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

In this study, we designed a new flex-on-board test vehicle with a 250-μm-pitch coplanar waveguide (CPW) structure for high frequency measurement. The microstirp 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. The designed PCB has a characteristic impedance of 50 ohm, which leads to low reflection condition. When electric signal faces the different characteristic impedance, some energy of the signal is reflected. Therefore, the PCB may affect the electrical performances due to the signal reflection. So 2 ACFs joints were passed by a 300 MHz to 20 GHz high frequency signals, and S-parameters and physical modelling were performed to characterize transmission property. The S-parameters are assessed by a vector network analyzer (VNA) and microprobe tips. The microprobe tips can evaluate the electrical performances for the ACF. The SMA connector has parasitic elements, which results in accurate measurement result in GHz range. Thus, microprobe tips are used for the electrical performance evaluation instead of the SMA connector. And by extraction of the bare measured FPC and PCB property, only 1 ACF joint insertion loss property was achieved. From the obtained S-parameter, we converted to a resistance (R) to compare the electrical loss by the ACF. During the conversion, the insertion loss, S21, is directly converted to the resistance from 300 MHz to 20 GHz.