Encapsulated Conduction Cooling

There are three types of conduction cooling:

  1. Baseline Aluminum Plate, which is the least expensive cooling method, when suitable
  2. Heat Spreaders/Thermal Doublers, which are materials like diamond and diamond composites.  They are mounted directly under the chip, and are very thin and have high thermal conductivity.  They have a much smaller scale than the other technologies discussed in these web pages, hence won’t be discussed further.
  3. Encapsulated Conduction Plates, which have a layer, typically aluminum, encapsulating a material with higher thermal conductivity.  They are the most expensive passive cooling technology, and are discussed below

If cost was no object, then diamond or diamond composites would be used for conduction cooling over large areas, however, their high cost means that they are only used as very thin heat-spreaders/thermal-doublers under specific high power chips.  Instead, metal composites or pyrolytic graphite is used to enhance the conductivity.  While less expensive, these material typically cannot be used by themselves, since they are often brittle, hygroscopic, and/or have relatively low strength.  To minimize these problems, encapsulated conduction cooling plates have been developed.  A core of high conductivity material is encapsulated in a metallic shell to provide protection and strength.

Figure 19. Thermal vias are required when the encapsulated material has poor out-of-plane thermal conductivity.

Figure 19. Thermal vias are required when the encapsulated material has poor out-of-plane thermal conductivity.1  1S. Kugler, “Aluminum Encapsulated APG High Conductivity Thermal Doubler” Spacecraft Thermal Control Workshop, El Segundo, CA, March 11-13, 2008

Anisotropic thermal conductivity is another factor that must be considered for some encapsulants.  For example, the in-plane thermal conductivity of pyrolytic graphite is 1000 to 1500 W/m K, but the out-of-plane thermal conductivity is only 10 W/m K.  As shown in Figure 18, aluminum thermal vias are located under the high power electronics components.  These vias provide an interface that supplies the heat to the pyrolytic graphite in the high conductivity plane.  Adding the pyramidal thermal vias to deal with the low out-of-plane thermal conductivity reduces the overall effective thermal conductivity.   Due to the effects of the thermal vias, independent measurements have documented thermal conductivities around 550W/m-K with an aluminum encapsulant.

Encapsulated Conduction Cooling Benefits and Limitations

The benefits and limitation of encapsulated conduction cooling are shown in Table 10.   The benefits include operation over a wide temperature range, long thermal transport lengths are possible, and that encapsulated conduction cooling cards are not affected thermally by acceleration or gravity.

The limits are higher cost and lower thermal conductivity compared to passive two-phase devices and the need to fix chip locations early in the manufacturing process.  Another limitation is the drop in effective thermal conductivity after several hundred thermal cycles, as the encapsulant moves relative to the thermal vias.  One set of tests reported that the effective conductance dropped by up to 12% (8% average) after ~220 thermal cycles between 100°C and -40°C.

An expensive manufacturing process with long lead times is required for encapsulated conduction cards, due to the encapsulation and the need for thermal vias.  The following steps are required:

  • Set location of electronics
  • Machine aluminum plate with vias, add graphite
  • Add aluminum top sheet
  • Hot isostatic press to bond the components
  • Final machining

Table 10.  Benefits and Limitations of Encapsulated Conduction Cooling

Benefits

Limitations

Thermal conductivity of 550 W/m K

Lower thermal conductivity than two-phase systems

Thermal performance unaffected by acceleration

Highest cost for passive thermal management

Lower density than two-phase systems

Lower specific thermal conductivity  than two-phase systems

Operates over a wide temperature range (Water heat pipes have low effective conductivities below 25°C)

Mounting locations for high power chips are fixed during the design

No length limitation in the vertical direction

Effective conductance can drop by up to 12% (8% average) after ~220 thermal cycles between 100°C and -40°C

Thickness from 0.060” (1.5 mm)

Mounting locations for high power chips are fixed during the design

Longer lead time than other passive devices

 

Encapsulated Conduction Cooling Selection Parameters

Selection parameters for Encapsulated Conduction Cooling are given in Table 11.  There is no set maximum heat flux that can be applied to an encapsulated conduction plate.  Instead, the maximum heat flux is set by the maximum allowable temperature of the component to be cooled (Note that due to its lower thermal conductivity, this heat flux is less than the maximum heat flux for two phase devices).

Encapsulated conduction cooling is much more expensive, has longer lead times, and a lower effective thermal conductivity than HiK™ plates, or vapor chambers. Because of this, it is normally chosen when none of the passive two phase devices are acceptable due to intrinsic limitations.    Encapsulated conduction cooling is recommended for the following cases:

  • Sustained high accelerations, when a two-phase device (heat pipe) can’t be oriented favorably
  • When heat must be transported below 25ºC
  • When thin sections are required (the minimum thickness for encapsulated conduction cooling is 1.5 mm (0.060 in.), while the minimum thickness for a HiK™ plate is 1.83 mm (0.072 in.)).
  • Over distances longer than 50 cm (20 inch) vertically

Table 11.  Encapsulated Conduction Selection Criteria

Parameter

Maximum Heat Flux

Depends on Geometry

Effective Thermal Conductivity

550 W/m-K

Density vs. Al

0.9 to 1.0 (Al and Mg)

Spreading

2 dimensional

Minimum thickness

1.5 mm (0.060 in.)

Maximum Dimensions

N.A.

Maximum Acceleration

Depends on stresses

Minimum Temperature

N.A.

Maximum Temperature

N.A.

Envelope Materials:

Al, Be, Al-Be, Al-Si, Mg, Cu

Envelope Material for Direct Bond:

Kovar

Typical Delivery Times

> 10 weeks

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