Until 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:
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.
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.
• 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.
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.