Vapor Chambers

Standard heat pipes only transfer heat along the axis of the heat pipe, so they are best suited to cooling discrete heat sources.  Vapor Chambers or High Conductivity (HiK™ ) Plates are used to collect heat from larger area sources, and either spread the heat, or conduct it to a cold rail for cooling.  Vapor Chambers are generally used for high heat flux applications, or when genuine two-dimensional spreading is required.  The lower cost HiK™ plates are used when only high conductivity in a tailored direction is required.

As shown in Figure 1 and Figure 2, a vapor chamber is a planar heat pipe, which can spread heat in two dimensions.  They are typically used when high powers and heat fluxes are applied to a relatively small evaporator area.  During operation, the heat input into the evaporator vaporizes liquid within the evaporator wick.  The vapor then flows throughout the chamber, creating an isothermal heat spreader.  The vapor then condenses on the condenser surfaces, where the heat is removed by forced convection, natural convection, or liquid cooling.  Capillary forces in the wick then return the condensate to the evaporator.  Note that most vapor chambers are insensitive to gravity, and will still operate when inverted, with the evaporator above the condenser.

One of the advantages of vapor chambers is that they can be used as heat flux transformer, cooling a high heat flux from an electronic chip or laser diode, and transforming it to a lower heat flux that can be removed by natural or forced convection.

Figure 1.  Vapor Chambers can be used for heat flux transformation.

Figure 2 below shows the components for a typical copper/water vapor chamber.  The vapor chamber has a series of copper posts to support the top lid at temperatures below 100°C (where the pressure in the vapor chamber is below atmospheric).  The fins shown in Figure 2 are used to remove heat by forced convection.

Figure 2.  Typical vapor chamber components, from top to bottom: 1. Fin Stack, 2. Vapor Chamber Evaporator, 3. Vapor Chamber Lid.

Figure 2.  Typical vapor chamber components, from top to bottom: 1. Fin Stack, 2. Vapor Chamber Evaporator, 3. Vapor Chamber Lid.

Wicks have been developed for high heat flux vapor chambers that can remove up to 2000 W over 4 cm2, or 700 W over 1 cm2.  An example of one of these wicks is shown in Figure 3. As shown in Figure 4, the wicks provide high heat flux cooling with a very low thermal resistance.

Figure 3.  Specially designed vapor chamber wicks can remove 2000 W over 4 cm2, or 700 W over 1 cm2

Figure 3.  Specially designed vapor chamber wicks can remove 2000 W over 4 cm2, or 700 W over 1 cm2

Figure 4.  Innovative wick designs reduce the overall resistance of the vapor chamber to less than 0.12 °C-cm2/W.

Figure 4.  Innovative wick designs reduce the overall resistance of the vapor chamber to less than 0.12 °C-cm2/W.

Benefits

  • Multi-component mounting
  • C.T.E. matching allows direct bonding of electronics to the vapor chamber.  Aluminum Nitride Ceramic with Direct Bond Copper  has a C.T.E of approximately 5.5 ppm/°C
  • Thickness from 0.12” (3 mm)
  • Excellent Heat Spreading (Resistance < 0.15 K/W)
  • High heat flux to low heat flux transformation, see Figure 35.
  • Ideal for high heat flux/high performance applications
  • Very high heat fluxes:  500 W/cm2 demonstrated over 4 cm2 (2 kW total power)

Limitations

  • Increased cost compared to Standard Heat Pipes and HiK™ Plates.  HiK™  plates are normally chosen over vapor chambers, except for high heat flux applications, or where true two dimensional heat spreading is required.
  • Cannot be used as structural members without paying a weight penalty

See also: