Conductive adhesives for high-frequency applications

In this paper, we present results from measurement and simulation of epoxy-based anisotropically conductive adhesive joints (ACA). We studied two different types of connection: flip-chip bonded Si test chip and a transmission line gap bridged by Cu foil. Test chips were mounted on three substrates: rigid FR-4 PCB, flexible PCB and high frequency teflon-based duroid substrate. Equivalent electrical models are discussed based on physical considerations and parameters were fitted to measurement data in the HP MDS high-frequency CAD tool. A HP8510 network analyser was used to measure S-parameters on rigid FR-4 and flex boards in the 500 MHz-8 GHz frequency range for both flip-chips and bridges, and for duroid mounted flip-chips and bridges over 1-30 GHz. ACA microstructures were studied by cross-sectioning and SEM. LF results for 500 MHz-8 GHz indicated that ACA flip-chip joints and bridge joints on FR-4 or flex can be used. For HF applications (1-30 GHz), ACA flip-chip joints and bridge joints on duroid can be used. For flip-chip joints and bridge joints, ACA was as good as or better than the solder joint for FR-4 over 45 MHz-2 GHz and for duroid substrates over 1-30 GHz. The largest contribution to transmission loss is due to Si chip resistivity in the flip-chip assembly. HP momentum analysis showed that for duroid, Si-chip crosstalk also gave power losses, which is not the case for FR-4 or flex. HP momentum analysis of flip-chip on FR-4 and flex showed that FR-4 and flex substrate losses also should be considered. Different ACA particle sizes and materials made little difference to ACA joint electrical behaviour