Conductive adhesives for high-frequency applications

In this paper we present results from measurements and simulations on epoxy-based anisotropic electrically conductive adhesive joints (ACA). We have studied two different types of connections, flip-chip bonded Si test chip and a simple transmission line gap bridged by a copper foil. The test chips were mounted on three different substrates, a rigid FR-4 board, a flexible board and a high-frequency teflon-based duroid substrate. Equivalent electrical models are discussed and is based on physical considerations and the parameters were then fitted to measurement data in the high-frequency CAD tool HP MDS. A HP8510 network analyzer was used to measure S-parameters on rigid FR-4 and flexible boards in the frequency range 500 MHz-8 GHz for both flip-chips and bridges. For the duroid mounted flip-chips and bridges, the frequency range was 1 GHz-30 GHz. The microstructures of the ACAs were studied by cross-sectioning and then using Scanning Electron Microscopy (SEM). For low frequency applications (500 MHz-8 GHz) the results indicated that the ACA flip-chip joint and the bridge joint mounted on the FR-4 board or flexible board can be used. For high-frequency applications (1 GHz-30 GHz) the results showed that the ACA flip-chip joint and the bridge joint mounted on the duroid substrate can be used. For duroid and FR-4 mounted flip-chip joints and bridge joints ACA has proved to be as good as or even better than the solder joint, for FR-4 in the frequency range 45 MHz-2 GHz and for the duroid substrate I GHz-30 GHz. The largest contribution to the transmission losses is probably due to finite resistivity in the Si chip in the case of flip-chip assembly. HP momentum analysis showed that for the duroid substrate the cross talk in the Si-chip also give power losses, which is not the case for the FR-4 board or the flexible board. HP Momentum analysis of the entire flip-chip structure on the FR-4 board and the flexible board indicated that losses in the FR-4 substrate and the flexible substrate also should be taken into consideration. Different particle sizes and materials in the ACAs gave little difference in the electrical behaviour of the ACA joints.