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November 2017 • The PCB Design Magazine 71 HEAT TRANSFER AND THERMAL CONDUCTIVITY: THE FACTS A heat sink is usually made from a solid piece of metal that has a much higher bulk thermal conductivity than a thermal interface material. At a microscopic level, the interface between the component and the heat sink is likely to be un- even and air will be trapped within this space. Air is a very poor conductor of heat so an in- tervening thermal interface material layer is ap- plied to remove these air pockets and therefore improve the interface's thermal conductivity. The thermal interface material reaches op- timum thickness when all the air is expelled and a uniform film is achieved, maximising the contact area of the component to the heat sink. Anything in excess of this is relying upon the thermal conductivity of the thermal interface material itself and as we know, this is nowhere near as thermally conductive as solid metal. So, the golden rule is: apply only as much as is re- quired to remove air and improve the quality of the mating surfaces; that way you will achieve the most efficient heat transfer between a com- ponent and its heat sink. As always, I strongly recommend you get some expert advice before you settle on any par- ticular material or processing method. PCBDESIGN Jade Bridges is the European technical support specialist for Electrolube. A KAIST team reported ultra-flexible organic flash memory that is bendable down to a radius of 300μm. A joint research team led by Professor Seunghyup Yoo of the School of Electrical Engi- neering and Professor Sung Gap Im of the Depart- ment of Chemical and Biomolecular Engineering said that their memory technology can be applied to non-conventional substrates, such as plastics and papers, to demonstrate its feasibility over a wide range of applications. Flash memory is a non-volatile, transistor-based data-storage device that has become essential in most electronic systems in daily life. With straight- forward operation and easy integration into NAND or NOR array architecture, flash memory is the most success- ful and dominant non-volatile memory technology by far. The solution processing used for the preparation of most of the polymeric dielectric layers also makes it difficult to use them in flash memory due to the complexity involved in the formation of the bilayer dielectric structure, which is the key to flash memory operations. The research team tried to overcome these hurdles and realize highly flexible flash memory by employing thin polymeric insulators grown with initiated chemical vapor deposition (iCVD), a vapor-phase growth technique for polymers that was previously shown to be promising for the fab- rication of flexible TFTs. The KAIST team produced flash memory with programming voltages around 10 V and a pro- jected data retention time of over 10 years, while maintaining its memory per- formance even at a mechani- cal strain of 2.8%. This is a sig- nificant improvement over the existing inorganic insulation layer-based flash memory that allowed only a 1% strain. Highly Flexible Organic Flash Memory for Foldable and Disposable Electronics Why do the majority of thermal interface materi- als have to be applied in very thin layers?

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