Conventional and microwave-assisted processing of Cu-loaded ICAs for electronic interconnect applications

Isotropically conductive adhesives (ICAs) and inks are widely used 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. In addition, there is a need to develop systems compatible with temperature-sensitive substrates through the use of alternative materials and heating methods. Copper (Cu) is considered as a more cost-effective filler for ICAs and in this work, Cu powders were treated to remove the oxide layer and then protected with a self-assembled monolayer. The treated Cu powder was combined with one of two different adhesive resins to form ICAs that were stencil printed onto glass substrates before curing. The use of conventional and microwave-assisted heating methods under an inert atmosphere for the curing of the Cu-loaded ICAs was investigated in detail. The samples were characterised for electrical performance, microstructure and shrinkage as a function of curing temperature (80–150 °C) and time. Tracks with electrical conductivity comparable to Ag-filled adhesives were obtained for both curing methods and with both resins. It was found that curing could be accelerated and/or carried out at lower temperature with the addition of microwave radiation for one adhesive resin, but the other showed almost no absorption indicating a difference in mechanism for the two formulations.