Electronics Protection - Winter 2014 - (Page 16)

Feature A Better Alternative to Heat Pipes: Integrating Vapor Chambers into Heat Sinks George Meyer, CEO Celsia, Inc. Often in high power density or low profile heat sink applications, the spreading resistances in the base of the heat sink limits the performance of the design. Once it is determined that normal heat sink materials are either insufficient or too bulky to meet the design objectives, the obvious next step is to look at two phase spreading devices, such as heat pipes or vapor chambers. Either technology is often an improvement in these applications. The use of vapor chambers offers two distinct advantages over heat pipes, direct contact to the heat source and uniform spreading in all directions. Integrating heat sinks and vapor chambers is simpler than most people think and this integration often allows for further improvements in performance. Integrating Vapor Chambers into Heat Sinks One typical design incorporates three basic parts: the vapor chamber, an aluminum frame for mechanical attachments and a fin pack, which is often made of aluminum. These three parts are soldered into one assembly. An alternative to this design is to simply add the vapor chambers to the base of an extruded heat sink. In Figure 1, several standard-size vapor chambers Figure 1. are shown imbedded into the base of a large heat sink to provide an isothermal base. The heat sink in Figure 2, a cooling solution for highbrightness light emitting diodes (HBLEDs), shows how a vapor chamber can be integrated into a Figure 2. fin stack directly. Vapor Chamber Thermal Resistance The most commonly asked question relating to the design of a vapor chamber cooling solution is what is the effective thermal conductivity (W/m-K) of the vapor chamber? Because two phase devices do not exhibit a linear heat transfer behavior, this number is application specific. There are two main resistances within all two phase heat transfer devices: the evaporator resistance and the vapor transport resistance. The third resistance, the condensation resistance, is much smaller than the other two. In the vast majority of applications, the evaporation resistance is the dominate resistance; therefore, making these devices somewhat length independent. This means that a vapor chamber with a transport distance of 75 mm will have almost the same Tsource -Tsink as one with a 150 mm transport distance. This, in effect, doubles the effective thermal conductivity for the longer devices. Evaporator resistance is expressed in units of degree C/W/ cm2. At lower power levels, 5 to 10 W/ cm2, this resistance is on the order of 0.1 C/W/ cm2. As power densities increase, the evaporator resistance decreases until a performance limit is reached. This limit can extend to 200 W/cm2 and higher, depending on the vapor 16 Winter 2014 * www.ElectronicsProtectionMagazine.com chamber design. Figure 3 shows the evaporator resistance for one particular vapor chamber design. The vapor transport resistance is expressed in similar terms, but refers to the cross sectional area of the vapor space. Keep in mind, changes in temperature or working fluid will change these values. The values presented are typical values for a water-based vapor chamber operating at electronics cooling temperatures. Figure 3. This resistance is 0.01 C/W/ cm2. Figure 4 shows common vapor chamber cross sections of 2.0 mm to 3.5 mm thicknesses and widths from 20 mm to 80 mm. The cross sections are calculated and the terms expressed Figure 4. in simple C/W for each size. [1] The thermal models in Figure 5 compare a copper-based 1U heat sink with a vapor chamber-based 1U heat sink. In this type of application, where the heat is being spread uniformly more than it is being transported a long distance, the typical effective thermal conductivities are on the order of 1,000 to 1,500 W/m-K. In a small form factor such as a 1U heat sink where the transport length is short the effects of the vapor chambers is Figure 5. an improvement of 3°C to 4°C or about a 10 percent improvement over a copper base. This improvement is often critical in high ambient applications or where the gain is used to lower fan speeds for noise considerations. In summary, vapor chambers are easily integrated into thermal solutions and can offer thermal performance improvements on the order of 10 percent to 30 percent over copper and heat pipe based solutions and can often be lighter in weight than equivalent extruded or copper based heat sink. These improvements allow for designers to design for higher ambient or lower noise due to low required fan speeds. References - 1. Garner, S.D., "Heat Pipes for Electronics Cooling Applications", Electronics Cooling, Vol. 2, No. 3, 1996. For more information visit www.celsiainc.com. http://www.celsiainc.com http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Winter 2014

Editor's Choice
EMI Compliance: Choosing the Right Shielding and Gasketing
Thermal-Fluid Modeling for Flat Thin Heat Pipes/Vapor Chambers
Increase Rack Cooling Efficiency and Solve Heat-Related Problems
Seven Essential Cabinet Design Considerations for Protecting 19 Inch Electronics
A Better Alternative to Heat Pipes: Integrating Vapor Chambers Into Heat Sinks
Common IP Testing Failures and How to Avoid Them
Industry News

Electronics Protection - Winter 2014