Ultrasonic Anisotropic Conductive Films (ACFs) Bonding of Flexible Substrates on Organic Rigid Boards at Room Temperature

In this study, a novel anisotropic conductive film (ACF) bonding method for flexible substrate-organic rigid board bonding was investigated using an ultrasonic vibration. And the ultrasonic (U/S) ACF bonding was characterized in terms of adhesion strength, electrical continuity in comparison with those of thermo-compression (T/C) bonding. An ultrasonic bonder was used to produce ultrasonic vibration in ACF joints between flexible substrates and organic rigid boards. Test boards were 25 um-thick polyimide based flexible substrates and 1 mm-thick FR-4 organic rigid boards. The effect of process conditions, such as U/S powers, bonding pressures, and bonding times on the ACF temperature were investigated. The in-situ temperature at the ACF layer was measured during U/S bonding to determine the relation between the ACF temperature and the bonding characteristics of ACF joints. The optimized U/S bonding time was obtained within 5 sec at room temperature. The significant meaning of this result is that the ACF bonding process times can be remarkably reduced by U/S bonding compared with conventional 15 sec T/C bonding at 190degC. The ACF joints with the optimized U/S bonding conditions showed similar bonding characteristics as those with T/C bonding in terms of the adhesion strength and the daisy-chain contact resistance. The fourier transformation infra-red (FTIR) spectroscopy showed that the ACF degree of cure was achieved 90 % at 3 sec and 95 % at 4 sec. Reliability test results showed that the U/S bonded test boards had stable daisy-chain contact resistances in 85degC/85 % RH test and 125degC high temperature storage test for 1000 hours and -55degC~125degC thermal cycling test for 1000 cycles.