Strategies for thermal performance improvements of polymeric thermal ground planes (TGP)

Abstract

This paper presents a study on the thermal performance and enhancement techniques of polymeric thermal ground planes (TGPs). The literature features a range of polymeric TGPs that exhibit an effective thermal conductivity up to twice that of copper. However, the TGP effective thermal conductivity does not provide insight into the performance and potential improvements that could be made to a specific TGP design and configuration. Relying only on this metric can be limiting to compare different TGPs performances, since effective conductivity is affected by the device size, layout of heated and cooled zones, and the fabrication materials. In this paper, we adapted a model by connecting the TGP pumping capacity to its thermal resistance to predict the polymeric TGP thermal performances as a function of their geometry, test configuration and material used. This approach enables the development of strategies to optimize the polymeric TGPs performances and identify their limitations rather than relying on effective thermal conductivity as a benchmark for comparison. The present model considers vapor flow through a mechanical structure for the first time in TGPs. Indeed, the polymeric TGP in contrast to the metallic one requires a denser mechanical structure to support the vapor core thickness during vacuum due to the flexibility of the polymeric shell. The generated vapor pressure drop along the TGP has a dual impact on its performance as it affects both the capillary limit and vapor resistance resulting in a tradeoff between the TGP pumping capacity and thermal resistance, for a given TGP thickness. To back up the proposed model, experimental characterization was carried out on a one-step assembled polymeric TGP, to extract simultaneously the total and vapor thermal resistances. The model highlighted a trade-off in designing the vapor core and wicking structure, balancing the polymeric TGP pumping capacity and thermal resistance, for a target TGP thickness. Hence, the current study provides guidelines for defining improvement strategies for TGPs thermal performances based on the geometry and the type of material used during the fabrication process. The recommended approach also permits the identification of polymeric TGPs capabilities and limits rather than depending on the thermal metrics, which may be misleading while comparing performances.