Spot Cooling Heat Pipes

Heat pipes are used for moving heat from point A to point B, spreading, and isothermalization.  Spot cooling refers to cooling discrete components by moving heat off the chip to a remote heat sink.  The primary use of spot-cooling heat pipes is to cool high power chips to decrease temperature and to increase maximum power output.

Consider spot cooling heat pipes whenever you need to cool individual components by transferring heat to an external sink.  They are low cost, and can be bent as required.  Several typical spot cooling heat pipes are shown in Figure 8, where the heat pipes transfer heat between different elevations and orientations.  Heat pipes can also be used in “block-pipe-block” configurations, where the evaporator and condenser of the heat pipe are embedded in blocks attached to the electronics and the heat sink; see Figure 9.

Figure 8.  Spot cooling heat pipes cool discrete components by moving heat off the chip to a remote heat sink.

Figure 8. Spot cooling heat pipes cool discrete components by moving heat off the chip to a remote heat sink.

Figure 9.  Many spot-cooling heat pipes are used in a block-pipe-block configuration.

Figure 9. Many spot-cooling heat pipes are used in a block-pipe-block configuration.

Spot Cooling Heat Pipe Uses and Benefits

The benefits and limitations of spot cooling heat pipes are shown in Table 4.   The benefits include passive operation, very high thermal conductivity and low cost.  Effective thermal conductivities range from 10,000 to 100,000 W/m K, higher than any other passive thermal device except vapor chambers.  They are typically not structural elements, and transfer heat one-dimensionally.  Other benefits include flexibility (allowing the heat pipes to be routed around other components), freeze/thaw tolerant, and shock/vibration tolerant.

Spot cooling heat pipe limitations are the same as for other passive two phase heat transfer devices.  In addition, while they are not suitable for direct bonding to electronics, they can be embedded in a suitable block.

Table 4.  Spot Cooling Heat Pipe Benefits and Limitations.

Benefits

Limitations

Cool high power chips to decrease temperature and to increase maximum power output

Standard heat pipe limitations
Effective Thermal Conductivities of 10,000 to 100,000 W/m-K

Maximum temperature of 150°C for Standard heat pipes, 270°C for enhanced heat pipes

Cheaper than HiK™ plates, Vapor Chambers, and Encapsulated Conduction Cooling.  Comparable to aluminum conduction cooling

Cannot be used as structural member

Very Flexible – Can be formed to fit countless geometries

No direct bonding to electronics, but can mount in suitable material
Not Affected by thermal cycling

Shock/Vibration Tolerant

Freeze/Thaw tolerant
Heat flux up to 60-70 W/cm2, 500 W/cm2 with special wicks

Information on minimum radii and tolerances for copper water heat pipes can be found in our Heat Pipe Design Guide, and our brochure on Heat Pipe ReliabilityACT’s Heat Pipe Calculator allows you to calculate the amount of power a given heat pipe can carry as a function of temperature and diameter.

Spot-cooling heat pipes transfer heat in a single dimension, while vapor chambers transfer heat in two dimensions.  HiK™ plates transfer heat in 1.5 dimensions:  They have a very high effective thermal conductivity in the direction of the heat pipes, but also spread heat perpendicular to the heat pipes by conduction in the plate material.

Spot Cooling Heat Pipe Selection Parameters

Spot cooling heat pipes are used to cool individual, high power components by transferring heat to an external sink.  Selection criteria are given in Table 5.  The important things to remember about spot cooling heat pipes are that they have very high effective thermal conductivities, transfer heat in one-dimension, and are shock/vibration and freeze/thaw tolerant.  They are extremely flexible, so can be bent around other components, and can accommodate different source and sink planes.

Table 5.  Spot Cooling Heat Pipe Selection Parameters.

Parameter

Maximum Heat Flux

~ 60-70 W/cm2, 500 W/cm2 with special wicks

Effective Thermal Conductivity

10,000 to 100,000 W/m-K

Density vs. Al

~1.3

Spreading

1 dimensional

Minimum thickness

3 mm (< 1.8 mm flattened)

Maximum Dimensions

~ 10 in. (25 cm) high

Maximum Acceleration

2-3 g

Minimum Temperature

-55°C, Conduction Heat Transfer only below 0°C

Maximum Temperature

~150°C for Copper, 270°C for Titanium and Monel envelopes

Materials

Copper, Titanium and Monel for high temperatures

Typical Delivery Times

3-4 weeks


More information on When to Use Heat Pipes, HiK™ Plates, Vapor Chambers, and Conduction Cooling: