Understanding 3D Vertical Vapor Chambers

Understanding 3D Vertical Vapor Chambers

3D Vapor Chamber ChristmasUntil the mid-2000’s heat pipes were really the only option when designing a high heat-flux heat sink whose condenser was oriented vertically above a horizontal heat source. That’s because they’re easily bent post-production into a variety of shapes including “L” or “U” configurations. Despite the advantages of vapor chambers, better spreading and less thermal resistance, these devices were for all practical purposes non-manufacturable in these shapes. Why?

A traditional vapor chamber, what we at Celsia call a two-piece design, is manufactured by stamping an upper and lower plate (thus the two-piece moniker), sintering copper powder to each side, and diffusion bonding each plate to one another on all sides.

Typical two-piece vapor chamber designs look like this:

Vapor Chamber Two Piece Examples

As seen in the images above, this type of vapor chamber is customizable into virtually any shape along the X&Y planes (length and width). Additionally steps or pedestals can designed into the stamped upper and/or lower plates, making them great for recessed heat sources or those that need to conform to a surface of varying heights. But, how could a two-piece production method create a 3D vertical vapor chamber like an “L” or “U” shape? Well, it wouldn’t be easy, fast, or very cost effective.

•  First, the upper and lower plates would need to be stamped into that shape or stamped flat and bent.

•  Second, specialty L/U shaped fixtures would need to be fabricated to hold the copper powder to the walls of each plate as they’re being oven sintered, and don’t forget these fixtures will take longer to heat up increasing the overall sintering cycle time. Another potential problem here is that the fixtures need to be made such that they insure a very even distribution of copper powder across the surface and also at the bend. Variations in sintered copper can lead to lower yield rates and/or finished parts that don’t perform according to specification.

•  Third, the two plates need to be attached together either through the traditional diffusion bonding process (back into an oven) or through a four sided TIG welding process. The former is not known to work well on complex, especially bent, shapes while the latter would require an operator to manually weld all sides together.

In the 1980’s, I remember working on some military applications that overcame these issues by combining vapor chambers for the horizontal portion of the sink with integrated copper tubes for the vertical portion (see my crude drawing below L). Here we cooled two 2kW traveling wave tubes for a radar application. The second image incorporated a 75x75mm vapor chamber with a one meter fin stack for a 3kW application.

Vapor Chambers with Vertical Copper Tubes

Vapor Chamber with Copper Tubes – 4kW Application (L), 3kW Application (R)

Although technically not a 3D vertical vapor chamber, this solution is excellent for very high heat-flux applications provided adequate structural support is done at the condenser/ tube areas to address shock and vibe concerns. Structural issues arise because a lot of leverage can be placed on the vertical pipes/condenser causing the copper tube braze joints to fail or cause damage to the tubes or vapor chamber itself.

So, is there a cost effective, process-optimized method of producing a true 3D vertical vapor chamber? About ten years ago, we began work on a design (manufacturing process) that in addition to cost savings provided designers with some unique options; most relevant to this article, the ability to bend it post-production to create L/U shapes.

The design we created was fairly straightforward. We call it a one-piece vapor chamber and it’s available from a growing number of suppliers.

Low Cost Vapor Chamber

One-Piece Vapor Chamber Bent Into Various Shapes

•  Rather than using the traditional two-piece approach beginning with upper and lower stamped plates, we start with a very large diameter copper tube to create a heat pipe of between 15-75mm OD.

•  A simple cigar shaped mandrel is inserted and copper power is layered in between it and the inside wall of the tube. This insures a very even sintering process and keeps oven cycle times to a minimum.

•  Next, the tube is flattened to the desired thickness, usually between 2-3.5mm, and a perforated wave-shaped spacer is inserted into the tube. This spacer allows the device to withstand clamping pressures up to 90psi and provides internal support for bending post-production.

•  Deionized water is added and the tube is vacuum sealed and TIG welded – only on the two narrow sides of the device. So now we have something that looks like an extremely wide flat heat pipe, but with an internal support structure.

•  Lastly, the device can be bent to form a variety of shapes including L/U configurations.

Here’s an example of a high power LED application we did in 2008. The horizontal portion of the vapor chamber makes direct contact with the heat source, eliminating a mounting block and TIM layer as would be typical with a heat pipe implementation. The vertical portions of the vapor chamber run directly into the condenser with a continuous and evenly distributed sintered copper layer along the inside wall. For the this application, vapor chamber width was only 30mm or so, but one-piece vapor chambers can be created as wide as 110mm.

Vapor Chamber One-Piece for UHBLED

One-Piece 3D Vertical Vapor Chamber – HBLED Application

 

In summary, a traditional two-piece design is really limited to steps or pedestals of 4-5mm in height. A flat vapor chamber with vertical copper tubes have some structural issues that need to be addressed yet are great for very high heat-flux applications. The newer one-piece vapor chamber is probably the truest form of a 3D vertical vapor chamber and offer cost savings over other designs.

Should I Use Heat Pipes or Vapor Chambers to Cool My Electronics Application

Should I Use Heat Pipes or Vapor Chambers to Cool My Electronics Application

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.

moving heat use heat pipe spreading heat use vapor chamber

 

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.

vapor chambers cool high power laser

450W RGB Laser Diode Solution for a 3D Projector

 

 

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.

heat pipes vs vapor chamber

 

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.

vapor chamber vs heat pipes