Electrical conduction stability of copper-filled adhesives in high-moisture environments

<p dir="ltr">Isotropic Conductive Adhesives (ICAs) are widely used as a substitute for solders and as conductive inks in order to create circuit patterns for printed electronics (such as sensors) and electrical interconnects to components, because their processing is low temperature and compatible with a range of substrates. Good electrical conduction is achieved through the inclusion of a high volume fraction of the filler particles, to ensure a large number of particle to particle contacts, but also requires the contacts between particles to be of low resistance. Finding low-cost substitutes for silver (Ag) based fillers has become a trend due to its high cost and toxicity, and the much lower cost of copper (Cu) combined with a similarly high bulk conductivity is attracting more attention as a replacement in ICAs. However, Cu surfaces can form non-conductive oxides that inhibit their use without appropriate pre-treatment. Self-Assembled Monolayer (SAM) coated Cu filled ICAs have been produced and shown to have similar electrical conductivity to Ag filled materials. However, in previous studies, the long-term reliability of the Cu-ICAs was found to be relatively poor compared to Ag-ICAs, and an incomplete understanding of the stability of the Cu-ICAs restricts their further development and commercial exploitation.</p><p dir="ltr">In this research, copper powder was modified with an ODT (Octadecanethiol) oxidation-prevention protective coating, and then mixed with either 1- or 2-part epoxy resins to make ICAs that were printed and subsequently thermally cured in an Argon atmosphere. The degradation of the electrical conductivity, microstructure and surface composition of the cured Cu filled ICAs in different accelerated aging environments were studied to determine their stability and a typical commercial Ag filled conductive adhesive was also studied for comparison. In addition, the influence of the protective coating process, the initial oxidation state of the modified copper fillers, adhesive resin type, the ratio of filler/resin, and curing conditions on the properties of the Cu filled conductive adhesives were also investigated.</p><p dir="ltr">The results of X-ray Photoelectron Spectroscopy (XPS) demonstrated that the SAM coating was successfully applied to the Cu particle surfaces, leading to low levels of oxide and restricted their re-oxidation. Cu-ICAs made with both epoxy matrices achieved similar electrical conductivities to commercial Ag-ICAs. Both the Cu-ICAs and Ag-ICAs were found to be electrically stable in room temperature ambient storage, but only the Ag-ICAs were electrically stable in high moisture environments. The electrical conduction of 1-part epoxy Cu-ICAs degraded more rapidly than 2-part epoxy Cu-ICAs when stored in deionised water at room temperature (RT-DIW). Scanning and Transmission Electron Microscopy (SEM and TEM) were used to examine the effects of DIW storage on the morphology, internal structure and composition of Cu-ICAs, along with their surface composition (from XPS). A number of electrical conduction and failure mechanisms of the SAM coated Cu filled conductive adhesives are proposed and discussed. The formation of Cu (I) oxides on the Cu particles in ICAs during storage in RT-DIW were believed to be the main reason for the initial conductivity degradation and thick non-conductive Cu (II) oxides were shown to be the main cause of conductivity failure.</p>