DEVELOPMENT OF HEAT TRANSFER TOOLS FOR SIZING FLEXIBLE GRAPHITE SPREADERS IN MOBILE APPLICATIONS

摘要

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.