Heat spreaders can be made from natural graphite sheet materials. These spreaders take advantage of the anisotropic thermal properties of natural graphite. Natural graphite exhibits a high thermal conductivity in the plane of the sheet combined with a much lower thermal conductivity through the thickness of the sheet. As a result, a natural graphite sheet can function as both a heat spreader and an insulator and can be used to eliminate localized hot spots in electronic components. In some cases, a natural graphite heat spreader can replace a conventional thermal management system consisting of a heat sink and cooling fan. This paper examines the properties of natural graphite heat spreaders and the application of these spreaders to thermal management problems in laptop computers. The thermal and mechanical properties of natural graphite heat spreaders are presented along with a discussion of how those properties are measured. The use of a natural graphite heat spreader to reduce the touch temperature in a laptop computer is presented. Both experimental techniques and numerical models are used to examine performance of the heat spreader in this application.
Screen temperature reduction and improved screen temperature uniformity are ongoing objectives for flat panel display manufacturers as they seek to improve image quality, increase brightness and luminous efficiency, and extend screen life. This paper describes the development of new natural graphite-based heat spreaders for both plasma and liquid crystal displays. Reductions in peak hot-spot temperatures of up to 9 ºC have been demonstrated in commercial flat panel displays, significantly reducing the potential for temperature-induced performance degradation mechanisms in all types of display. IntroductionScreen temperature reduction and improved screen temperature uniformity are ongoing objectives for flat panel display manufacturers as they seek to improve image quality, increase brightness and luminous efficiency, and extend screen life.In plasma display panels, the primary sources of the heat are the discharge cells in the plasma display itself, with the temperature of the glass panel being related to the amount of white in the image. As will be described in this paper, peak temperatures up to 61 °C have been measured in some commercial 42-inch high definition plasma display panels at room temperature using whiteline test images, with temperature gradients of up to 23 °C.The method traditionally used to keep the glass cool is to sink the heat to the aluminum chassis that is attached to the back of the screen. The heat spreaders most commonly used are silicone or acrylic based materials that serve as both thermal interface materials to transfer heat from the glass to the chassis and also the adhesive attachment method to secure the glass to the chassis. The issue with these materials is that they are relatively thick (~ 1.8 mm) and have low thermal conductivity (~ 1 W/mK) resulting in relatively poor heat transfer into the chassis. This is compounded by the poor thermal conductivity of the glass itself and can lead to significant hot spots in the glass screen which increases the risk of image burn-in, reduced phosphor life as well as reducing image quality [1]. Adhesive coated aluminum spreaders have been used in combination with silicone-based pads in an attempt to reduce thermal resistance, but further performance improvements are still required.In liquid crystal displays with CCFL backlights, the source of the heat is both the CCFL and the display electronics, particularly the inverter circuitry on the back of the chassis. As will be described in this paper, peak temperatures of up to 50 °C have been measured in commercial 40-inch LCD TVs under room temperature test conditions with temperature gradients up to 15 °C.Natural graphite is a naturally occurring material that can be formed into thin sheets of flexible graphite, in the range of ~ 0.075-1.5 mm [2]. Natural graphite derived flexible graphite is thermally anisotropic, having very high in-plane thermal conductivity and low through thickness thermal conductivity. This anisotropy can be varied by controlling the manufacturing process, with in-plane ther...
Even as the use of flexible graphite heat spreaders becomes ubiquitous in mobile electronics, numerically quantifying the heat dissipation remains a challenge. The rapid pace of development of mobile devices has deterred the industry from establishing standards, and rules of thumb are few, as are closed-form solutions. Users have requested numerical methods and tools to simplify the selection of flexible graphite heat spreaders from among the standard thicknesses and grades, as well as to quantify the effect of changing heat transfer area and configuration. In the presence of adjacent layers — adhesives, dielectrics, or still air gaps — the thin nature of the materials and the high, orthogonal thermal conductivity ratios of the graphite combine to create a complex conjugate heat transfer problem. Although the thinnest of these sheets constitute but a tiny fraction of the thickness of a cell phone or tablet, their dominant role in the heat transfer requires that they not be neglected in the calculations. Some CFD software guidelines advise using multiple meshing layers to capture fully the heat transfer in these spreaders, while others (primarily FEA based) provide a plate element that negates the need for discretization. In the former, a fully meshed spreader confounds the goal of a quick calculation, but the flexibility of 3D solution also demands meticulous attention to the details, provides “an answer” that is easy to misinterpret, and in the hands of an unskilled user, invites error. The goal of this project is to establish the guidelines for computing heat spreading in graphite, including cell dimension ratio, mesh density, spreading radius, and transport capacity and to marry the orthogonal properties of the material with the row-column format of a spreadsheet or matrix software. It also reviews methods for addressing the non-orthotropic situations such as angled plates, and the curved surfaces seen in the case of graphite wraps and flexible hinges. There are cases in which a simple contact resistance value adequately represent a graphite thermal interface material, but others that require an accounting for the lateral conductivity that increases the efficacy of the TIM. Finally, the error of the calculation is assessed for a simple representative geometry.
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