Formation of self assembled monolayer (SAM) on metal surfaces for high performance anisotropically conductive adhesives

Abstract

To improve the electrical property of the anisotropically conductive adhesive (ACA) joints, self-assembled monolayer (SAM) compounds are introduced into the interface between the metal filler and the substrate bond pad. The formation of the SAM on various metal surfaces and their thermal stability are investigated by measuring the contact angles of SAM compounds with a hydrophilic or hydrophobic tail groups such as octadecanethiol (ODT) and mercpatoacetic acid (MAA) on Au, Cu, Sn and SnPb surfaces. Goniometer testing and grazing FTIR spectra demonstrate that SAM molecules are readily adhered to metal surfaces. The concentration of SAM solutions, immersing time, thermal treatment temperature and time were varied as experimental parameters. ODT adhered to copper surface the best, while gold was the best metal for MAA. The ODT SAM coatings on the metal surfaces used were thermally stable for 2 hrs at room temperature and 100 °C. However, they were slightly degraded at 150 °C. The MAA SAM coatings on the metal surfaces are unstable at 150 °C and are degraded after 2 hrs at 150 °C. The dithiol SAM compound (1,4-Benzenedithiol) was applied in ACA joints, where conductive fillers (polymer particles coated with gold) and Ni/Au bond pad surfaces were treated with the dithiol SAM compound. Epoxy resins with two different curing temperatures were used as polymer matrices of the ACA formulations, respectively. From the current-voltage (I-V) measurement, it was found that the SAM treated ACA joints showed a lower resistance at the same applied current than nontreated joints.