When to Use Heat Pipes, HiK™ Plates, Vapor Chambers, and Conduction Cooling

Electronics must be maintained below a specific temperature for mission success.   Within the thermal design engineer’s toolbox are passive technologies that are often considered for electronics cooling.  These include:

These webpages provide information on these technologies, including operating principles, thermal performance, thermal conductivity, size, weight, operational and storage temperature, mechanical strength, and cost. The objective is to provide the thermal engineer with the information necessary to conduct trade studies involving these technologies.

Conduction cooling is the standard for less challenging cooling applications.  It is also used in certain high-end applications, where passive two-phase cooling is not suitable.  Spot Cooling Heat Pipes can provide low thermal resistance paths between critical electronic components and heat sinks.  HiK™, or high conductivity plates, with embedded heat pipes can provide highly effective heat spreading or transport at low mass and cost.  Vapor chambers can act as flux transformers, spreading heat out in two dimensions. They are used when temperature uniformity is important.  These are the standard methods for moving heat from electronics over short distances, to a location where it can be removed by liquid or forced air cooling.

Table 1 summarizes each of the technologies, evaluating effective densities, spreading, effective thermal conductivity, maximum heat flux, minimum thickness, maximum height, and relative cost.  The information used to derive the table is discussed in the links above.  Note that the table progresses from least to most expensive.  In general, two phase heat transfer devices weigh more than aluminum or encapsulated conduction cards, but have higher effective thermal conductivity.  As a result, they have higher specific thermal conductivity.

Table 1. Comparison between Baseline Conduction Cooling, Spot Heat Pipes, HiK™ Plates, Vapor Chambers, and Encapsulated Conduction Cooling.

Table 1. Comparison between Baseline Conduction Cooling, Spot Heat Pipes, HiK™ Plates, Vapor Chambers, and Encapsulated Conduction Cooling.

 

  • Maximum Heat Flux for Heat Pipes, HiK™ Plates, and Vapor Chambers is set by the wick design
  • Maximum Heat Flux for Aluminum and Encapsulated Conduction Plates is set by the maximum allowable chip temperature, and is lower compared to heat pipe solutions

Selection Criteria Summary

Benefits, Limitations, and Selection Criteria are explained in detail on the individual web pages, and summarized below.

Baseline Aluminum Plates  Aluminum plates are both the lowest performing, and the cheapest solution for thermal management.  The thermal conductivity is roughly 200 W/m K, and they provide spreading in two dimensions.  Due to their lower conductivity, they are generally suited for low power systems.  When analyses show that the peak temperature or mass is too high for a simple aluminum plate, then HiK™ Plates, Vapor Chambers, or Encapsulated Conduction Plates should be used.

Spot Cooling Heat Pipes are used for 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.  They are inexpensive, their effective thermal conductivity ranges from 10,000 to 100,000 W/m-K, and the heat transfer is one-dimensional.

Figure 1. Spot cooling heat pipes cool discrete components by moving heat off the chip to a remote heat sink.

Figure 1. Spot cooling heat pipes cool discrete components by moving heat off the chip to a remote heat sink.

 

HiK™ Plates are plates with embedded heat pipes to increase the effective thermal conductivity.  Their effective thermal conductivity ranges from 600 to 1200 W/m K.  They are used for strategic thermal spreading.  The heat transfer is 1.5-D, since there is preferential thermal spreading along the heat pipes axes.  They are relatively inexpensive, and there are plate materials that allow for direct bonding of electronics.

Figure 2. HiK™ plates provide an inexpensive way to increase the effective thermal conductivity of baseline aluminum plates by two to four times.

Figure 2. HiK™ plates provide an inexpensive way to increase the effective thermal conductivity of baseline aluminum plates by two to four times.

 

Vapor Chambers are planar heat pipes for heat spreading and isothermalizing.   Like conventional cylindrical heat pipes, vapor chambers transport heat from a heat source to a heat sink with a very small temperature gradient.  Vapor Chambers are generally used for high heat flux applications, or when genuine two-dimensional spreading is required.  The effective thermal conductivity ranges from 5,000 to 100,000 W/m K, with two-dimensional spreading.  Vapor cambers can be designed for High Heat Flux (up to 750 W/cm2) capability, are isothermal to within 1-2°C or less, and allow for direct bonding.

Figure 3. Vapor Chambers spread heat in two dimensions, with very high effective conductivity.

Figure 3. Vapor Chambers spread heat in two dimensions, with very high effective conductivity.

 

Encapsulated Conduction Plates protect a fragile, high conductivity material by encapsulating it, typically with aluminum.  They have an effective thermal conductivity around 550 W/m-K, with two dimensional spreading.  In some encapsulated conduction plates, the effective thermal conductivity can decrease with thermal cycling.

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
Temperature profiles on plates with identical dimensions, calculated with a 50 W heat source at the top, and forced air convection cooling on the entire back surface of the plates. The maximum temperature of the aluminum plate was 55°C.

Figure 4. Temperature profiles on plates with identical dimensions, calculated with a 50 W heat source at the top, and forced air convection cooling on the entire back surface of the plates. The maximum temperature of the aluminum plate was 55°C.

 

We hope that these webpages help Design Engineers by providing direction as to the “Use and Benefits” and “Selection Criteria” for these technologies.   Please contact ACT if you have any questions, or need to discuss an application in more detail.

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