Thermal Conductivity Enhancement of Soft Polymer Composites through Magnetically Induced Percolation and Particle–Particle Contact Engineering

Abstract

DOI: 10.1002/admi.201801857 cross-linked polymer matrix (i.e., a TIM “pad”) or grease. The resulting composite thermal conductivity (kc) of the TIM is typically up to an order of magnitude higher than the base material (e.g., up to 2 W m−1 K−1 vs 0.2 W m−1 K−1). However, as transistor density has increased over the last decade, TIMs persist as a major thermal bottleneck in the thermal management of modern ICs.[3,4] Consequently, considerable research effort is dedicated to improving thermal performance of TIMs. The thermal conductivity of TIM composites is traditionally improved by increasing the particle thermal conductivity (kp) and the overall particle volumetric fraction (φ).[5–8] The most significant improvements to kc are achieved by increasing φ to well above the percolation threshold (φp, typically around φp = 0.3 for spheroidal particles).[9–11] Unfortunately, increasing the particle volume fraction to φ ≥ 0.5 in TIM pads, where significant kc enhancements are made, also stiffens the composite.[12] This produces undesirable effects for microelectronics packaging applications, including increasing Rc at the surface of the TIM.[2] Recent advances have shown that using liquid metal inclusions in a polymer matrix can prevent composite stiffening, however, very high φ along with additional processing that elongates the inclusions is required to achieve notable thermal conductivity.[12–15] Percolation can be enforced at φ ≪ φp by imposing a magnetic field on an uncured polymer composite with magnetically permeable fill particles. This simple and cost-effective process creates composites with highly anisotropic electrical and thermal conductivities that are significantly enhanced along the direction in which the particles are aligned.[15–20] For example, magnetically aligning spherical nickel particles in a polymer matrix provides a twofold improvement for kc in the direction of particle alignment.[22,23] Furthermore, aligning particles with higher aspect ratios provides an even higher enhancement to kc—up to threeand fourfold for particles with aspect ratios between 7 and 20.[24–27] The direct contact between particles in these percolating networks decreases the interfacial thermal boundary resistance Despite major advancements in the performance of thermal interface materials (TIMs), contact resistance between components persists as a major thermal bottleneck in electronics packaging. In this work, the thermal performance of composite TIMs is enhanced through a synergistic coupling of magnetic alignment and engineered particle coatings that reduce the thermal resistance between particles. By itself, magnetically induced percolation of nickel particles within a cross-linked silicone matrix doubles the thermal conductivity of the composite. This process significantly increases contact between particles, making the interfacial particle–particle resistance a major contributor to the composites thermal performance. The resistance at these interfaces can be reduced by introducing soft metal (silver) or liquid metal coatings onto the nickel particles. Compressing powder beds of these hybrid particles reveals that, dependent on coating thickness, the contact engineering approach provides multifold increases in thermal conductivity at mild pressures. When dispersed in a polymer matrix and magnetically aligned, the coated particles provide a threefold increase in composite thermal conductivity, as compared to unaligned samples (up to nearly 6 W m−1 K−1 with volumetric fill fraction of 0.5). For equivalent coating thicknesses, silver coatings achieve better performance than liquid metal coatings.