Thermo-mechanical properties of underfills at partial and full filler percolation - sub-layering the underfill

Underfill materials protect electronic package interconnects. They are designed to bridge the thermal expansion behavior of die, substrates and interconnects and couple their stiffness. However 3D integration requires higher thermal conductivities than conventional underfills can provide. Because percolating thermal underfills promise to offer substantially higher conductivities, this study investigates property gradients within percolating thermal underfills. All sphere-filler composites in electronic packages have limited properties close to the interface: Fillers cannot penetrate the interface. Thus, structure and properties show a interface vicinity gradient. Although the composite behavior dependence on filler content and distribution has been well described for both disperse fillers and heterogeneous materials in general, especially of the CTE, moduli, and conductivity, the transition to communicating fillers both in bulk and contrained spaces is only covered insufficiently. The effective medium approach to thermo-mechanical underfill modelling faces a lack of geometry representation, especially when the ratio of filler size to gap size increases to 0.5 and beyond. Therefore we sub-layer the underfill and apply unit cell model results to the ratios and fill fractions of the layers. A central layer of the linear buckling phase shows a high filler volume fraction and is approximated by a face centric cubic unit cell. It reveals a 30 % drop in Young's modulus where the share of the central layer is more than one third of the entire underfill thickness. To evaluate the model errors we compare a stacked-layer effective modulus to an effective modulus from a constrained space unit cell that is being published separately. We present bulk specimen preparation and relate first experimental results to the unconstrained space unit cell model. All values are shown for an example of monodisperse 45 μm diameter silica fillers in a 2 GPa matrix. With this study we suggest that the ratio of filler size to gap height could be utilized to tailor the modulus along the height axis. Package simulations could potentially benefit from sublayering an underfill layer.