I’m often asked which two-phase cooling technology, heat pipes or vapor chambers, is best suited for a particular electronics application. While there are some good rules of thumb the answer isn’t always simple. Nonetheless, this blog post will cover the basics with additional information covered in ann online webinar found in the Resources section of this site.
Let’s start by saying that a transition from traditional heat sinks to either type of two-phase technologies should only be considered when the design is conduction limited and/or when non-thermal goals such as weight or size can’t be achieved with other materials such as solid aluminum or copper.
Once it’s determined that traditional heat sinks just won’t meet thermal requirements, how do mechanical engineers begin to determine the next best option?
Are You Moving or Spreading the Heat?
While there’s no hard line of distinction, think of the difference like this.
Space constrained applications and/or those with two or more heat sources sharing a common condenser (fin stack) are good candidates for moving the heat to a remote location of more than around 40mm distance. In these cases, the design flexibility of heat pipes, by virtue of their bendability in all directions, are the default choice. And this is especially true if only a single heat pipe is needed. For reference, a 3mm sintered metal heat pipe using water as the working fluid is adequate for cooling 5-20 watts of power while an 8mm pipe can carry 20-90 watts. The range is affected by the porosity and thickness of the wick structure, the amount of working fluid, the degree of bending required and the amount of flattening done to the round pipe.
In my experience heat pipes are used in 95%+ of applications where heat needs to be moved to a remote condenser. However, when conduction loss needs to be reduced further, such as in the laser diode example below, vapor chambers become a viable option. Here three heat sources share a common condenser, reducing temperature rise by roughly 15% over a similar heat pipe alternative.
When it comes to spreading heat to a local condenser, the choice between heat pipes and vapor chambers becomes a lot more complicated….
Heat pipes are probably a best choice if:
- There’s plenty of air flow
- You’ve got lots of room for fins
- Nominal power densities are <25 w/cm2
- Ambient temperatures are normal – let’s call this below 45oC
- Cost is a key consideration – every penny counts!
A vapor chamber should be considered if:
- Power densities are high – certainly by the time they hit 50 w/cm2
- Reducing hot spot across the die is a key concern
- Z-direction (height) is constrained, yet fin area needs to be increased
- Atypical ambient temperatures and/or low air flow
- Performance is a key consideration – every degree counts!
Before I review a couple of examples, it’s important to note some of the reasons vapor chambers can perform better than heat pipes for spreading heat. First, vapor chambers make direct contact with the heat source, whereas heat pipes usually require a solid base plate increasing conduction loss –from the base plate itself and from an additional TIM layer between the base plate and the heat pipes. Second, heat spreading via vapor chambers is multi-directional while with heat pipes it’s linear. Third, a vapor chamber solution often allows for additional fin area. These advantages typically allow vapor chamber solutions to have between 10-30% better performance than their heat pipe counterparts. This translates to around 3-9oC for most applications.
Despite the performance difference, vapor chambers were regarded as a very niche solution due in large part to their cost premium over heat pipes, especially for non-consumer (low volume) applications. But manufacturing innovations from an increasing number of suppliers helped narrow the gap.
Let’s take a look at a couple of examples. The first is for a telecom infrastructure manufacturer that wanted to understand both the cost and performance difference between competing heat pipe and vapor chamber designs. The heat source was in the 85 watt range with ambient temperature of 55 degrees Celsius. We compared a four 8mm heat pipe solution to a single vapor chamber design. As the image below shows, the former required a base plate as secondary machining of the heat pipes was not possible due to the wall thickness of the pipes. Testing showed the vapor chamber design to outperform the alternative by a full 4oC, allowing the assembly to meet specifications.
The second example is from one of the first two graphic card solutions to use vapor chambers. Increasing GPU power required that the current heat pipe based solution be redesigned while maintaining the heat sink stack height. Here two 8mm pipes were replaced with a vapor chamber. Elimination of the base plate allowed us to increase fin height and overall fin area while the better heat spreading of the vapor chamber all contributed to a 6oC better solution.