Copper conductive adhesives for printed circuit interconnects

Isotropically conductive adhesives (ICAs) and inks are seen as suitable candidates for interconnecting components and for printed circuits. Silver (Ag) filled ICAs and inks are the most popular due to their high conductivity and good reliability. However, the price of Ag is a significant issue for the wider exploitation of these materials in low cost, high volume applications such as printed electronics. Copper (Cu) is considered as a more cost-effective filler with greater abundance. However, one of the major barriers to the implementation of Cu loaded ICAs is the poor oxidation resistance of Cu particles. The copper oxide is non-conducting and hence, the conductivity of untreated Cu loaded ICAs is limited. In this work, an efficient method for the removal of the copper oxide from copper powders and application of a protective organic coating to prevent re-oxidation was developed. The coating was found to be able to limit the re-oxidation of the copper before mixing and curing of the adhesive. The treated copper powder was combined with an adhesive resin and tracks were stencil printed onto glass substrates, and cured by heating under an inert atmosphere. The effect of different adhesive resins was investigated and the samples were characterised for electrical performance and microstructure. Tracks with electrical conductivity comparable to Ag filled adhesives were obtained. The cured materials showed good stability of resistivity during storage under ambient conditions, but when exposed to 85°C and 85% relative humidity, the resistivity of exposed tracks increased substantially after 12 hours storage and methods to reduce this were identified. Functional circuits were fabricated by printing a pattern of Cu paste, placing surface mount components into the wet paste and curing.