Microwave model of anisotropic conductive film flip-chip interconnections for high frequency applications

Microwave model and high-frequency measurement of the anisotropically conductive film (ACF) flip-chip interconnection was investigated using a microwave network analysis. The test integrated circuits (ICs) were fabricated using a 1-poly and 3-metal 0.6 μm Si process with an inverted embedded microstrip structure. As flip chip bumps, electroless Ni/Au plating was performed on Al input/output (I/O) pads of test IC chips, As an interconnect material, several ACFs were prepared and flip-chip bonded onto the Rogers(R) RO4003 high frequency organic substrate. S-parameters of on-chip and substrate were separately measured in the frequency range of 200 MHz to 20 GHz using a microwave network analyzer HP8510 and cascade probe, and the cascade transmission matrix conversion was performed. The same measurements and conversion were conducted on the test chip mounted substrates at the same frequency range. Then impedance values in flip-chip interconnection were extracted from cascade transmission matrix. The extracted model parameters of the 100 μm×100 μm interconnect pad show the resistance increases due to skin effect up to 8 GHz. Above this frequency, conductive loss of epoxy resin also increases. Reactance is dominantly affected by inductance of Ni/Au bumps and also conductive particles in the ACF interconnection over the measured frequency range. The inductance value of ACF flip chip interconnection is below 0.05 nH and the contact resistance is below 0.9 R. In addition, the effects of different ACF conductive particle materials on high frequency electrical behavior in GHz range were also investigated, Different ACF conductive particle materials show difference in the reactance, resistance, and resonance frequency behavior up to 13 GHz. Our results indicate that high frequency electrical performance of ACF combined with electroless Ni/Au bump interconnection is acceptable for use in the high frequency flip chip application up to 13 GHz. Finally, 80-ps rise time digital signal transmission with small dispersion low loss reflection was demonstrated through the flip-chip interconnection with combination of ACF and Ni/Au bump