Indium Electrodeposits

Indium electrodeposits provide excellent solderability, low electrical contact resistance, friction resistance, and atmospheric corrosion resistance when plated on aluminum, copper-base alloys, and steel, which are typically selected for their engineering properties. Indium can be readily electrodeposited from either acid or alkaline solutions. It is particularly useful for coating aluminum and other amphoteric metals; its alkaline corrosion resistance provides a wider measure of corrosion protection for these metals than that provided by cadmium, tin, or zinc.

Indium can be plated without special apparatus. Any shop or laboratory that has plating equipment can set up an indium plating tank without costly equipment. Any technician familiar with the plating of silver, copper, and so on finds indium plating quite easy to handle. However, barrel plating of small, lightweight items (e.g., ring lugs, wire terminations, and threaded fasteners and washers) may present a problem on occasion. This type of part may cold weld during the tumbling action of the barrel and end up as a solid indium-plated mass. The problem is easily overcome by adding gelatin or glue to the bath to increase its viscosity.

Plating Baths. The four most commonly used indium plating baths are indium cyanide, indium fluoborate, indium sulfamate, and indium sulfate. Table 1 compares these processes. The details of the processes are shown in Tables 2, 3, 4, and 5.

Table 1 Comparison of indium plating baths

Parameter

Bath salt

Cyanide

Fluoborate

Sulfamate

Sulfate

Throwing power

Excellent

Good

Excellent

Poor

Quality of plate

Excellent

Good

Excellent

Passable

Ease of solution analysis

Difficult

Easy

Easy

Easy

Critical temperature

No

21-32 °C (70-90 °F)

No

Controlled

Color of solution

Clear

Clear

Clear

Clear

Wettability

Easy

Difficult

Easy

Difficult

Anode

Insoluble

Indium

Indium

Indium

Cathode efficiency

40-50%

40-50%

90%

30-70%

Tendency to pit

No

No

No

Yes

Control of solution

Cyanide and metal

Metal and pH

Metal and pH

Metal and pH

Table 2 Indium cyanide plating bath

Constituent or parameter

Value or condition

Indium as metal

33 g/L (4.4 oz/gal)

Dextrose

33 g/L (4.4 oz/gal)

Total cyanide (KCN)

96 g/L (12.7 oz/gal)

Potassium hydroxide (KOH)

64 g/L (8.5 oz/gal)

Temperature (static)

Room temperature

Cathode efficiency

50-75%

Anodes

Plain steel

Throwing power

Excellent

Quality of plate

Excellent

Ease of solution analysis

Difficult

Critical temperature (working)

None, with or without agitation

Color of solution

Clear, pale yellow to dark amber

Wettability

Easy

Tendency to pit

None

Control of solution

Cyanide and metal by additions

Use

General

Current

162-216 A/m2 (15-20 A/ft2)

pH

High

Notes: (1) Because insoluble anodes are used, it is necessary to replace the indium metal content of this alkaline bath. Under normal conditions, addition of cyanide will not be required; however, it is best to keep the cyanide concentration at about 100 g/L (13.4 oz/gal) for efficient operation. (2) Plating efficiency of the bath will be maintained within a range suitable for normal plating until the indium content is reduced. The plating rate should be checked at regular intervals, because as the bath is depleted a decrease in rate of deposition is to be expected.

Table 3 Indium fluoborate plating bath

Constituent parameter

Value or condition

Indium fluoborate

236 g/L (31.5 oz/gal)

Boric acid

22-30 g/L (2.9-4.0 oz/gal)

Ammonium fluoborate

40-50 g/L (5.3-6.7 oz/gal)

pH (colorimetric)

1.0

Temperature (static)

21-32 °C (70-90 °F)

Cathode efficiency

40-75%

Anode efficiency

Indium, 100%

Throwing power

Good

Quality of plate

Good

Ease of solution analysis

Easy

Critical temperature (working)

21-32 °C (70-90 °F), with or without agitation

Color of solution

Clear

Wettability

Difficult

Tendency to pit

None

Control of solution

Metal and pH

Use

Experimental

Current density

540-1080 A/m2 (50-100 A/ft2)

Notes: (1) The pH of this bath is controlled by the addition of 42% fluoboric acid. (2) Some insoluble anodes (platinum or graphite) should be used because the anode and cathode efficiency are not in good relation.

