Vapor Chamber vs Heat Pipe Cheat Sheet
In the battle of two-phase devices, vapor chamber vs heat pipe, there’s no clear winner. Each has attributes that make one superior to the other. This engineering guide covers two-phase differences and usage rules of thumb. Its goal is to be concise while providing useful vapor chamber and heat pipe links throughout the text.
Heat Transport of Vapor Chamber vs Heat Pipe
As a stand-alone product, heat pipes and vapor chambers do a horrible job of dissipating heat. In other words, they are NOT heat sinks. They simply don’t have enough surface area. Rather, these devices are used to spread and/or move heat from the heat source to the fins of the heat sink. Consequently, they are part of a heat sink assembly.
When considering which two-phase device best fits an application, it’s best to begin with this generally accurate rule of thumb. Use vapor chambers to spread heat to a local heat sink; use heat pipes to move heat to a remote heat sink.
In the end, both heat pipes and vapor chambers do an excellent job of transporting heat. After all, the manufacturing process and working principles are functionally identical.
Heat Transport Winner: Tie
Design Flexibility of Vapor Chamber vs Heat Pipe
Think of this as the ability of vapor chambers and heat pipes to be used in a myriad of ways, depending on the thermal challenge. Heat pipes can slither left to right, up and down, be used alone or in combination and in different directions. In short, they are an indispensable thermal option, especially for thermal challenges involving a difficult path from the heat source to the heat sink.
The historical design flexibility of vapor chambers was limited to the X and Y planes, not up and down. Because their outer layer is made from two stamped copper plates, almost any contiguous shape along those axes is possible.
Fortunately, there is another type of vapor chamber with design flexibility in the up and down (Z) direction. Knows as 1-piece vapor chambers because they begin the manufacturing process as an enormous tube (20-70mm diameter), they can be bent post-production into L and U-shapes. However, their starting shape is limited to a rectangle or a rectangle with a corner removed.
Design Flexibility Winner: Heat Pipes (but it’s a close call)
Heat Carrying Capacity of Vapor Chamber vs Heat Pipe
Also known as Qmax, heat carrying capacity is the maximum power input (in watts) that can be applied to a heat pipe or vapor chamber and have it work properly.
By virtue of their contiguous cross-sectional area, a single vapor chamber designed for electronics cooling can handle power input upwards of 450 watts. By contrast, the largest generally available heat pipe tops out at around 125 watts when used in the horizontal orientation.
However, heat pipes are often used in combination to divvy up the heat load, whereby increasing total heat carrying capacity. To do this so each heat pipe has a relatively equal load, the pipes must be positioned directly above the heat source. Typically, a multiple heat pipe configuration will be close to its Qmax limit in operation while a single vapor chamber will have plenty of room to spare.
Heat Carrying Capacity Winner: Vapor Chambers
Isothermality of Vapor Chamber vs Heat Pipe
Whether spreading or moving heat, the goal for most higher-performance thermal applications is to minimize the temperature differential (delta-T) in the base of the heat sink and/or to reduce hot spots across the die face.
There are two commonly implemented ways heat pipes improve isothermality over solid copper, both of which relate to how the heat pipes interface with the heat source.
- Indirect Interface – The most common method is a base plate of either aluminum or copper that’s mounted to the heat source which in turn conducts heat to embedded heat pipes.
- Direct Interface – The second method is to mount the heat pipes directly to the heat source. This will invariably require the heat pipes to be machined to ensure good direct contact with the heat source. This method, while generally more expensive, performs better as the base plate and additional solder are removed from the heat sink assembly.
As mentioned earlier, vapor chambers have a very large internal cross-sectional area, even when compared – in practice – to multiple heat pipes. Moreover, a vapor chamber can ‘connect’ multiple heat sources to the same heat sink and in the process create a situation where temperature differences between and around the heat sources is minimized.
Lastly, shrinking microprocessor die size has resulted in ever-increasing power density that needs to be dispersed quickly. Heat pipes are typically used for applications with power density of less than 25 W/cm2, while vapor chambers are almost a certainty when the density approaches 50 W/cm2.
Isothermality Winner: Vapor Chambers
Cost of Vapor Chamber vs Heat Pipe
Commercial use of heat pipes began in the 1960s at a time when, relative to today, heat loads and power densities were low. Often a single heat pipe sufficed. A vapor chamber would have been ‘overkill’. Consequently, the manufacturing process was refined sooner and competition increased – driving prices down.
The traditional – two stamped copper plates – method of manufacturing vapor chambers is inherently more costly than the heat pipe method of production. Additionally, demand for vapor chambers only began to dramatically grow at the turn of the millennia due to higher power density devices.
The advent of 1-piece vapor chambers, in conjunction with the higher demand, has driven vapor chamber pricing close to parity with multiple heat pipe designs. While a few consumer applications have spawned standard size vapor chambers, the majority of the designs are custom lower volume projects.
Cost Winner: Heat Pipes
Clearly, we have a tie when comparing vapor chambers to heat pipes if all the mentioned criteria are weighted equally. In practice, thermal applications require that design engineers’ weight these differently. Most often, heat pipes prevail – that’s why they represent the bulk of two-phase choices. But, when every degree counts and cost becomes slightly less important, vapor chambers win the contest.
Heat Pipes Are the Best Choice If:
- Nominal power densities are <25 w/cm2
- Ambient temperatures are nominal – let’s call this below 45oC
- Cost is a key consideration – every penny counts!
Vapor Chambers Should be Considered If:
- Power densities are high – certainly by the time they hit 50 w/cm2
- A high degree of isothermality is a critical design element
- Atypically high ambient temperatures and/or low air flow
- Performance is a key consideration – every degree counts!
Winner: Every Thermal Engineer