Reworkable No-Flow Underfills for Flip Chip Applications

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Wong, C. P.
Wang, Lejun
Li, Haiying
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Underfill is a polymeric material used in the flip-chip devices that fills the gap between the integrated circuit (IC) chip and the substrate (especially on the organic printed circuit board), and encapsulates the solder interconnects. This underfill can dramatically enhance the reliability of the flip-chip devices as compared to the nonunderfilled devices. No-flow (compress-flow) underfill is a new type of underfill that allows simultaneous solder bump reflow and underfill cure, which leads to a more efficient no-flow underfilling process as compared to the standard capillary-flow underfilling process. Reworkable underfill is another type of underfill that allows the faulty chips to be replaced individually. It is the key material to address the nonreworkability issue of the current flip-chip devices. Reworkability is especially important to the no-flow underfill because electrical test of the assembled chips can only be done at the end of the no-flow underfilling process. The goal of this study is to demonstrate the feasibility of a no-flow reworkable underfill. Two approaches are taken to develop this new type of underfill. The first one is to add a special additive into a standard no-flow underfill formulation (underfill 0) to make it reworkable, called underfill 1. The second approach is to develop a no-flow underfill based on a new thermally degradable epoxy resin that decomposes around 240℃, called underfill 2. Comparing to underfill 0, these two underfills have similar properties including glass transition temperature (T[subscript g]), coefficient of thermal expansion (CTE) and modulus. Underfill 1 has similar curing and fluxing capability as to underfill 0. Underfill 2 cures faster than underfill 0, and it has slightly weaker fluxing capability than underfill 0, but it still allows 100% of solder bumps wetting and collapsing on the copper board. Moreover, underfill 1 and underfill 2 allow the flip chips to be reworked using a developed rework process while underfill 0 does not.
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