High speed touch screen panels (TSPs) assembly using anisotropic conductive adhesives (ACAs) vertical ultrasonic bonding method

Abstract

In this study, the effects of vertical ultrasonic (VUS) bonding parameters such as vibration amplitudes and bonding pressures were investigated and optimized in terms of thermal deformation of TSP polymer substrates, electrical continuity, and pull adhesion strength of anisotropic conductive adhesive (ACA) joints. And the reliability of VUS bonded TSP ACA joints were evaluated at various test conditions. As the vertical ultrasonic vibration was applied, ACA temperatures rapidly increased due to the spontaneous heat generation in the ACA itself and surrounding polymers. The ultrasonic vibration showed significant effects on the peak temperature of the ACA layers, and the bonding pressure affected the heating rates. By adjusting both ultrasonic vibration and bonding pressure, the ACA temperature could be successfully controlled. In terms of thermal deformation, VUS bonding showed no severe thermal deformation of TSPs up to 120°C which is much higher than the Tg of polyethylene terephthalates (PETs) which are the base material of TSPs. In terms of electrical continuity of the ACA joints, VUS bonded TSPs showed stable electrical resistances at 2 MPa bonding pressures, and there were no significant effects of vibration amplitudes and bonding times on contact resistances. At the same time, VUS bonded TSPs showed strong adhesion at the ACA joints with 1 second bonding time at 7.5 μm vibration amplitude and 2 MPa bonding pressure. During the FPCB pull test, VUS bonded TSPs showed higher than 3 kgf pull strengths. Therefore, VUS bonding parameters were optimized at 7.5 μm vibration amplitude, 2 MPa bonding pressure, and 1 second bonding time in terms of PET thermal deformation, electrical continuity and pull adhesion strength of the TSP ACA joints. With the optimized parameters, various reliability tests were conducted such as thermal shock test, salt spray test, high temperature/high humidity test, high temperature storage test and writing test. After each test, the VUS bonded TSPs showed no significant changes in electrical resistance compared with those of thermo-compression bonding. As a summary, the VUS method can be successfully used in TSPs assembly with no thermal damages, higher speed assembly and good reliability.