Fine Pitch Compliant Bump Interconnection for Flip chip on Flexible Display Packaging by Anisotropic Conductive Film

Today, there is an increasing demand for flexible liquid crystal display (LCD) in many applications such as mobile phone, smart cards, electronic newspaper and integrated display. The requirement for better display resolution and image quality necessitate the use of advanced packaging technology because of higher number of inputs/outputs to be interconnected. Flip chip technology can meet some key points of the demand. However, due to the temperature limitation of substrate material and manufacture process, the traditional flip chip technology are not feasible at all to be used in packaging of flexible display and smart card even though low cost, high density and high reliability are the advantages of flip chip process. To establish fine pitch driver IC packaging process with low temperature for flexible display and smart card, flip chip on high Tg plastic substrate with anisotropic conductive fdm (ACF) was studied in this paper. Bonding temperature was designed by thermal analysis such as DSC and peeling strength. Bonding pressure was also defined by relationship between contact resistance and applied loading. In addition, novel bump with compliant buffer layer was investigated for the thermo-compression process. Interestingly, lower contact resistance interface was obtained by using compliant bump bonding than traditional Au bump bonding due to the compliancy of the bump structure. Cross-section view from SEM was also studied to show the deformation relationship of conductive particles, plastic substrate and bump. Finally, the reliability tests of, 85degC /85% RH thermal humidity storage test (THST), and -40degC ~125degC thermal cycling test (TCT) were carried out for evaluating electrical properties. Increasing contact resistance after environment tests were mainly due to the loss of contact by thermal stress effect and moisture absorption by epoxy.