Vapor Chambers

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.  As shown in Figure 16, vapor chambers can accept heat from a high heat flux source, and spread the heat uniformly over a large area.

Figure 17.  Vapor Chambers can be used for heat flux transformation

Figure 18.  (a) Typical vapor chambers.  (b) Vapor chamber internals. (c) Vapor chamber with DBC envelope, for direct die attach of vertical cavity surface emitting laser chips.

Figure 18. (a) Typical vapor chambers. (b) Vapor chamber internals. (c) Vapor chamber with DBC envelope, for direct die attach of vertical cavity surface emitting laser chips.

 

Figure 17(a) shows several typical vapor chambers, while Figure 17(b) shows an example of vapor chamber internals and the faceplate.  Figure 17(c) shows a vapor chamber with a Direct Bond Copper (DBC) envelope, for direct mounting of vertical cavity surface emitting laser (VCSEL) chips.  (See also, “Low CTE, High Heat Flux, High Power, Low Resistance Vapor Chambers or Thermal Ground Plane”).

The primary difference between a vapor chamber and a spot-cooling heat pipe is that a vapor chamber transfers heat in two dimensions, while a spot-cooling heat pipe only transfers heat in one dimension.  HiK™ plates have a lower thermal conductivity and cost, and spread heat in 1.5 dimensions. Vapor chambers are roughly 2.3 times the density of a HiK™ plate, but have an effective thermal conductivity that is 10 to 100 times higher.

Vapor Chamber Benefits and Limitations

Table 8 gives the primary benefits and limitations of vapor chambers.  Benefits of vapor chambers include that they are isothermal to 1-2°C, can be used to cool multiple components, can be made as thin as 3mm, and have a low thermal resistance.  Heat fluxes for standard wicks are similar to heat pipes, but can be increased significantly with wick enhancements.

In addition to the Standard heat pipe limitations, the primary limitations are that vapor chambers have a higher cost compared to HiK™ plates, and they cannot be used as structural members.  The maximum temperature is 105°C for standard vapor chambers, since the facesheets tend to bow when the water pressure is higher than atmospheric.  This can be increased to 150°C with special vapor chamber designs.

Finally, the other devices discussed here can have the heat input and heat output surfaces at any orientation with respect to each other.  In contrast, the evaporator and condenser of a vapor chamber are always parallel, either in-plane, or on opposite sides of the vapor chamber.

Table 8.  Benefits and Limitations of Vapor Chambers

Benefits

Limitations

Thickness from 0.12” (3 mm)

 Standard heat pipe limitations

Cheaper than encapsulated conduction cooling

Increased cost compared to HiK™ plates
Excellent Heat Spreading

Higher Minimum thickness than other options

Excellent Heat Spreading

Cannot be used as structural members without paying a weight penalty

Resistance < 0.15 °C/W, < 0.08 °C/W for special wicks

Maximum temperature of 105°C for standard vapor chambers, 150°C for enhanced vapor chambers

Excellent Isothermalization

Evaporator and Condenser must be in the same plane, or on parallel planes

High heat flux to low heat flux transformation

Freeze/thaw tolerant

Ideal for high heat flux/high performance applications.  Heat flux > 60 W/cm2, up to 750 W/cm2 for special wicks

Direct Bond to electronics possible

Not Affected by thermal cycling

Vapor Chamber Selection Parameters

Vapor chambers are best suited for:

  • Very high heat fluxes, up to 750 W/cm2
  • Flux transformation in a thin structure
  • Very uniform temperature profiles

Selection criteria are given in Table 9.  The important things to remember about vapor chambers are that they have similar benefits to heat pipes: high effective thermal conductivities, passive operation, the ability to handle very high heat fluxes, and can be used for direct die attach with AlN DBC.

 Table 9.  Vapor Chamber Selection Criteria.

Parameter

Maximum Heat Flux for standard systems

~ 60-70 W/cm2

Maximum Heat Flux for Optimized Wick, specific Location

 500 W/cm2 over 4 cm2                                                   750 W/cm2 over 1 cm2

Effective Thermal Conductivity

5,000 to 100,000 W/m-K

Density vs. Al

~2.8

Spreading

2 dimensional

Minimum thickness

3mm (0.120”)

Maximum Dimensions

10 in. x 20 in. (25 cm x 50 cm)

Maximum Acceleration

2-3 g

Minimum Temperature

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

Maximum Temperature

~105°C, 150°C for non-standard designs

Envelope Materials:

Copper

Envelope Material for Direct Bond:

AlN Direct Bond Copper

Typical Delivery Times

8-10 weeks

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