High productivity and damage-free ultrasonic anisotropic conductive film (ACF) bonding for touch screen panel (TSP) assemblies

In this study, bonding time for touch screen panel (TSP) assemblies were reduced by two third using an ultrasonic (U/S) horn which fits the shape of the entire TSP bonding area. Most TSPs have at least two bonding areas on the substrates with different heights due to structural design of touch sensing. Using conventional thermo-compression anisotropic conductive film (ACF) bonding, each bonding area should be separately bonded to perform suitable interconnections with uniform pressure and a temperature due to the step height among the bonding areas. However, in U/S bonding, the bonding area could be bonded all at the same time using the U/S horn which fits the bonding area. The test vehicles were capacitive TSPs which consist of three layers of polyethylene terephthalate (PET) substrates. The TSP had three separated bonding areas. Two of them were on the double layer PET substrate. The other one located in the middle of the whole bonding area was on the single layer PET substrate. Therefore, the middle bonding area was 210 μm lower than the other two bonding areas. This complicated TSP structure requires three separate ACF bonding using conventional ACF bonding method. When using a fitted U/S horn, the in-situ ACF temperatures for all bonding areas showed negligible deviation less than 5°C. After U/S bonding for 15 seconds at 2 MPa bonding pressure and 150°C ACF temperature, the adhesion strength of the ACF joint was higher than 650 gf/cm. No damage was observed on the electrode and the substrate. Also, the ACF joints had stable electrical continuities. In conclusion, U/S ACF bonding with fitted horn was successfully demonstrated for high productivity TSP assemblies.