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

Microwave model and high-frequency measurement of the ACF flip-chip interconnection is investigated using a microwave network analysis. The test IC was 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 test IC chips and as an interconnect material, several ACFs were prepared and flip-chip bonded onto the Rogers(R) R04003 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. 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 resistive increases due to skin effect up to 8 GHz; after this frequency, conductive loss of epoxy resin also increases. Reactance is dominantly affected by inductance of Ni/Au bumps and conductive particles in the ACF interconnection at whole measured frequencies. ACF flip chip interconnection has only below 0.1 nH. The effects of different conductive particle materials on electrical behavior in GHz were investigated. Different particle materials in ACF show the difference in the reactance and resistance behavior up to 12 GHz and in the resonance frequency. Our results indicate that the electrical performance of ACF combined with electroless Ni/Au bump interconnection is comparable to that of solder joint