RF performance of flip chip ACA joints for CPW transmission lines

Abstract

In earlier work, radio frequency (RF) performance of flip-chip anisotropic conductive adhesive (ACA) joints has been studied both in experiments and simulations. Nowadays, coplanar waveguide (CPW) transmission lines are being used widely as its lower loss compared to conventional MS lines. In this paper, we studied RF performance of the ACA joints for CPW lines using the simulation software HFSS. Firstly, we investigated the high frequency performance by varying distance and number of ACA particles. It shows that the S11 parameter greatly deteriorated at the lower frequency band. To further study this phenomenon, we extracted the equivalent circuit. The extracted circuit value showed that when particle number increased, the electromagnetic interaction between each particles gradually dominant the overall performance. As a result, the mutual inductance increased the equivalent inductance, and the equivalent capacitances was also increased in the interconnect region. The particle distance was found to have little influence on the RF performance. Secondary, we studied the influences of resin compound (epoxy matrix) with different dielectric constant. Resin compound greatly influence the effective constant and impedance at the output port, thus induced impedance mismatch and slightly changed S11 response in the frequency range. Finally, we performed ANOVA analysis to evaluate the impact factors among ACA resin, joint height, signal to ground bump distance, conductor overlap and particle number. The dielectric constant of resin was found to be the most important factor in flip chip CPW-CPW ACA joints. Conductor overlap and ground to signal distance are still the two main factors. To optimize the performance, the dielectric constant of resin should be as lower as possible, and the conductor overlap length and ground to signal distance should be minimized. Moreover, particle number and bump height proved to have least effect. To achieve best performance, the particle number in single joint need to kept at a lower value.