Fine-pitch chip-on-flex packaging of optoelectronic devices using low temperature optodic bonding

With the growing demand for mechanically flexible circuits or systems in consumer electronics, e.g. mobile phones, portable computers, TFT displayers, RFID tags etc., chip-on-flex packaging has been developed to a great extent and is increasingly being implemented in a variety of applications. High-performance colored polymer films, such as polyimide, are most commonly employed as flex substrates. However, taking the feasibility for a wide range of optical applications and a low-cost procurement into account, transparent polymer foils, such as polymethyl methacrylate (PMMA), exhibit a greater prospect. A major challenge in using these polymer foils as flexible carriers lies in the thermal loading control depending on their low glass transition temperatures (Tg). Thus, we propose flip chip-based optodic bonding for chip-on-flex packaging by employing these cost-effective transparent polymer foils. In this bonding technology, conventional thermal loading is minimized since UV curable adhesives are used as bonding materials and an optode is correspondingly adopted. Additionally, the fine-pitch circuits on the flex substrates are designed according to the chips and fabricated by screen printing technology using silver nanoparticles. Considering the effects on the bonding performance, which result from the rheological behavior of the adhesives during dispensing and applying, contact angles between the adhesives and polymer foils are measured for determining the appropriate volume. In order to protect the silver interconnects as well as the applied adhesives from oxidation and thus attaining a long-term stability in respect to the electrical conductivity, a functionalized coating against ambient moisture is implemented onto the flex circuit. Through the performance tests of the final packages for a test chip and a bare optoelectronic component, this novel technology is verified as a promising solution for fine-pitch chip-on-flex packaging.