High thermal conductivity of boron nitride filled epoxy composites prepared by tin solder nanoparticle decoration
Introduction
Thermally conductive filler-based epoxy composites have been extensively studied for semiconductor packaging applications. These composites can be prepared with epoxy molding compound (EMC), under-filling substances, thermal interface materials (TIMs), and glob tops [[1], [2], [3], [4]]. In the last few decades, multi-stacked chips and small package sizes have been trending, driven by the need for portable consumer products and lower manufacturing costs. In addition, the device speeds have continued to increase, leading to the generation of more heat within a smaller chip volume. These factors limit the high power dissipation or high “clock” frequency of semiconductor chips because of the low thermal conductivity of presently available epoxy resin-based composites. Unfortunately, a cured epoxy resin has poor thermal properties such as a high coefficient of thermal expansion (CTE) and low thermal conductivity [5].
The incorporation of high-thermal-conductivity fillers into an epoxy resin is the most effective way to enhance the thermal conductivity of epoxy resin-based composites. The fillers require only phonon conduction with electrical insulation; examples of fillers include diamond, beryllium oxide (BeO), aluminum nitride (AlN), aluminum oxide (Al2O3), and boron nitride (BN), the crystalline structure of which can be hexagonal (h-BN) or cubic (c-BN) [5,6]. Diamond and c-BN are ideal fillers for heat conduction; however, they are expensive, and beryllium oxide is toxic. The theoretical thermal conductivity of Al2O3 is approximately in the range of 38–42 W m−1 K−1, which is lower than those of AlN and h-BN. Hexagonal boron nitride (h-BN), including some of its nanoscale moieties, namely BN nanosheet (BNNS), BN nanoplatelet (BNNP), and BN nanotube (BNNT), has a high thermal conductivity, reasonably low CTE, and low dielectric constant [7,8]. Many researchers have conducted studies on improving the thermal conductivity of epoxy polymer composites by adding high-thermal-conductivity solid fillers such as h-BN, BNNS or graphene [[9], [10], [11], [12], [13], [14], [15]]. Many factors affect the thermal conductivity of composite materials, including the intrinsic thermal conductivity of the fillers, the filler dispersion state in the matrix, the interfacial interaction between the filler and matrix, and the orientation degree of the fillers in the matrix [[16], [17], [18]]. Among these factors, the inter-filler contact and the filler alignment in the polymer matrix play the most important roles in improving the thermal conductivity of polymer composites, given the inevitable phonon scattering at the filler–polymer interface; moreover, the interfacial thermal resistance deteriorates the thermal conductivity [[19], [20], [21]].
In the last few years, research has focused on connecting a 3D network of thermally conductive fillers to reduce phonon scattering within the polymer matrix [[22], [23], [24]]. Several methods have been implemented, including surface modification with organic molecules or silver nanoparticles (Ag NPs) [[25], [26], [27]], external pressure application, and freeze-casting or use of aerogels followed by polymer infiltration [22,24]. Liu et al. [28] spin-coated and hot-pressed BNNS treated with aminopropyl triethoxysilane (APTES) at 5 MPa. The thermal conductivity was found to be 5.86 W m−1 K−1 under a BNNS loading of 40 wt.% (25 vol.%), with improved BNNS in-plane orientation. The formation of covalent bonds on the BNNS surface improved the filler–epoxy interface contact and resulted in a high thermal conductivity under a small amount of BNNS loading. Chen et al. [29] fabricated a multilayer epoxy composite composed of BNNS and AlN platelets under a 10 MPa hot-pressing condition. They realized a high thermal conductivity of 8.53 W m−1 K−1 owing to the improvement in the inter-filler contact and alignment by applying a relatively high pressure and optimizing the filler particle shape. Xiao et al. reported a maximum thermal conductivity of 17.61 W m−1 K−1 (in-plane direction) and 5.08 W m−1 K−1 (out-of-plane direction) at a BNNP loading of 65.6 vol.% [30]. They fabricated a thermally conductive composite by synthesizing boron nitride microbead (BNMB) hollow spheres and applying a compressive force followed by infiltrating the epoxy resin. The high thermal conductivity could be attributed to the high surface area of BNNP, leading to better filler contact area with the bead shape of BN, while reducing the friction during compression. These factors make it possible to achieve a high filler loading for epoxy resin and high thermal conductivity of the composites. Zhang et al. [31] introduced Ag NPs on h-BN surfaces by wet-impregnation methods and aligned the orientation of Ag NPs on h-BN fillers by hot pressing under 0.2 MPa, representing an effective approach to improve the thermal conductivity. With their method, owing to the improved contact facilitating the heat flow at the interface by the Ag NPs and maximized overlapping area of h-BN by hot-pressing, the in-plane (λ∥) and out-of-plane (λ⊥) thermal conductivity values were reported to be 23.1 and 3.6 W m−1 K−1, respectively, under a 60 vol.% loading of Ag NPs on h-BN. Thus far, this has been the best thermal conductivity value reported for electrically insulated filler-based epoxy systems. Although the deposition of Ag nanoparticles on the filler surface plays an important role, it is difficult to achieve a high thermal conductivity for epoxy composites without applying an external pressure for filler orientation and alignment. Chen et al. [32] constructed a 3D BNNS network, where the fillers were treated with AgNO3 and a reducing agent, followed by in situ curing without applying any pressure. They reported a very low thermal conductivity of 1.13 W m−1 K−1, suggesting that an effective 3D filler network cannot be formed without applying external forces. However, applying an external pressure or constructing a pre-made 3D filler framework severely restricts the commercial application of such composites in the electronic device packaging industry. Therefore, it is important to realize a high thermal conductivity for epoxy composites under pressure-less curing conditions.
