High performance anisotropic conductive adhesives for lead‐free interconnects

To study the interface properties of anisotropic conductive adhesives (ACAs) and improve the electrical properties of ACA joints as a replacement for Sn/Pb solder in the electronics industry.

In this study, different types of self‐assembled monolayer (SAM) compounds were introduced into the interface between the metal filler and the substrate bond pad. The formation of these SAMs on gold and silver surface and their thermal stability were investigated by measuring the contact angles with water of these SAM coated surfaces. The SAM compounds used had either hydrophilic or hydrophobic tail groups, and included octadecanethiol (ODT), mercpatoacetic acid (MAA), 1,4‐benzenedithiol (dithiol) and malonic acid (acid M). Epoxy resins with two different curing temperatures were used as polymer matrices for the ACA formulations. The electrical properties of ACA joints containing these SAMs were studied by measuring their current‐voltage relationship.

The results show that SAMs can be successfully coated onto specific metal surfaces, depending on the affinity of their functional groups with the specific metal. The SAM treated ACA joints show much lower resistance at the same applied current than non‐treated joints, and the effect on the low curing temperature epoxy matrices was more significant. Nano‐Ag filled ACAs show more significant improvements in their electrical properties due to the greater surface area and higher surface energy of nano‐particles and consequently their higher thermal stability when coated with SAMs.

This paper is the first to use functional organic monolayers to enhance the interface properties of electrically conductive adhesives and, in particular, for ACAs.