Development of a curable conductive copper paste in air

Silver is currently the most widely-used material for conductive adhesives applied to electronic components. However, silver has some problems such as price volatility and electromigration under high temperature and humidity. In recent years, copper has great attention as an alternative to silver because copper is much lower in manufacturing cost and more resistant to electromigration than silver. However, one of the drawbacks of copper is that it is easily oxidized in air. Thus, most of the copper pastes are cured under nitrogen atmosphere. In this study, we are going to report on development of conductive copper paste curable in air. The copper paste was prepared by a simple and facile method using copper powder (electrolyte copper powder) with resin (resol-type phenolic resin) and additives. Additives play an important role on the development. We examined tri-ethanolamine (TEA) and liquid fatty acid (e.g., oleic acid, linoleic acid and linolenic acid). The prepared copper paste was screen-printed on an alumina substrate, then cured at 210°C for 10 min in air to obtain cured copper product. The resistivity of the cured copper product with additives was less than 6.0 × 10 ^-5 Ω·cm. Without the additives, the resistivity was 7.1 × 10 ^-4 Ω·cm, it was higher than the case of with additives. When the additives were used individually in each sample, the one with oleic acid showed a resistivity of 2.9 × 10 ^-4 Ω·cm and the one with TEA showed 4.7 × 10 ^-4 Ω·cm, respectively. These results indicate that TEA and liquid fatty acid interact with each other in the copper paste, and then the resistivity of the cured copper product is decreased. We found the combination of these two additives leads to low resistivity without copper surface oxidation. Furthermore, the copper pastes using a liquid fatty acid and TEA had longer "pot life" than that of the other copper pastes. This finding offered a valuable insight to our development.