Effects of curing agents on the conductivity of isotropical conductive adhesives (ICAs)

Curing agents greatly influence the electrical conductivity of Epoxy resin/Ag composed isotropic conductive adhesives (ICA). In this paper, three kinds of latent curing agents-dicyandiamide (DICY), boron-amine complex (594) and imidazole derivate (MZ) were used for formulating one-package epoxy-based ICA, respectively. The investigation showed that the 70% silver-containing conductive adhesive with imidazole derivate as curing agent has excellent conductivity and its bulk resistivity was 5 x 10(-4) Omega.cm. boron-amine complex also show good conductivity in the same formula. However, the ICA with DICY as curing agent gave poor conductivity performance. The different curing mechanisms were linked to the different conductivity of ICAs cured by different agents. In the ICA with DICY as curing agent, the agent has two strong polar N and -CN group, it has strong molecular adhesion to silver. Basically one DICY molecule can link 4 epoxy groups. During the curing process, the active functional groups (-CN, -N and -OH) link with silver and possibly form a layer of dense polymer capsulate covering on the surface of silver. This polymer capsulate is the key reason to increase the tunnelling resistance. So the ICA with DICY as curing agent gave poor conductivity. When use imidazole derivant or boron-amine complex as curing agent, which follows the anion or cation-catalytic-polymerization curing mechanism, the curing products were polyether gloss network. In such situation, the interaction between resin and silver is weak, therefore decreases the tunnelling resistance, thus can achieve good conductivity.