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Answer: Electrically conductive adhesives are made up of a polymer resin that is filled with conductive particles. The resin provides a mechanical bond between two substrates, while the conductive filler particles provide the desired electrical connection.
The answer to your question really depends on the level of conductivity you require. The most conductive cost-effective metals are silver and copper. Silver is generally used where high conductivity (low resistivity) is required and there is a possibility of atmospheric aging. Silver is unique in that not only is the metal highly conductive, but its oxide is electrically conductive as well. This is in contrast to copper, which can start off as highly conductive but loses its conductivity on oxidation.
Nickel can also be used, but it is not as inherently conductive as silver or copper and it is typically used for EMI and RF shielding applications rather than current-carrying applications. The conductive metals are used at high loadings (typically 60-80%), and they are used in flake form rather than spheres to ensure good particle contact. To lower the cost of your formulations, it is possible to use flakes made of copper or nickel that is coated with silver.