Insights into the role of surface hydroxyl of the silica fillers in the bulk properties of resulting underfills

The advances of flip-chip technology have driven the development of both underfilling processes and underfill materials. Current underfill material is mainly silica filled liquid epoxy encapsulant and their properties depends to a large extent on the filler factors such as loading, size and size distribution, shape, and especially surface chemistry state. To illustrate this point, in our study, monodisperse spherical SiO2 particles with plenty of hydroxyl groups have been synthesized via the Stöber process and further thermally treated to obtain -OH-free SiO2 particles. Then the anhydride based underfill filled with the two types of silica filler were prepared and their performances were evaluated comparatively in order to provide insights into the role of intrinsic surface hydroxyl of the silica fillers in the bulk properties of resulting underfills. We found that as compared to the -OH-free SiO2 filled epoxy underfills, the presence of the surface -OH groups of silica lead to negative effects on the rheological, thermomechanical as well as reliability properties of resulting underfills, such as a sharper viscosity increase with time, an obvious decrease in glass transition temperature (Tg), the interfacial thermal stresses harder to eliminate, a much higher moisture absorption. Therefore it was necessary to remove the surface -OH groups of SiO2 fillers through pre-heat treatment to develop high performance underfills.