Abstract
Due to the strong demand of electronic products with light-weight, flexibility and portability and rollability from consumers, the development of flexible electronics with flexible interconnects is vigorously underway. Flexible electronics is a technology that extends electronics devices beyond the rigid form factor. It is highly recognized that it can bring a very vigorous drive toward the new flourish of economic growth in the electronics industry. In the study, a flexible interconnect technology based on an ultra thin chip and a very thin flexible polyimide (PI) circuit substrate is first introduced. The electrical interconnects and also thermal-mechanical connections of the technology are formed through a piece of anisotropic conductive adhesive (ACA). It is thus termed ACA-typed ultra-thin-chip-on-flex (UTCOF) interconnect technology (i.e., ACA-UTCOF in abbreviation) throughout the study. The technology is promising and potential for a variety of flexible electronics applications, such as flexible display, paper-thin smart labels, particularly the RFID labels, miniaturized medical electronic systems, E-paper, E-label and memory chip stacking etc. Before the full and successful realization and implementation of the ACA-UTCOF technology, many technical challenges need to be resolved, including reliability and bendability. For example, the technology is very vulnerable to temperature and humidity variations because of the epoxy-based interconnect technology. Thus, the study aims at the investigation of the hygro-thermo-mechanical behaviors of the advanced ACA-UTCOF technology during high temperature and humidity conditions through numerical analysis and experimental validation. The considered hygro-thermal behaviors include moisture saturation time, moisture diffusion rate, moisture concentration, moisture diffusion coefficient, saturated moisture concentration. To achieve the goal, an ACA-UTCOF test vehicle is first constructed for subsequent testing and characterization, and the humidity properties of the PI substrate are experimentally determined by measuring the weight gain during moisture absorption. Using the derived humidity properties, three-dimensional (3D) moisture transient finite element (FE) analysis based on Fick law is carried out to calculate the moisture distribution and passage in the package. Subsequently, by the calculated moisture distribution, 3D hygro-thermo-mechanical FE analysis is performed to predict the strains/stresses behaviors of the package at high temperature and humidity conditions. At last, the effects of the stress relaxation behaviors of the ACA on the hygro-thermo-mechanical behaviors are assessed through nonlinear viscoelastic FE analysis. FE simulation results reveal that moisture diffusion into the ACA layer is mainly through the substrate rather than the periphery side of the ACA. In addition, the ACA-UTCOF test vehicle reaches moisture saturation after about twenty hours under an 85℃/85% relative humidity condition. Most importantly, from the experimental results, it is found that moisture would play a much more significant role in the interconnect reliability of the ACA-UTCOF technology than temperature.