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Monday, December 22, 2008

Electrically conducting adhesives

Incorporating electrically conducting particles in an epoxy adhesive represents a special use of filler. The idea is not new; other conducting cements-thermoplastic as well as thermosetting-are marketed, but more recently there has been an increasing interest in epoxy conducting adhesives.

The function of electrically conducting adhesives is really the same as that of metal solder, to bond and provide electrical contact. But because epoxy adhesives are more versatile, they are able to bond a wide range of metals, and also dissimilar materials, including porcelain and most plastics; one example is the joining together of two conductors, while at the same time bonding them to an isolator.

The common conducting fillers are silver and copper, including silver-plated copper and in some special cases, gold. The amount of metal filler added depends on particle size and shape. At least a part of the filler is usually in the form of flake, and in this form it is generally considered that a smaller amount is needed; but even so, the quantity is large if good particle-to-particle contact and low resistance is to be obtained. Additions of two or three times the weight of resin are not uncommon. Volume resistivity of from 0.01 to 0.001 ohm/cm and shear strength in bonding steel-to-steel of 3,200 p.s.i. have been quoted.

Normal hot or cold curing agents may be used although it is evident that some must be superiors to others from an electrical standpoint. At least one epoxy conducting solder is supplied as a two-tube package.

The whole subject of incorporating metal-fillers in plastics has been dealt with in a book by DELMONTE.