Nanocomposite underfills for flip-chip applications

Abstract

The no-flow underfill process is an attractive alternative to the use of capillary underfills in flip-chip attachment. A limitation of typical no-flow underfills is their high coefficient of thermal expansion (CTE) compared to that of capillary unddills. Conventional micron-sized inorganic fillers that are used to decrease the CTE in capillary underfills have not been successfully incorporated into no-flow formulations because they interfere with the solder interconnection process. This paper describes nanocomposite no-flow underfill adhesives, a promising new class of materials that addresses this liiitation. Nanosilica fillers can be incorporated into self-fluxing no-flow underfill adhesives to lower the CTE of the adhesive formulation. The investigations reported in this paper indicate that unlike conventional fillers, nanosilica filler does not interfere with solder interconnection when present between the solder bumps and bonding pads prior to solder reflow. This nanocomposite technology has provided liquid adhesives that show promise as no-flow underfill adhesives having material properties and thermal performance more like capillary underfills. For a selected nanocomposite underfill adhesive, the CTE, modulus, and viscosity will he compared to an untilled analogue, and its use in the no-flow underfill process will he described. Finally, preliminary thermal cycling data that demonstrate the potential for achieving a significant reliability enhancement with a nanocomposite noflow underfill will be presented.