solutions that use phosphate or nitrate salts as conducting media and use insoluble platinum or platinized titanium or niobium anodes have been developed to meet this requirement. Silver is present as potassium silver cyanide, and its concentration must be maintained by making periodic additions of this double salt. Careful attention must be paid to buffering because of the tendency to produce low pH values at the insoluble anodes. If this occurs, an insoluble silver salt will rapidly coat the anode and plating will cease. A typical formula is shown below.

Component/Parameter

Value

Silver, as KAg(CN)2,g/L (oz/gal)

40-75 (5-10)

Conducting/buffering salts,g/L (oz/gal)

60-120 (8-16)

pH

8.0-9.5

Temperature, °C (°F)

60-70 (140-160)

Current density, A/dm2(A/ft2)

30-380 (275-3500)

Noncyanide formulas that have been reported include those based on simple salts such as nitrate, fluoborate, and fluosilicate; inorganic complexes such as iodide, thiocyanate, thiosulfate, pyrophosphate, and trimetaphosphate; and organic complexes such as succinimide, lactate, and thiourea. A succinimide solution and a thiosulfate/metabisulfite solution have been commercialized, but the volumes used are very small compared with the cyanide solutions.

Specifications. Federal specification QQ-S-365D gives general requirements for silver plating. Using this specification it is possible to define the type of finish needed: matte (type I), semibright (type II), or bright (type III), and with chromate film for added tarnish resistance (grade A), or with no film (grade B). A minimum thickness of 13 ^m (0.0005 in.) is required for functional coatings.

ASTM B 700 specifies electrodeposited coatings of silver for engineering uses and defines purity (types 1, 2, and 3: 99.9, 99.0, and 98.0%, respectively); degree of brightness or mechanical polish (grades A, B, and C: matte, plated bright, and mechanically polished, respectively); and absence or presence of a chromate film (class N or S). Thickness must be specified by the purchaser.

The aerospace industry refers to four aerospace material specifications: AMS 2410G, AMS 2411D, and AMS 2412F, each of which applies to specific undercoats and bake temperatures; and AMS 2413C, which defines requirements for silver and rhodium plating on microwave devices.

International standard ISO 4521 defines silver coatings on metallic and nonmetallic substrates. Thicknesses are not specified but preferred thicknesses are quoted.

Users of silver plating for decorative purposes will find guidance in "Guides for the Jewelry Industry," originally issued by the Federal Trade Commission.

Gold Plating

Alfred M. Weisberg, Technic Inc.

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