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Development on wafer level anisotropic conductive film for flip-chip interconnection

With the fast growth of multimedia, the packaging has to satisfy high interconnection density, high data throughput, miniaturization, easy thermal management and reliability needs while keeping a low cost. Flip-chip, which probably offers the best solution to meet these requirements, has been chosen by major companies in various configurations: FCOB (flip-chip on board), WLCSP (wafer level chip scale package) and WLP (wafer level packaging). Flip-chip interconnection using micro-balls is a common interconnection technique and major developments are being carried out on wafer bumping. In this paper, a new bumping approach on the IC wafer with z-axis ACF (anisotropic conductive film) is presented. This WL-ACF concept proposes a universal solution for bumping as there is no specific rule design to be taken into account. Moreover, the z-axis ACF developed at CEA-LETI gives the possibility of achieving lower pitch and pad area as well as good electrical contact and reliability. WL-ACF is evaluated on a specific test vehicle assembly and compared with localized micro insert bumping and hybridization with standard ACF.