Minimizing Thermal Resistance with Direct Attach Heat Spreaders

Wednesday, February 01 2012

Low CTE AlSic HiK Plate

An embedded heat pipe plate (HiK Plate), is a machined plate (typically aluminum) that has heat pipes either soldered or epoxied in place to create an enhanced heat transfer path from the source to the sink.

Aluminum silicon carbide (AlSiC) is a metal matrix composite that combines the highly conductive properties of aluminum with the favorable CTE of silicon. The CTE value can be matched to that of the electronic device by adjusting the composition of Al and SiC. The isotropic controlled thermal expansion values can range from 7-12 ppm/°C. AlSiC typically has a thermal conductivity of 200 W/m-K. While this can be effective in many cases, high power or concentrated heat loads will require greater heat spreading capability. By embedding heat pipes into the AlSiC, bulk thermal conductivity values ranging from 500 to 1,000 W/m-K can be realized (dependent on overall size of the heat spreader). The increased conductivity is attributed to the exceptionally high thermal conductivity of heat pipes. Heat pipe operation is illustrated in Figure 1.

Heat pipes operate by vaporizing the working fluid at the heat source or evaporator end, moving the vapor by internal pressure difference and condensing at the heat sink or condenser end. The liquid is then returned to the evaporator by capillary force provided by an internal wick structure. For HiK plates used in electronics cooling, the typical heat pipe envelope material/working fluid combination is copper-water. Water heat pipes operate over the range of 20°C - 150°C, making them an ideal choice for electronics cooling. The sealed heat pipe creates a passive closed 2loop system, which generates long life and high reliability.

Figure 2 shows the temperature profile of two heat spreaders of identical dimensions comparing a HiK AlSiC plate with a standard AlSiC plate. A heater was placed in the middle and the edge temperature of each plate was controlled. The hot spot temperature at the source for the AlSiC HiK plate was less than half that of standard AlSiC. This decreased hot spot temperature can translate into higher allowable electronics power or increased thermal margin.

AlSiC HiK plates are particularly effective in systems with multiple heat loads. The heat pipe pattern can be optimized to take advantage of the system geometry and ambient conditions. When designing a HiK plate it is important to consider individual heat pipe limitations to assure reliable operation for a specified system. Whether the final design relies on conduction through the plate to the liquid cooled edge or spreading heat over a larger area to enable an air-cooled heat sink, the AlSiC HiK plate can provide advantageous heat dispersion, lightweight packaging, and direct component attaching for lower interface resistance.

Vapor Chambers
A vapor chamber is a planar, twodimensional heat pipe with exceptional heat transfer capability and very low thermal resistance. Like HiK plates, vapor chambers are used for heat spreading and heat transport applications.

Low CTE vapor chambers were designed primarily for high power, high heat flux electronic components. Often individual components are directly attached to the vapor chamber surface. Heat fluxes of 700 W/cm2 to over 1cm2 and total power of 2000W over 4cm2 have been demonstrated. Vapor chambers operate similarly to heat pipes, utilizing the benefits of two-phase heat transfer and liquid return via an internal wick structure.