Electrically conductive adhesives at microwave frequencies

Abstract

In this paper we present results from measurements on epoxy-based anisotropic and isotropic, electrically conductive adhesives, ACAs and ICAs respectively. We study two different types of connections, a flip-chip bonded silicon test chips and a simple transmission line gap bridged by a copper foil. These measurements are referenced to equivalent solder joints. The silicon chip is a standard test chip. The test chips are mounted on three different substrates, a rigid FR-4 board, flexible board and a high-frequency Teflon-based duroid substrate. We also discuss two electrical models for the connections, an equivalent RC model and a stochastic model, based on random particle distribution. The equivalent electrical model is based on physical considerations and the parameters are then fitted to measurement data in the high frequency CAD tool HP MDS. All the adhesive and solder interconnections are measured before and after temperature cycling and humidity tests. The temperature dependence of the connections S-parameters are also studied in a temperature controlled environment. A HP8510 network analyzer is used to measure scattering parameters. On rigid and flexible boards, the frequency range investigated is 500 MHz to 8 GHz, and on duroid mounted flip-chips and bridges, the range is 1 GHz to 30 GHz. The Thru-Reflect-Line calibration procedure is used to get the best possible calibration. Power testing is used with the Cu bridge assembly to find the maximum power transmission through an electrically conductive adhesive interconnect. This is done at relevant microwave frequencies. Results indicate that the isotropic adhesive interconnections handle high power throughput well. The adhesive joints are subjected to a maximum peak pulsed power of 250 W. Maximum work factor of the pulsed signal is 10%. The effects of different particle sizes and materials in the ACAs are systematically investigated. Three different particle sizes and two particle materials are examined. All adhesives considered show similar electrical properties and are all suitable for adhesive electrical interconnections. The microstructure of the adhesive joints is studied by cross-sectioning using Scanning Electron Microscopy (SEM) before and after temperature cycling.