Thermal Interface Material and the role it plays in Thermal Management
If you ever thought of upgrading your computer by replacing a CPU, you may have noticed a packet of white thermal paste with the processor; it is sometimes spread on the heat sink, between the CUP and the heatsink to improve thermal conductivity. The proper operation of the silicon processor, one of the most sophisticated feats of engineering achieved by humankind to date, depends to a large extent on that critical assembly piece, the thermal interface material. IC thermal management, generally performed by mechanical engineers, is a critical aspect of both consumer products and high performance computing. Often an engineering team is involved in power management, or IC thermal management. Sometimes the heat removal is called backend power management to distinguish from the software aspect of the power management.
In general, materials closest to the regions of heat release have the most importance on the IC or transistor operating temperature. Crystalline silicon has excellent an thermal property, its thermal conductivity being in the same order of magnitude as the materials of which heat sinks are usually made, aluminum or copper. Silicon crystal has the same structure as diamond, diamond cubic. This high thermal conductivity of single crystal silicon, in addition to its excellent oxide properties, has helped keep Moore’s Law going strong for close to five decades.
Finite Element Analysis used to determine heat transfer
A seemingly easy task of engineering a heat sink on an IC turns out to be extremely difficult. The heat flow passes from the IC, through the thermal paste, and must next enter the metallic heat conductor, or heat sink, on its way out to the environment. This heat transfer involves thermal conduction, or heat transfer through solids. Usually a fan moves air over the heat sink. Thus, heat transfer by convection and fluid dynamics comes into play. Convection is the heat transfer through a fluid, either gas or liquid. Finite element analysis or computational fluid dynamics (CFD) is very useful in analyzing the convective heat transfer to the ambient or room air.
There are many challenges with both thermal and thermo-mechanical performance: reliability, manufacturability and finally, cost, that enter into the equation. To highlight one aspect, one must sacrifice thermal conductivity of a thermal interface material sometimes to the point of reducing its value nearly a hundred-fold compared to the first medium heat flow encounters, silicon. And this is the second most important material, from a thermal perspective, required for efficient power dissipation. Packaged thermal heat spreaders were a breakthrough a few years back that decreased the thermal chokepoint by several fold. However, the addition of a thermal spreader on the chip package requires changes internal to the CPU package and manufacturing process. What happens internally in the CPU is carefully gauged by their makers to make life easier for the product designers and end users. Not all CPU form-factors can use the internal heat spreader, and this increasingly is becoming the case, with mobile computing being the preferred consumer choice.
If you decide to explore putting a heat spreader into your package, Glew Engineering can provide the necessary advice and engineering work from our experienced engineers. We can help with sorting out all the elements in the hierarchy of the heat path out to the environment, so the final product is not subject to pre-mature thermal degradation. We can also provide innovative suggestions and technical support for heat removal for mobile CPUs, those used in smart phones, tablets and notebook computers.
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