Advanced integration technology for fabricating high-speed electro-optical sub-assembly

Here, we report flip-chip bonding processes based on Anisotropic Conductive Film (ACF) and Sn-Ag-Cu (SAC) solder implemented for bonding three test chips (two Silicon, Si ICs on either side of an Indium Phosphide, InP IC) in series on 25 µm thick polyimide foil substrates. Si ICs were bonded only with SAC solder, whereas both SAC and ACF were applied for the flip-chip integration of the fragile InP chips. RF measurements were then performed on the impedance controlled paths across the polyimide foil of the fabricated assemblies to evaluate as well as to compare the RF performance of ACF and SAC solder in terms of the differential S21 parameter. The measurements revealed that when ACF is employed for interconnecting the InP chip and SAC solder for the two Si chips, a higher 3 dB bandwidth of 42 GHz was obtained whereas a 3 dB bandwidth of only 31 - 34 GHz was measured when only SAC solder was used for bonding all three ICs. These measurement results confirm that a higher bandwidth can be obtained from the assemblies when ACF is used as the interconnection material. Therefore, it can be concluded that the ACF based integration technology could be favored over solder processes for bonding high performance multi Gbit/s subassemblies, especially with higher density of contact pads.