Electrical property of anisotropically conductive adhesive joints modified by self-assembled monolayer (SAM)

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 4, 4´-thiodibenzenethiol (MPS), 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. MPS gave mixed results because it could align in various configurations on the metal surfaces due to its flexible molecular structure and the orientation on this molecule. The ODT and MPS 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 four metal surfaces are unstable at 150°C and most of these coatings are degraded after 2 hrs. 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 (IV) measurement, it was found that the SAM treated ACA joints showed a lower resistance at the same applied current than non-treated joints.