Chemical Modification of High Performance Metal-based Nanocomposite Thermal Interface Materials Toward Efficient Cooling in Electronic Systems (2018)

Overheating is a crucial problem limiting the reliability and performance of electronic systems. Since these systems have become continuously smaller, more powerful and sophisticated in recent decades, they produce much larger amounts of heat. As such, effective thermal management, in other words, thermal interface materials (TIMs) used in these devices for heat dissipation has gained utmost significance in terms of lifetime and reliability. Thermal greases, polymer-based composites, phase change materials (PCMs) and solders are currently the most commonly used TIMs. In this work, we describe next-generation metal matrix-based nanocomposite TIMs produced by chemical integration of organic ligand-functionalized boron nitride nanosheets (BNNS) into copper matrix. These TIMs possess much higher thermal conductivity than existing solder TIMs and comparable mechanical compliance with existing epoxy-based TIMs. BNNS, prepared by mechanically assisted cleavage of h-BN flakes through ultrasonic dispersion, was functionalized with three groups of organic ligands through nucleophilic addition to form functionalized BNNS (f-BNNS). These ligand groups were categorized by their chain type, and compatibility with the BNNS and metal matrix, and the functionalization was verified via Fourier Transform Infrared (FTIR) Spectroscopy. Then, the f-BNNS was dispersed in a copper matrix using a novel electrocodeposition method where copper and f-BNNS are deposited on a cathode in the presence of an electric field and potential difference to form nanocomposite TIMs. The thermal properties of the nanocomposite TIMs were investigated using the laser flash analysis, and the thermal conductivies were found to be between 160 W.m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 320 W.m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Once the thermal properties were measured, mechanical properties of the developed TIMs were investigated via nanoindentation technique and tensile testing. The elastic moduli of the nanocomposite TIMs were determined to be as low as 14 GPa as these values were much smaller than that of the electroplated pure copper thin films (99-125 GPa). There was nearly a six-fold reduction in elastic modulus of copper matrix in presence of functionalized BNNSs. The hardness value corresponding to this sample was about 0.18 GPa, which is about five-to-fifteen times lower than the measured hardness values of pure copper (1.1-2.8 GPa). Briefly, BNNS were functionalized with various ligands, and it was shown that electrocodeposited nanocomposite TIMs with thermal conductivies as high as 320 W.m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and elastic modulus values less than 20 GPa can be produced. Considering two extreme cases of epoxy-based and pure copper shim TIMs, our results are significant in advancing the current state of art for TIMs.