We’re running a two part series at the ATS Thermal Blog this week on vapor chambers, what they are, how they work, and their use in thermal management. In part 1 we’ll cover what a vapor chamber is and their effectiveness.
Increasing heat fluxes and decreasing sizes pose a major challenge for keeping electronic components below their critical junction temperatures. To cool a high power device with a small footprint requires a heat sink larger than the component. This size difference creates an added thermal resistance, called spreading resistance, which is usually in the same order of magnitude as the heat sink thermal resistance.
Spreading resistance is observed in combinations of very high performance heat sinks and small heat source sizes, e.g. 10 x 10 or 15 x 15 mm. There are many ways to reduce this added resistance. One is to use a highly conductive material, such as copper. It has a smaller spreading resistance than aluminum, although it is much heavier and costlier. Other ways to lower spreading resistance include the use of heat pipes, liquid cooling, vapor chambers, micro TECs (thermoelectric coolers), and the recently developed Forced Thermal Spreader (Advanced Thermal Solutions, Inc.). The focus of this article, however, will be the use of vapor chambers.
A vapor chamber (VC) is basically a flat heat pipe that can be part of the base of a heat sink. It is vacuumed and then injected with just enough liquid, e.g. water, to wet the wick. The theory of operation for a vapor chamber is the same as for a heat pipe. The heat source causes the liquid to vaporize on the evaporator side. The resulting pressure increase in this area forces the vapor into the condenser side, which is the base of the heat sink. Here, the vapor transfers the heat to the heat sink, and it then condenses back to liquid. The liquid is pumped back to the base through the capillary action of the wick structure.
It has been shown that for electronics cooling applications, sintered copper and water are the best choices for the chamber material and its internal liquid. The advantages of water are its high thermal conductivity, high surface tension, and non-toxicity.
A vapor chamber is generally used to spread heat more uniformly than with a solid metal block. This is due to the chambers high equivalent thermal conductivity. Figure 1 shows the temperature distribution in two heat sinks: one has a solid base and the other has a vapor chamber in its base. The heat sink without the VC shows temperature concentrated on top of the heat source, while the major portion of the heat sink is running cold. The result is low thermal performance. On the other hand, the heat sink with the VC in its base shows a very uniform temperature distribution. The very high equivalent thermal conductivity of the vapor chamber has spread the heat uniformly, leading to more efficiency from the heat sink.
The effective thermal conductivity of heat pipes and vapor chambers is estimated to be 5,000-20,000 W/moC . It should be noted that when a heat pipe is attached to the base of a heat sink the extra interfacial resistance from this bonding can degrade the effective thermal conductivity to about 4,000 W/moC. A vapor chamber, however, has the advantage of being part of the heat sink base, eliminating the interfacial resistance.
In part 2 of our two part series on vapor chambers, we’ll cover more about why vapor chambers are effective, as well as their application in thermal management of electronics.
- Mehl, D., Dussinger, P., Use of Vapor Chambers for Thermal Management, ThermaCore Inc.
- Chi, S., Heat Pipe Theory and Practice, Hemisphere Publishing, 1976.
- Prasher, R., A Simplified Conduction Based Modeling Scheme for Design Sensitivity Study of Thermal Solution Utilizing Heat Pipe and Vapor Chamber Technology, J. Electronics Packaging, Vol. 125, 2003.
- Wei, X, Sikka, K., Modeling of Vapor Chamber as Heat Spreading Devices, ITHERM 2006.