Indium Plating
INDIUM is a soft, low-melting-point, silvery white metal with a brilliant metallic luster and a color resembling that of
platinum. It alloys with most other metals to form a series of unique alloys, many of which are used as solders. It is soft
enough to be readily marked by light fingernail pressure. Indium can be easily extruded at very low pressures: solders
containing 50% In can be extruded as 1 mm (0.04 in.) wire at a pressure of 83 MPa (12 ksi). The hardness of indium is 0.9 to 1.0 on the modified Brinell scale, and it has a melting point of 156.7 °C (314.1 °F), a boiling point of 2000 °C
(3632 °F), and a low vapor pressure.
Indium is ductile, malleable, crystalline, and diamagnetic. The pure metal gives a high pitched "cry" when bent. It wets
glass and finds application in low-melting alloys and solders. It is used in making alkaline batteries, automotive trim,
bearing alloys, electronic assemblies, germanium transistors, photoconductors, rectifiers, thermistors, vacuum seals, and
group III-V compound semiconductors such as indium phosphide and indium arsenide. When rubbed together, two
indium-plated parts will "cold weld" (autogenously join). This can be easily accomplished with freshly plated parts, but as
surface oxides build up with time, more vigorous rubbing is required. This cold welding phenomenon is being explored
for use in the surface mount technology of the electronics industry. Indium is electropositive to iron and steel and
electronegative to tin. In an aqueous 3% sodium chloride solution of pH 6.7 to 7.2, indium has a half-cell static potential
of -0.56 V referenced to that of a silver electrode given the value of zero. This places indium between cadmium and tin in
the electromotive series of metals, which is used by materials and design engineers to identify and avoid potential
galvanic corrosion problems.
Indium is particularly useful in making reliable electrical contact to aluminum. When indium-plated steel wire terminals
are secured to aluminum, the high-resistance surface aluminum oxide cracks under the pressure and the indium extrudes
into the oxide cracks, making direct metal-to-metal contact with the underlying aluminum. This application, which was
widely used in the telephone industry, has diminished in use with that industry's switch to fiber optics. However, it is used
for aluminum wire terminals in the electronics industry, particularly where the use of terminal fluids is undesirable. One
relatively new use is for the plating of steel internal dished-tooth star-washer-ring-lug terminals for attachment to
aluminum capacitors.

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