Formulation of percolating thermal underfill by sequential convective gap filling

Abstract

Iii this work, a methodology for creating highly-packed percolating thermal underfills is presented. The methodology involves four sequential steps, namely, convective particle filling, drying of the earner fluid, epoxy refilling, and curing. In our experiments, silica and aluminum oxide spherical particles of different diameters (10 to 53 um) dispensed from water or isopropanol at concentrations of 1/8 vol% are used to create the particle bed within a confined space (representing the solder-ball cavity between two IC dies in a 3D chip stack). The resulting composite material is characterized in terms of filling fraction and corresponding thermal conductivity. The fillina fraction was obtained using imaging systems, such as optical microscopy, scanning electron microscope, and X-ray computational tomography, whereas the thermal conductivity is measured in silicon-silicon cavities. We also report on the dynamics of particle bed formation and the capillary epoxy filling of the particle bed to ensure that processes are compatible with higli-volume- manufacturing. In addition, the different experimental steps of the methodology are analyzed with numerical simulations. We have found that the flexibility provided by decoupling the different steps of our sequential filling method, in terms of material selection and dimensions, results in composite materials with high particle filling fractions (40 to 60% in volume) whose thermal conductivity (i.e. 1.3 +/- 0.1 W/m-K) outperforms that of commercially available capillary-based underfills by 60%. Therefore, this method may become an important part of efficient heat removal in future 3D chip stacks.