Thin and ultra-thin heat pipe vapor chambers offer excellent choices for space-constrained applications where heat needs to be moved to a remote location or spread quickly to a larger heat sink surface area. However, mechanical engineers should be mindful of the concessions that come with choosing these devices.
In this blog, we’ll break down the design variables of a heat pipe or vapor chamber, discuss each part’s purpose, and examine the implications of making each element thinner: enclosure wall thickness, wick thickness, wick type, vapor space, and internal support structures. Then, we’ll look at how each component can be changed to achieve a thinner device along with the performance impact of each change. Here, we’ll use the following definitions:
- Thin two-phase devices
- Heat pipes of thickness down to 1.1mm
- Vapor chambers of thickness down to 1.55mm
- Ultra-thin two-phase devices
- Heat pipes of thickness down to 0.6m
- Vapor chambers of thickness down to 1.3mm with some experimental designs reaching 0.2mm
Standard and Thin Heat Pipe Vapor Chamber Component Parts
Before getting a handle on the thinned versions of heat pipe vapor chambers, let’s take a closer look at the design variables and key metrics for standard two-phase devices. If you’re feeling comfortable with this section, skip ahead.
Enclosure Wall Thickness
Whether made from copper (the most common), stainless steel, titanium or aluminum, a heat pipe’s enclosure wall is what gives it most of the strength needed to maintain structural integrity during nominal clamping forces of 20-60 psi against the heat source. While standard sintered wick copper heat pipes have a wall thickness between 0.1-0.3mm, vapor chamber walls are slightly thicker at between 0.3-0.5mm. The thicker walls along with an internal support structure allow vapor chambers to maintain structural integrity despite higher; aspect ratios.
Internal-Support Structure (vapor chamber only)
With sintered heat pipes, the width-to-height aspect ratio rarely exceeds 3:1 but for vapor chambers, we regularly produce parts with a 10 :1 ratio. The great advantage is that heat is spread in all directions over a large surface area. The disadvantage is that its structural rigidity against clamping force or accidental bump is diminished. To solve this problem, thicker walls and an internal support structure, usually in the form of vertical columns, are added to span the upper and lower divide of the internal enclosure walls. While support structures are designed to limit the reduction in vapor space, they often result in slightly reduced Qmax for many applications.
The empty space inside the thin heat pipe vapor chamber enclosure is what allows the vaporized liquid to move to areas of lower pressure – and really what gives a two-phase device its potential to move high power density with very low thermal resistance. Round heat pipes, especially those diameters above 5mm, have enough excess vapor space that flattening them slightly has no effect on Qmax. However, after a certain flattening point, Qmax will start to decrease.
Wick Thickness & Type
Heat pipe wick thickness, structure, and type all play key roles in helping move the condensed liquid from the heat sink area to the heat source area so that the two-phase cycle can continue. If the device needs to operate against gravity (heat source above heat sink) you must have a wick. The most common, most efficient, and thickest is a sintered wick followed in all categories by bundled fiber, mesh and grooved/etched wicks. As heat pipe wick thickness increases, the amount of vapor space, in many cases, is diminished resulting in reduced thermal performance.
Thin Heat Pipe Vapor Chambers
When heat pipe vapor chamber performance and flatness across a wide range of power densities and orientations are needed, a sintered wick is likely a necessity. Typical configurations for sintered wicks include, a) attached all the way around the inner walls of a thin heat pipe and b) attached to both the upper and lower plates of a thin vapor chamber. This configuration circulates the most condensed liquid back to the evaporator, an especially important consideration for operation against gravity and/or with higher power density heat sources.
As seen in the table above, thin heat pipes and vapor chambers range in thickness from 1.5mm to nearly 2.0mm if a full coverage wick is required to ensure the most effective fluid return if the device is operating against gravity.
If power density, top Qmax performance, and working against gravity are less of a concern, it’s possible to make a thinner device while keeping the same vapor space by applying the sintered wick to only the evaporator side of the device. For thin heat pipes, this means a sintered wick on slightly less than half the inner wall (before flattening) and for thin vapor chambers it means just the lower plate that contacts the evaporator will have a wick structure. See image below.
By removing the sintered wick on the top side of the thin heat pipe or vapor chamber, we’re able to reduce the overall device thickness by 0.4mm while maintaining the vapor space. In these cases, Qmax is reduced by almost half because the device is unable to return liquid to the evaporator even while operating in the horizontal orientation. An important consideration when reducing the amount of wick material is that Qmax will drop very fast as the device is required to work harder against gravity.
Ultra-Thin Heat Pipe Vapor Chambers
Further changes in wick structure and wall thickness are the only ways to manufacture ultra-thin heat pipes and vapor chambers. While sintered wicks allow for higher power densities (100’s of watts/sq cm2) and the ability to work against gravity, certain applications may not require these levels of performance. Let’s look at alternative wicks: bundled fiber and mesh.
Fiber bundles in the shape of a cylinder whose diameter is smaller than that of the inside diameter of the copper pipe are typically used for ultra-thin heat pipes. As the tube and bundle is squashed, vapor pockets are formed along the edges of the heat pipe, while the top, bottom, and middle contain the wick. One distinct advantage of bundled fiber over mesh is that the former allows ultra-thin heat pipes to be bent.
Mesh is often favored for vapor chambers, which are typically not bent, because it allows for greater design flexibility. The ultra-thin mesh screens have a variety of weaves to choose from as well as the ability to be used alone or in conjunction depending on the application. Further, if the device is operating with gravity (evaporator above condenser) or in the horizontal, mesh can allow for higher Qmax figures.
As we can see from the table below, we see ultra-thin heat pipes down to 0.6mm with vapor chambers achieving as little as 1.3mm.
Of potential concern for engineers looking to use an ultra-thin variant of heat pipes or vapor chambers is the inability of the device to carry liquid back to the evaporator beyond a 5 or so degrees adverse slope where the heat source is above the heat sink. For two-phase devices to be thinner than 0.6mm in the case of heat pipes and 1.3mm in the case of vapor chambers, further compromises must be made to both wick structure and wall thickness.
Choices for ultra-thin heat pipes and vapor chambers lead to high strength materials such as titanium or very low strength composites such as mylar/metal foil composites (image below). Both technologies have been around for decades but recently the need for spreading hot spots in low-powered consumer devices has led to the adoption of these ultra-thin, low-capacity devices.
Kelvin Thermal is an offshoot of the University of Colorado and was formed in 2014 with the goal of delivering an ultra-thin vapor chamber, which it calls a thermal ground plane and has a claimed thickness of as little as 0.15-0.25mm. A project venture between Cooler Master and Murata has touted vapor chambers down to 0.20mm. In both cases, a metal-covered plastic foil, similar to a potato chip bag, is used for the enclosure with the wick likely just an etched surface only thick enough to separate the vapor and liquid and provide a capillary path. While Qmax data is not available for specific configurations of these two-phase alternatives, it’s safe to say that power handling drops to somewhere between a few watts to perhaps as much as ten watts.