What is a Vapor Chamber?
A Vapor chamber is constructed from sealed copper plates, and filled with a small amount of fluid such as de-ionized water that allows heat to be rapidly dispersed away from the source. Inside a vapor chamber there is an internal support structure to prevent buckling of chamber walls. Classed formally as a heatpipe, a Vapor chamber is one of the best heat spreading option at the base of a heatsink, and is typically used for high power devices. Vapor chambers are typically combined with stamped fins to create a high-end thermal management device that can rapidly spread heat from a small source to a large area.
A vapor chamber can be integrated with either aluminum or copper heatsinks. The simplest method is to solder a vapor chamber to the base of an extruded heatsink. A more thermally efficient method is to solder a stack of stamped fins directly to the surface of a vapor chamber. To improve the dimensional integrity, these fins are often interconnected by locking tabs called zipper fins.
How a Vapor Chamber works?
A vapor chamber consists of a sealed vacuum vessel, with an internal wicking structure, and a small amount of working fluid that is in equilibrium with its own vapor. The vacuum vessel is typically made of copper, and sealed around the perimeter. The wick can be made of many different substances. The most common way is to sinter copper powder to the inside wall of the vessel. Many fluids can be used as the working fluid of the vapor chambers. But in most CPU, GPU and LED cooling applications, water is selected as the working fluid, because of its high latent heat, high surface tension, high thermal conductivity and suitable boiling temperature, not to mention the cost and environmental concerns.
The low pressure inside the chamber allows the fluid to vaporize at a temperature much lower than its normal boiling temperature. When heat is applied to the vapor chamber, the fluid near that location immediately vaporizes and rushes to fill the entire volume of the chamber (driven by pressure difference). When the vapor comes into contact with a cooler wall surface, it condenses, and releases its latent heat of vaporization. The condensed fluid returns to the heat source by capillary action of the wick structure. As the vaporization and condensation cycle repeats, heat is moved for the heat source to the entire volume of the chamber, resulting in a uniform temperature distribution on its surface. The return of fluid to its boiling location is primarily driven by the capillary force, but gravity and centrifugal force can also contribute to some degree. To utilize the external forces, it is important to design the vapor chamber, such that the gravity or centrifugal force, is working in the direction that drives the fluid from its cold side to the hot side. For example, in a spinning system, the heat source (hot spot) should be located in the outer side of the PCB, and the fins should be located closer to the spinning center.
The benefits of using a Vapor Chamber
A properly designed vapor chamber combined with a stamped fin heatsink can improve the thermal performance by 10- 30% over copper, and heat pipe based solutions. In mission-critical applications, it not only lowers the temperature by a number of degrees, but also sometimes eliminates the need for a fan on top of the heatsink, which improves the reliability of the system and eliminates noise. It is also perfect for low profile applications where height vs. performance is critical. In addition, a vapor chamber is much lighter than solid copper, due to its internal chamber structure. In many cases, a vapor chamber based heatsink weighs similar to an extruded aluminum heatsink, but works much better than a copper heatsink.
The effective thermal conductivity of a vapor chamber is usually 5 to 100 times the conductivity of copper – but it is application specific. The thermal resistance of a vapor chamber comes from many sources. The two major factors are the evaporation resistance and transport resistance. Transport resistance is distance dependent, but it is relatively small compared to the evaporation resistance. Since the dominating evaporation resistance is independent of size, the larger the vapor chamber is compared to the heat source, the greater is the effective thermal conductivity. In a coarse calculation or CFD simulation, it is not uncommon that a uniform and isotropic thermal conductivity, say 10000 W/m-K which is 25 times the thermal conductivity of copper, is assigned to the entire volume of the vapor chamber.
Reliability of a Vapor Chamber
- Thermal Shock Test
- Accelerated Life Test
- Freeze Thaw Test
- Burst Test
- Cosmetic Degradation Test
The following table shows the suggested operation conditions for typical applications. They are not necessarily the maximum capabilities of vapor chambers.
|Vapor Chamber Ambient temperature||0 – 85 ºC|
|Power||20 – 300 W|
|Heat Flux||Up to 300 W/cm2|
|Size (width and length)||50 to 200 mm|
|Vapor Chamber Thickness||3 mm and up|
|Vapor Chamber Flatness||0.1 mm in every 25×25 mm area|
|Vapor Chamber Life (MTBF)||80000 hours|
|Thermal cycling||Tested 200 cycles between -40 and 85 ºC|
Vapor Chamber Examples
In this example a copper stamped fins were attached to a vapor chamber to create a very high performance heatsink. Two zipper fins were used to create a solid structure, and a cut out was created to allow for the push pin location.
In this example, a light engine was mounted onto a custom vapor chamber.