Electrical property enhancement of electrically conductive adhesives through Ag-coated-Cu surface treatment by terephthalaldehyde and iodine

Abstract

One of the largest obstacles for Ag based electrically conductive adhesives (ECAs), as an alternative for Pb-containing solders in electronic packaging, is that the conductivity of ECAs is lower than that of solders due to the limited physical contact area between/among conductive fillers and the insulated organic lubricant and metal oxide layers on the surface of the conductive fillers. What’s more, the high cost of Ag fillers is also restricting the wide use of Ag based ECAs. In this study, Ag-coated-Cu flakes were chosen as a substitute for Ag flakes to reduce the cost. At the same time, the coating of Cu with an Ag layer could protect the Cu-based fillers from oxidation and corrosion. A mixture of weak reducing agents and substituting agents was selected to treat the Ag-coated-Cu flakes to increase the conductivity of the Ag-coated-Cu based ECAs. During the treatment process, the weak reducing agents can reduce the metal oxides on the filler surfaces, enabling more metallic contacts. Meanwhile, the substituting agents can partially remove or replace the long chain fatty acid lubricants on the metal flakes, improving the electron tunneling between/among neighboring flakes. As such, the multiple effects of the reducing agents and substituting agents can improve the conductivity of the ECAs. By using an appropriate amount of terephthalaldehyde and iodine treated Ag-coated-Cu flakes, the resistivity was reduced to as low as 1.28 × 10⁻⁴ Ω cm for the ECA with 75 wt% content of treated fillers, which is comparable to that of commercially available Ag-filled ECAs (×10⁻⁴ Ω cm). This work suggests that a surface chemical method can enhance the electrical conductivity of metal filler based ECAs.