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Use of Wafer Applied Under Fill for 3D Stacking
Keywords: Wafer-level Under Fill, 3D Stacking, micro-bump
A key element for improving 3D stacking reliability is the choice of the right Under Fill materials to be integrated during the dies stacking process and the final packaging. The Under Fill is a specialized adhesive that has the main purposes of locking top and bottom dies; it must fill the gap between bumps and dies, while reducing the differential movement that would occur during thermal cycling. Traditional under filling processes are based on local dispensing after solder bump reflow (Capillary dispensing), or before flip chip operation with no need of reflow (No Flow Under Fill, NUF). In case of 3D stacking, such processes present some limitations: need of a dispensing area (die size increase); material flowing (spacing between dies) and cost (low throughput). In this paper we report the work done to assess the properties of several Wafer Applied Under Fill (WLUF) materials and their integration in 3D stacking. These materials have been initially applied on silicon wafers in order to assess the minimum achievable thickness and the material uniformity. The wafers have been coated by using different methods: spin coating and film lamination. After this initial assessment, the most promising materials have been used for 3D stacking. The test vehicle used has Cu/Sn µbumps with a pitch of 40µm. The quality of the materials is judged by electrical test, SAM (Surface Acoustic Microscope) and X-SEM (Scanning Electron Microscope).
Antonio La Manna,
Leuven, B-3001,

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