The Great Underfill Race
By Dr. Ken Gilleo & David Blumel
Alpha Metals/Cookson, 250 Culver Ave.
Jersey City, NJ 07304
Abstract
Most Flip Chip assemblies require underfillto bestow reliability that would otherwise be ravished by stress due tothermomechanical mismatch between die and substrate. While underfill can beviewed as “polymer magic” and the key to modern Flip Chip success, many see itas the process “bottleneck” that must be eliminated in the future. Both viewsare accurate. A substantial amount of R&D is being focused on makingunderfill more user-friendly. Electronic materials suppliers, variousconsortia, government labs and university researchers are working diligently toshatter the bottleneck and fully enable Flip Chip – the final destination formicropackaging. This paper will describe these efforts and provide a statusreport on state-of-the-art underfill technologies. We will also examine newprocessing strategies.
Today, three distinctly different underfillsystems vie for victory: capillary, pre-dispense and solid film. The incumbentcapillary flow underfill manufacturers attempt to win by offering snapflow/snap cure materials and new features. Can the new 5-minute cure underfillsdeliver enough throughput and performance? Will the so-called “No Flow”, orpre-dispense liquids, win by eliminating flux even though dispensing equipmentis still required? Which systems are REWORKABLE and how important is thisfeature?
Perhaps the ultimate solution is solidunderfill, but can the polymer chemists meet such a challenge wrought withparadox? How and where should solid underfills be applied and processed? Learnthe status of the most unusual approach – Reworkable Wafer-LevelFlux-Underfill. Success at wafer-level can have a profound affect on Flip Chipby eliminating all steps and equipment now associated with the underfillprocess. We will try to determine if wafer-level underfill can deliver on thepromise of making Flip Chip the Ultimate CSP!
Key words: Flip Chip, capillary flow, ChipScale Package, flux, polymerization, reworkable, thermoplastic, thermoset,underfill, wafer-level
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