Incorporation of inorganic filler into the no-flow underfill material for flip-chip application

The concept of no-flow underfill has drawn much interest for the past few years. The ideal simultaneous in-situ fluxing, interconnection of the solder joints and curing of the underfill material through a single flip chip reflow step exhibit many process advantages over the conventional underfills. However, these underfill materials are basically an-filled and with coefficient of thermal of expansion (CTE) in the range of 80 similar to 200ppm/degrees C. Considering that the application of the underfill material is to enhance the reliability of the solder joints of the flip chip assembly, close matching of material properties, such as CTE, is critical for device reliability performance. The most straightforward and effective approach for CTE reduction is to incorporate inorganic silica filler into the normal epoxy system. The preliminary pre-filled versions of the no-flow underfill materials were investigated with silica or solid glass bead or hollow glass sphere as the filler. The formulations were deliberately manipulated to reach nearly uniform filler suspension within the organic matrix even at the high reflow/curing temperature. However, the incorporation of the inorganic filler would hinder the solder joint interconnection formation and as such, limit the wetting of the chip bump onto the substrate pad. Thus, some specific quantity of the conductive alloys/powders of various melting points (95 degrees C, 138 degrees C, 183 degrees C and silver flake) were mixed together with the silica filler into the epoxy no-flow underfill system. The application of the formulations was tested with the typical solder reflow profile regarding the wetting of the solder balls onto the copper substrate. This could finally lead to optimization of the pre-filled no-flow underfill systems for highly demanding flip-chip applications.