AIM Specialty Materials Division

Due to its softness, indium is considered an ideal material to create a compression seal. AIM supplies indium preforms in various shapes including flat washers, “O” rings, square windows, discs and rectangles. Foil and wire are available for custom seal fabrications.

The Quality of an Indium Seal Depends On: The purity and cleanliness of the indium.

A minimum of 4Ns purity indium is preferred; some very critical applications may call for 5Ns purity. Heavy contamination with organic compounds should be removed by degreasing.  Surface oxide can be removed by using 50% hydrochloric acid (a 2-4 second dip) or acetone, followed by rinsing in deionized water.

Proper preparation of the surfaces to be sealed

Both seal surfaces and the indium should be kept as clean and dry as possible. Glass and ceramic surfaces to be joined must be cleaned with an appropriate cleaner.  Drying can be performed by passing the parts through a gas flame or in an oven.  For applications where extreme cleanliness is required, cleaning may be performed with electrical plasma or by exposure to ultra-violet light. Metal surfaces to be sealed can be bare, pre-tinned with indium, or plated with indium or silver.  Bare surfaces can be polished using abrasive powders or can be machine finished.  Machine finished surfaces should not be “improved” by using abrasive powders, cloths, etc.

Thickness and shape of the indium

The thickness of the indium may range from less than 0.2mm to 1-2 mm, depending upon the area of the surfaces to be joined and the compressive force on the seal. If the seal is positioned in an “O” ring groove, the seal should be sized so that it overfills the groove by five to fifteen percent when compressed. In some applications, the use of overlapping segments of indium can perform as well as a continuous washer. When making a seal out of indium wire, cut the wire to length, beveling the edges to make the joint.  The freshly cut edges will readily stick together. Correctly formed seals have leak rates of less than 2×10 -7 torr liter/sec -1

For further information contact AIM’s Specialty Materials Department.