Table 4 Indium sulfamate plating bath

Constituent or parameter

Value or condition

Indium sulfamate

105.36 g/L (14 oz/gal)

Sodium sulfamate

150 g/L (20 oz/gal)

Sulfamic acid

26.4 g/L (3.5 oz/gal)

Sodium chloride

45.84 g/L (6 oz/gal)

Dextrose

8.0 g/L (1 oz/gal)

Triethanolamine

2.29 g/L (0.3 oz/gal)

PH

1-3.5(a)

Temperature (static)

Room temperature

Cathode efficiency

90%

Anode efficiency

Indium, 100%

Throwing power

Excellent

Quality of plate

Excellent

Ease of solution analysis

Easy

Critical temperature (working)

None, with or without agitation

Color of solution

(b)

Wettability

Fairly easy

Tendency to pit

None

Control of solution

Metal and pH(a)

Use of solution

Experimental

Current density

108-216 A/m2 (10-20 A/ft2)(c)

(a) 1.5-2 preferred. The pH of this bath is controlled by the addition of sulfamic acid.

(b) Clear when new; after use will darken due to organic material breakdown. This has no effect on deposit. Filtering of bath can be done through activated charcoal to maintain clarity of bath.

(c) Optimum. If metal is increased, current density can be increased up to 1080 A/m2 (100 A/ft2).

Table 5 Indium sulfate plating bath

Constituent or parameter

Value or condition

Indium (as sulfate)

20 g/L (2.67 oz/gal min)

Sodium sulfate

10 g/L (1.3 oz/gal)

pH

2.0-2.5

Temperature (static)

Room temperature

Cathode efficiency

30-70%

Anode efficiency

Indium, 100%

Throwing power

Poor

Quality of plate

Passable

Ease of solution analysis

Easy

Critical temperature (working)

Controlled, with or without agitation

Color of solution

Clear

Wettability

Difficult

Tendency to pit

Yes

Control of solution

Metal and pH

Use

Experimental

Current density

216-432 A/m2 (20-40 A/ft2)

Notes: (1) The pH of this bath is controlled by the addition of sulfuric acid or sodium hydroxide as needed. (2) Some insoluble anodes (platinum or graphite) should be used because the anode and cathode efficiency are not in good relation.

Diffusion Treatment. The plating of indium on a clean, nonferrous surface does not necessarily end the operation. For some applications, such as bearing plating, the indium deposit is diffused into the base metal, forming a surface alloy. This is accomplished by placing the plated part in an oven or hot oil bath and heat treating it for about 2 h at a temperature slightly above the melting point of indium. Indium melts at 156.7 °C (314.1 °F), and the diffusion treatment is carried out at about 175 °C (350 °F). The processing time may be shortened by increasing the temperature, but only after the diffusion has actually begun. Failure to observe the proper temperature at the beginning of the diffusion process may lead to the formation of surface bubbles or droplets of indium, which are undesirable, particularly on a decorative finish. A number of factors govern the depth of diffusion:

• The amount of indium plated on the surface

• Temperature of heat treatment

• Time of diffusion treatment

• The diffusion coefficient for indium in the base metal Indium Alloy Electrodeposits

A variety of indium alloy deposits have been reported in the literature. Included are alloys with antimony, arsenic, bismuth, cadmium, copper, gallium, lead, tin, and zinc. Of these, only indium-lead has had any degree of commercial importance.

Indium-lead electroplated alloy was developed as an improvement over the diffusion alloy that is formed by plating a thin layer of indium over lead on lead-containing bearings and diffusing the indium into the lead in a hot, 150 °C (300 °F) oil bath. The alloy reduces the corrosion of the lead-containing bearings by lubricating oils. An alloy containing an average of about 4% In had high resistance to corrosion and was harder and had better antifriction properties than lead. However, the composition of the thermally diffused alloy was nonuniform. The electrodeposited indium-lead alloy provided greater uniformity of composition and showed only one-fourth the corrosion compared to the thermally diffused alloy.

Plating Baths. The two most successful indium-lead plating baths are indium-lead fluoborate and indium-lead sulfamate. Table 6 compares these processes. The details of the processes are shown in Tables 7 and 8.

Table 6 Comparison of indium-lead plating baths

Parameter

Bath salt

Fluoborate

Sulfamate

Indium content of deposit

11%

5%

Microhardness of deposit

2.5 kg/mm2

Table 7 Indium-lead fluoborate plating bath

Constituent or parameter

Value or condition

Indium fluoborate

25 g/L (3.4 oz/gal)

Lead fluoborate

90 g/L (12.0 oz/gal)

Free fluoboric acid

15 g/L (2.0 oz/gal)

Glue

1.5 g/L (0.2 oz/gal)

Current density

100-300 A/m2 (9-28 A/ft2)

Temperature

20 °C (70 °F)

Table 8 Indium-lead sulfamate plating bath

Constituent or parameter

Value or condition

Indium sulfamate

20 g/L (2.67 oz/gal)

Lead sulfamate

1 g/L (0.13 oz/gal)

Soluble coffee(a)

5 g/L (0.67 oz/gal)

PH

1.5

Current density

100-300 A/m2 (9-28 A/ft2)

(a) Regular instant coffee powder

(a) Regular instant coffee powder

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