Development of low stress no-flow underfill for flip-chip application

No-Flow Underfill (NUF) technology has been studied in the various ways about cure properties, self fluxing performance, and filler loading. Due to the filler entrapment between solder bump and contact pad on board, no-flow underfills are mostly no filler loading or filled with very low filler loading. The high coefficient of thermal expansion (CTE) of the polymer material has significantly lowered the reliability of flip chip assembly using NUF, and has limited its application to large chip assemblies. However, high-end flip-chip packages are heading towards a high pin count and large chip size with lead free issue and low-k's. [1] It is expected to develop a low stress underfill because of flip-chip such as low-k chip package has a brittle structure. This paper presents a novel approach to develop for low stress NUR We have realized flip-chip bonding with a high filler loading of NUF using a local reflow flip-chip bonding assembly process. We have evaluated a deformation of flip-chip package using Shadow Moire Technique during 260 reflow. This technique is one of the warpage measurement techniques that use a shadow of grating glass. Warpage of IC is decreasing during the reflow process after cooling it recovers initial warpage level. The effects of resin, filler size, filler loading level, fillet size is investigated Filler size, loading level, and fillet size is roughly the same result as reference. As a result, NUF is optimized for lead free soldering process with low stress.