As commercial and military electronics applications continue to “push the envelope” with higher powers and smaller packaging requirements, it is becoming more critical to minimize the thermal resistance from the heat load to the heat sink. Ideal packaging materials must have high thermal conductivity and coefficient of thermal expansion (CTE) values that are compatible with the integrated circuit device while remaining lightweight and affordable. Given these constraints, removing high heat loads and/or high heat flux from these electronics presents some interesting challenges for design engineers.

Figure 1. Heat Pipe Operating Principles

One approach is to spread the heat to a larger area for dissipation through high thermal conductivity bulk materials or custom designed heat spreaders. Bulk metallic components such as copper or aluminum have high CTE values compared to most electronic components. Because of this CTE mismatch, heat spreading materials require the use of stress reducing components such as intermediate substrates, thermal pads, or grease for attachment to the semiconductor device. These materials compensate for the expansion differences between the semiconductor electronics and the heat spreader. However, they often provide significant thermal resistance due to their poor thermal conductivity. Thermal gap pads have thermal conductivities ranging from 0.5 to 3 W/m-K. This is significantly less than the metallic spreaders which range from 180 W/m-K in aluminum to over 400 W/m- K in copper spreaders.

Packaging materials with device and substrate compatible CTE values minimize the thermally induced stresses during power cycling. Thermal stresses often result in the delamination of substrates disrupting the thermal dissipation path and causing premature electronics failure. Good CTE compatibility mitigates this issue by eliminating the need for intermediate substrates and allowing direct attachment using high conductivity solder.

Figure 2. Thermal Profile of an AlSiC Plate and AlSiC Hi-K Plate

An optimum low CTE heat spreader combines low cost, lightweight materials, with the high heat transfer rate of passive, two phase heat transfer technology (heat pipes). The two most common systems, with a proven record of high performance and reliability in industrial applications are embedded heat pipe plates and vapor chambers. Solutions for improved CTE for both systems have been developed. An aluminum silicon carbide (AlSiC) embedded heat pipe plate and an aluminum nitride /direct bond copper vapor chamber allow for direct component attachment as well as distinctive thermal advantages. The two device structures, applications and implementation are different and will be examined here.

Low CTE AlSic HiK Plate

Figure 3. Low CTE High Heat Flux Vapor Chamber

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

Figure 4. High Heat Flux Low CTE Vapor Chamber Test Results

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.

Vapor chambers for electronics cooling have typically been manufactured using copper for the envelope and water as the working fluid. As discussed previously, copper has a relatively high CTE and therefore substrate and compliant interfaces are required to attach to low CTE electronics chips. This results in unfavorable additional thermal resistances, lowering the allowable power and heat flux of the electronics.

To address this historical shortcoming, ACT developed a low CTE vapor chamber using aluminum nitride (AlN) ceramic plates with thin layers of direct bond copper (DBC). The aluminum nitride ceramic with DBC has a CTE of approximately 6 ppm/°C, which is similar to many electronic chip packages. The copper DBC on the inside of the vapor chamber provides material compatibility with the water working fluid. The copper DBC on the exterior allows for direct solder attach and electrical circuitry directly etched into the surface of the vapor chamber. Figure 3 shows a multiple evaporator, low CTE, high heat flux vapor chamber, etched and prepped for direct die attach of four (4) chips.

In addition to eliminating the need for an intermediate substrate and the associated interface resistances, the vapor chamber was also designed to handle high heat fluxes. This is made possible through advanced wick designs that enable effective separation of liquid and vapor phases. In this design, a thick wick structure, which can readily absorb the condensate, is co-located with a very thin wick that has very low evaporation thermal resistances. Resulting vapor chamber performances with these wick structures have exceeded 500 W/cm2 in heat flux. Heat flux limits of 30 to 50 W/cm2 are typical in commercially available off-the-shelf copper/water vapor chambers. Evaporator thermal resistance less than 0.05°C-cm2/W has been demonstrated. Test results in Figure 4 show the performance relative to the heat flux for a 7.6 cm × 12.7 cm, 3mm thick low CTE vapor chamber.

The wick structure design is scalable with heat source sizes from less than 0.6 cm2 to 10 cm2. This type of performance is favorable for high heat flux chips such as IGBTs and MOSFETs as well as high power laser diode arrays and phased arrays.

Conclusion

Both AlSiC Hi-K plates and high heat flux AlN/DBC Vapor Chambers have demonstrated the ability to be directly attached to electronic devices. An AlSiC HiK plate can provide heat spreading for multiple devices via a custom heat pipe layout that can be designed for manufacturability and thermal performance. The rugged, lightweight, AlSiC HiK plate can also be used structurally in a system. The low CTE, high heat flux vapor chamber is primarily for high, concentrated heat loads and creates a nearly isothermal base. The nearly isothermal vapor chamber can be used for spreading heat in air-cooled applications and for heat transport in edge cooled liquid systems.

This article was written by Bryan Muzyka, Sales Engineer, and Pete Ritt, Vice President, Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, Click Here .


Defense Tech Briefs Magazine

This article first appeared in the February, 2012 issue of Defense Tech Briefs Magazine.

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