A vapor chamber, sometimes called a planar heat pipe or a vapor chamber heat spreader, is a two-phase device used to spread heat from a heat source to a heat sink. For electronics cooling applications, the heat transfer is usually to a heat sink in very close proximity to the heat source; a local as opposed to a remote heat sink.
The use of vapor chambers has increased markedly as both the total power and, as a result of shrinking die sizes, the power density has skyrocketed. In terms of price and application flexibility, today’s vapor chambers are both more capable and lower cost than even a decade ago.
This page will cover the following topics:
- Vapor Chamber Technology
- How Does a Vapor Chamber Work?
- Types of Vapor Chambers
- Vapor Chamber Thermal Conductivity
Vapor Chamber Technology
In order to understand vapor chamber technology, it’s first important to understand the function of each of its component parts.
- Vapor Chamber Enclosure – the vapor chamber enclosure is a vacuum sealed envelope that houses the wick structure and a working fluid. Once the wick material and liquid are added, the device is vacuum sealed.
- Vapor Chamber Wick Material – affixed to the inner walls of the vapor chamber, the wick absorbs and helps distribute liquid from cooler to hotter areas of the device.
- Vapor Chamber Working Fluid – the working fluid turns to vapor that carries heat to cooler areas of the device. Because it’s in a vacuum, water turn to vapor at much lower temperatures.
- Vapor Chamber Vapor Space – the amount of internal space not taken up by water and wick that can be used to transport vapor. Not really a ‘part’ but a consequence of other ‘part’ choices.
The most popular vapor chamber technology combination is a copper envelope using a copper sintered wick with water as the working fluid.
How Does a Vapor Chamber Work?
In the most common vapor chamber heat sink configuration, the heat source makes direct contact with the underside center portion of the vapor chamber and a finned heat sink as attached to the top side. Some of the working fluid (water) vaporizes and travels to cooler areas. The fin array absorbs this heat causing the vapor to condense and return to liquid form. This liquid is reabsorbed by the wick material and distributed, through capillary action, to the heat source where the cycle repeats.
Vapor Chamber vs Heat Pipe
Vapor chambers operate under the same working principles as heat pipes. They have a metal enclosure which is vacuum sealed, an internal wick structure attached to the inside walls, and move liquid around the system using capillary action.
It’s the differences in physical dimension and, consequently, functionality that separate vapor chambers from heat pipes. Even when flattened to their limit, heat pipes maintain a width to height aspect ratio of about 4:1, less for smaller diameters. An 8mm heat pipe that’s been flattened to a width of 11mm will be 2.5mm tall. A mesh wick will allow for some additional flattening, but this rule generally holds true. On the other hand, a 2.5mm thick vapor chamber can have a width up to around 150mm. That’s a 60:1 width to height aspect ratio.
As seen in the figure above, vapor chambers can be used to spread heat to a local condenser or move it to a remote one. However, spreading heat is by far the most common use of vapor chambers. Heat pipes are usually used when heat needs to be moved to a remote condenser
The general advantages of vapor chambers over heat pipes include:
- Vapor chambers usually are in direct contact with the heat source, decreasing total thermal resistance and improving performance. Heat pipes, especially round ones, require a mounting plate between the heat source and the pipes.
- Due to their large, continuous surface area, vapor chamber solutions allow better isothermalization at the chip interface, reducing hot spots. They can be extended beyond the width of the heat source, allowing further performance benefits versus their heat pipe counterparts.
These two factors combine to allow vapor chamber solutions to outperform heat pipe solutions by 15-30%. Depending on the application, this typically translates to between 3-10 oC improvement in overall heat sink delta-T.
Types of Vapor Chambers
Vapor chamber heat spreaders can be manufactured using one of two methods: 1-piece and 2-piece designs.
Low Cost One-Piece Vapor Chamber
Celsia began the early work on the one-piece vapor chamber design over a decade ago. The goal was to meaningfully cost reduce a traditional two-piece vapor chamber heat spreader while maintaining thermal performance characteristics and adding some unique capabilities. During the course of development, our engineering teams experimented with hundreds of component and manufacturing process combinations to arrive at a device portfolio that has been used in millions of commercially available products since 2008. It was the first vapor chamber, of any kind, to be used in high-performance heat sinks for graphics cards, DRAM memory modules, and notebook computers.
Starting from large diameter tube stock, a wicking structure is bonded to the inside walls using various density and porosity formulations. Then a wafer thin micro-perforated spacer is inserted to aid in vapor flow and increase structural rigidity. As with all two-phase devices, a liquid is added and the device is vacuum sealed. This process requires no stamping or machining and reduces the number of secondary operations, driving cost down to near parity with heat pipe solutions.
Advantages of this type of vapor chamber versus a traditional two-piece design include:
- Bending in the Z-direction while maintaining nearly all of its thermal performance characteristics.
- Fast and comparatively less expensive prototype and production costs.
However, it should be noted that one-piece vapor chambers can only be made (before bending) in a rectangular shape. Designs that require other shapes along the X and Y axes will require a two-piece design.
Versatile Two-Piece Vapor Chamber
Most manufacturers of vapor chamber heat spreaders use a traditional two-piece design. While studies show that performance of heat sinks using vapor chambers can be enhanced by 15-30% over their heat pipe counterparts, a two-piece design has cost implications of roughly the same magnitude. Consequently, their use has historically been limited to high power densities and/or those applications where other requirements outweigh the cost.
Manufacturing this type of vapor chamber requires the use of an upper and lower plate which are usually stamped but sometimes forged or machined. A wicking structure is attached to the upper and lower inside plates. Wick type and porosity can be optimized depending on the application but could involve sintered copper powder, mesh, or a grooved internal surface. Numerous solid copper columns are added to reduce deformation from either higher clamping pressures of the module to the heat source or from high internal VC temperatures.
Finally, the two pieces are welded or diffusion bonded on all sides, a working liquid is injected, and the device is vacuum sealed. The device is now ready to have a fin stack(s) and the appropriate connecting hardware added. Specialty ‘column’ vapor chambers are also available. The image below shows a 25mm thick T-shaped vapor chamber which provides an isothermal surface for the mounting of multiple heat generating components.
Despite the added production time and cost, two-piece vapor chamber designs have advantages over a one-piece design.
- Complex shapes along the X and Y axes.
- Changes to the wick type and porosity within a single vapor chamber.
Vapor Chamber Thermal Conductivity
Like heat pipes, vapor chamber thermal conductivity depends on several factors, but mostly with the distance heat is moved away from the heat source. When comparing heat pipe thermal conductivity to that of vapor chambers for the same application, it’s important to remember that despite having a lower thermal conductivity, vapor chambers will outperform heat pipes. It seems counterintuitive, but the reason for this has to do with the substantial size difference in cross sectional wick material.
The image above shows heat sink options for the same application. One uses four flattened 6mm heat pipes while the other uses a single vapor chamber. With the heat pipe version, the four heat pipes share the heat load with each taking one quarter of it. Despite thermal conductivity three times lower than the heat pipe solution, the vapor chamber option was 6 oC cooler.