A Study on the High Frequency Performance of solder ACFs Joints for Flex-on-Board Applications using Coplanar Waveguide

摘要

Solder anisotropic conductive film (ACF) joint is in a micron scale and very hard to measure its high frequency performance vertically in a flex-on-board (FOB) structure. In this study, we designed a horizontal measurement through 2 FOB joints by a coplanar waveguide (CPW) method. Although in 2000s, we used microstrip to investigate high frequency performance of ACF joint, the signal dielectric loss largely occurred in the microstrip because the electromagnetic field widely spread out of the designed circuits. In order to solve it, coplanar waveguide structure is designed by adding two parallel ground electrodes and gapping the center microstrip to reduce the outer electromagnetic field. Therefore, we designed the Flexible printed circuit (FPC) board and printed circuit board (PCB) with 250 μm pitch CPW patterns. The microstrip is 150-um-width and 2-mm-length by Au/Ni surface finished Cu electrode and the ground electrode is designed by 4000-um-width and 2-mm-length. On PCB, 2 CPW patterns are placed with the gap of 0.5 mm. 1 CPW pattern is placed on FPC and aligned in the center of 2 CPWs on PCB by bonding process. 2 FOB joints were evaluated by 200 MHz to 20 GHz high frequency signals, but by extraction of the bare FPC and PCB performance and square root of 2 ACFs joints performance, only 1 ACF joint insertion loss property was achieved. In addition, the conventional Ni ACF joint and SnBi58 solder ACF joint were compared in terms of high frequency performance.