Vapor Chambers Explained

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.

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.

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

One of the advantages of vapor chambers is that they can be used as heat flux transformers, 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 below shows the components of 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 innovative fins shown in Figure 2 are used to remove heat by forced convection.

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 2. As shown in Figure 3, the wicks provide high heat flux cooling with a very low thermal resistance.

Figure 2.  Specially designed vapor chamber wicks can remove 2000W over 4 cm², or 700W over 1 cm²

Figure 3.  Innovative wick designs reduce the overall resistance of the vapor chamber to less than 0.12 °C-cm2/W.

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 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 of 10 to 100 times higher.

Figure 4.  (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.

Vapor Chamber Benefits and Limitations

Table 1 gives the primary benefits and limitations of vapor chambers.  The 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 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 two 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 face sheets tend to bow when the water pressure is higher than atmospheric.  This can be increased to 150°C with specially designed vapor chambers but is only warranted when the heat flux is sufficiently high to warrant the increased cost.

Finally, the other two-phase devices 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 1.  Benefits and Limitations of Vapor Chambers

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 2.  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 2.  Vapor Chamber Selection Criteria

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