The objective of this study was to synthesize low-melting-temperature (thus thermo-controllable at general epoxy curing temperatures under 250 °C) tin nanoparticles (Sn NPs) on h-BN surfaces by wet-impregnation methods followed by chemical reduction and to fabricate high-thermal-conductivity epoxy composites, while maintaining relatively low dielectric property values and coefficient of thermal expansion (CTE). With the introduction of hybrid fillers, i.e., Sn NPs on h-BN, into the epoxy resin matrix, the generation of a thermal conduction path via the action of tin fusion could effectively reduce the thermal contact resistance at the filler–epoxy matrix interface during in situ curing. Consequently, the thermal conductivity of the epoxy composite is significantly improved without having to apply an external pressure. The h-BN-decorated Sn-NP-filled epoxy composite proposed in this paper can broaden the application scope of electronic device packaging and has great potential for the thermal management industry.
Section snippets
Materials
The epoxy monomer ECC (3,4-Epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate), HHPA (hexahydrophthalic anhydride) as a curing agent, and 2-EMIP(2-Ethyl-4-methyl-1H-imidazole-1-propanenitrile) as a catalyst were purchased from Sigma-Aldrich. The solvents, namely DMF (N, N-dimethylformamide) and NMP (N-Methyl-2-pyrrolidone), purchased from Sigma-Aldrich, were used without further purification. PVP (Polyvinyl-pyrrolidone, M.W. ∼40 k), silver nitrate (AgNO3), Tin (IV) chloride pentahydrate
Synthesis of metal NPs on h-BN surfaces
Fig. 1 shows the results of synthesized Ag NPs on h-BN following a 144-h reaction, in which the SEM images show Ag NP morphologies and EDAX analyses of each element. The particle size distribution of the Ag NPs could be roughly estimated from a few tens of nanometers to the submicron range with an irregular shape. In Fig. 1g, the XRD peaks at 2θ values of 38.2°, 44.3°, and 64.5° can be assigned to the (111), (200), and (220) crystallographic planes of face-centered cubic (fcc) silver crystals,
Conclusions
A newly developed thermally conductive epoxy composite incorporated with hexagonal boron nitride (h-BN) decorated with tin nanoparticles (Sn NPs) was fabricated to investigate its applicability in electronic device packaging. Hybrid fillers (Sn NPs on h-BN) were successfully synthesized by NaBH4-mediated reduction reaction, where the particle size, shape, and homogeneity were completely controlled by the NMP/EtOH co-solvent ratios. With increasing NMP proportion, the size distribution of Sn NPs
Credit authorship contribution statement
Lee E. S: Conceptualization and Data curation, Investigation, Formal analysis, writing original draft, Writing - review & editing, Supervision. Kang J. G & Kang M. K: Investigation, Formal analysis. Kim K. H: Formal analysis. Park S. T: Formal analysis. Kim Y. S: Formal analysis. Kim I: Project administration. Kim S. D.: Project administration. Bae J. Y: Writing original draft & editing.
Data availability statement
The datasets generated and/or analyzed during this study can be made available from the corresponding author on reasonable request.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank Prof. Lee J-C (Department of Materials Science and Engineering, Sungkyunkwan University) for his help with the dielectric measurements of the epoxy composites. We also thank Baek W. J. (SAIT) for TEM measurement and mapping analysis.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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