Berkeley ELENG C245 - Active fin structure for electronic device cooling using gap closing actuator

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IntroductionFabrication process and configurationTheoretical design and thermal performance calculationResults and discussionConclusionReferenceActive fin structure for electronic device cooling using gap closing actuator Youngshik Shin and Tae-Kyu Lee* Department of Mechanical engineering, *Department of Materials Science and Engineering University of California, Berkeley The importance of cooling an electronics device increases these days with a vast amount of interest. According to this issue, a design of an active fin structure using the motion of gap closing actuator is suggested. A gap closing actuator is designed on a 5X5cm substrate. The thermal performance of this structure is calculated, based on a lumped capacitance method. 30 times increased performance is achieved through this structure compared to a plain heat sink with a mainstream air velocity of 5m/s. Because of the efficient heat pumping mechanism of active fin structure, both important factors, thermal performance and overall size of the heat sink is improved. Introduction Nowadays, heat generation from electronic devices is one of the critical problems that we have to solve. In a certain electrical circuit, a good performance in an aspect of thermal condition, requires a functional temperature limit. Out of this functional temperature limit, the probability of a cause of logic error, unexpected diffusion with phase transformation, thermal fatigue and plastic deformation in the system increases, which may result in a mis-operation of the component. The maximum functional temperature to ensure operation and safe performance for an electronic device is typically under 100°C. For example, the Intel 850 chipset allowed the operating temperature to 50°C, with a recommended airflow over the package for 150 LFM(linear feet per minute). The thermal solution, which is applied to this Intel 850 chipset, is a passive simple extruded heat sink with thermal and mechanical interfaces. Additionally to these kinds of cooling methods, to improve the system cooling characteristics, a redesign of fans, vents and ducts can be considered. But because of a vast development in the IC technology, electrical devices appears in smaller scale with a higher performance ability, which come along with a general aspect that, the more heat is generated as the size of electronic device is getting smaller. Fig.1 Generic configuration of active fin with heat source Which provides a need of dissipation of heat with an enhanced cooling method. The recent research in this area suggests a few ways for a solution. One is using a fin structure with am additional fan, another is using a heat pipe, which is considered specially for Heat source Thermal interface (high thermal conductivity)FanMain stream Heat sink(Active fin structure) Side viewlaptop cooling. A cooling system by using micro-fluidic channels is also suggested, which has the advantage that heat transfer coefficient can be increased. Since heat transfer coefficient is inversely proportional to hydraulic diameter of the channel or it can be larger than single phase convection heat transfer case by order of magnitude if phase changed is used. But all of the above methods of cooling are “passive”, which provided a limit in the effectiveness. Based on these aspects, a new method is suggested by using an “active” micro-fin structure. A gap closing actuator is applied for this active fin structure on a 5X5cm substrate. The substrate is attached to the heat spreader plate, which is attached to heat source with a fan as described in Figure 1. Hot air near the surface can be pumped out and cooler air fills in the space by the motion of the gap closer. Fabrication process and configuration Figure 2 shows the schematic of fabrication process. For a high aspect ratio of approximately 30:1, a deep reactive ion etching (DRIE) is applied. Using a plasma source, DRIE can achieve a high aspect ratio with a side wall angle of 90°±2°. The etching rate is on the order of 2 to 3µm/min. Additionally to a simple DRIE process a silicon fusion bonding (SFB) process is added. As shown in Fig.2 Schematics of the micro-fabrication process Fig. 3 Schematics of a basic unit active fin structure Fig2(d), a second wafer is fusion bonded onto the bottom wafer. After polishing down to approximately 200µm thickness. Followed by a patterning of a DRIE photoresist mask (e) and a DRIE etch through the second wafer (f). Figure 3 shows a basic unit of the final layout. Each basic unit is in a size of 2x2mm. Based on this geometry 190 units can be placed on a 5X5cm substrate. An electrically isolated wall is also established to double the number of gaps. Based on this geometry, the total number of gaps in a 5X5 cm substrate is ~100,000, and the total effective volume of gaps (Vtot) is the multiplication of this number of gaps with the individual gap volume, which is 1.68x10-7 m3. Theoretical design and thermal performance calculation Figure 4 shows the schematics of heat rejection during one cycle movement of gap closing actuator, the mechanism which is used in figure 3. Based on this geometrical design, the calculation is done with the following approximated properties of air at 300K as follows: Density (ρ) = 1.1614 kg/m3 Heat capacity (Cp) = 1007 J/kg.K Thermal conductivity (k) = 0.0263 W/mK 2mm1mm1000µm7µm14µm24µmStationary wall (stopper)Moving wall Stationary wall Stationary wall (electrically isolated)(a)(b)(c)(d)(e)(f)(g)Fig. 4 Schematics of heat rejection during one cycle movement of gap closing actuator To simplify the analysis, Biot number (Bi) is checked as follows: 1.0<=khLBi Because of the smallness of characteristic length, L (on the order of 10-5m) Bi is very small (Bi <0.1) for a wide range of h (heat transfer coefficient between the air and surface). It is, therefore, reasonable to assume a uniform temperature distribution across air in the gap at any time during a transient process (Lumped capacitance model). Then, the change in air temperature can be expressed as follows. ])(exp[ tVchATTTTssisiρθθ−=−−= where, Ts is the surface temperature and Ti the main stream temperature The above equation shows that the temperature difference between solid surface and air must decay exponentially to zero as t increases. The quantity (hAs/ρVc) can be interpreted as a thermal time


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Berkeley ELENG C245 - Active fin structure for electronic device cooling using gap closing actuator

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