Investigation of ultrasonic flip chip bonding on flex substrates

Most commonly used flip chip on foil technique for display applications is based on Anisotropic Conductive Film (ACF). In this technique, ACF is laminated onto the substrate before an IC is aligned and placed under high temperature. The ACF is then cured under pressure. Conductive particles trapped between the bumps of the IC and Cu tracks on the substrate forms electrical connections; while the adhesive ensures mechanical integrity. With the pitch becomes smaller and smaller, ACF technique is no longer capable to provide reliable connections due to the limitation in particle size and density and thus high risks in. open and short. Ultrasonic flip chip is developed as one of the potential techniques for ultra-fine pitch COF applications. In this technique, ultrasonic energy is applied through the bonding head to the bump/track interfaces. The relative moment results in localized high temperature which promotes the welding of two surfaces, to form mechanical and electrical connections. In this paper, the experimental results on ultrasonic flip chip on foil will be reported Au bumped test ICs were used The bumps were in rectangular shape with the length of typically 80/spl mu/m. The smallest pitch of the test ICs was 40/spl mu/m. Two layer adhesiveless polyimide based flex circuit substrates were de signed and used as the test vehicles for the investigation. Ni/Au were plated on the Cu tracks. The bonding process was optimized to achieve reliable interconnections. The critical parameters in ultrasonic bonding include ultrasonic energy, pressure, stage temperature, time and sequence of underfilling. High assembly yield was achieved with the optimized parameter settings. Thermal shock test and high accelerated stress test were conducted. It was shown that reliable interconnections can be formed using the ultrasonic bonding technique for fine pitch applications. However, considering the machine accuracy, bumps co-planarity, and displacement amplitude in the bonding proc- ss, further investigation is required for applications of less than 30/spl mu/m pitch.