While heat pipes are a common choice for difficult to cool electronics, they sometimes fall a bit short when trying to deliver a heat sink with the lowest possible thermal resistance. “If only we could eke out another 5 or 10 oC from our heat pipe design”, is something we hear regularly. But how do you do that? Pumped liquid cooling will do it easily but that involves power, cost and risk.
Sometimes, the only choice is to use a heat pipe’s planar cousin – the vapor chamber. This simple, relatively cost-effective, and very dependable device can be used alone or in combination with heat pipes to boost more traditional heat sink designs past the performance threshold.
Vapor chambers operate under the same working principles as heat pipes. They have a metal enclosure which is vacuum sealed, an internal wick structure attached to the inside walls, and move liquid around the system using capillary action. Unlike heat pipes, they can achieve an impressive 60:1 width to height aspect ratio (flattened heat pipes are on the order of 4:1).
Let’s take a look at the conditions most likely to result in vapor chambers becoming the best solution.
When the Thermal Budget is Very Tight
Heat pipes become a likely solution when the thermal budget is less than 40 oC, but as this budget shrinks vapor chambers become the likely hero. The main reason? Vapor chambers make direct contact with the heat source while heat pipes generally have a base plate between them and the heat source.
Note: thermal budget is the difference between the maximum semiconductor temperature (Max Tcase or Tjunction) and the maximum operating ambient temperature of the final system (Max Ambient).
And yes, you are correct that a heat source can make direct contact with a heat pipe solution. Two issues:
- The heat pipes have to be fly cut, adding an additional step and expense to the solution.
- The mounting block still has solid metal channels between the pipes, reducing thermal performance and possibly creating die face hot spots.
When “Isothermalize” is the Word of the Day
Vapor chambers are ideal for applications where high power densities need to be dispersed quickly, hot spots across the die face need to be minimized, or 2+ heat sources are required to be close in temperature. Below, 6 ASICS were required to remain within 2 oC of each other. The center cutout reduced weight.
When the goal is to achieve as uniform a temperature as possible, vapor chambers trump heat pipes by virtue of their large contiguous surface area that move heat in every direction. Heat pipes only move heat in a linear direction.
When Height is Constrained, Yet Fin Area Needs to Grow
Typically, heat pipes run through the center of a fin stack in order to maximize contact area and therefore transfer as much heat to the fins as possible. The downside – fin area is reduced. While this is not a problem if you’ve got the room to increase fin height, it poses a problem when that metric is constrained, as it is here in an add-in desktop graphic card application.
As mentioned in the opening to this article, heat sink performance improvement of 5-10 oC can be had by using a vapor chamber in place of heat pipes because they make direct contact with the heat source, can more evenly distribute heat across a large base, and allow for maximum fin area.
At Celsia, most of the applications for which we design benefit more from heat pipes, albeit those with application-driven changes to wick thickness and porosity to boost performance. But, that’s not to say we aren’t deeply invested in vapor chamber technology. The inherent performance benefits of these devices have been known since the 1960’s.
Vapor chamber cost relative to heat pipes, until a decade ago, was prohibitive for all but the most critical applications. Fortunately, the growing use of these devices coupled with innovative manufacturing techniques pioneered by us a decade ago (1-Piece Vapor Chamber) have driven the price down to near parity with 2-4 heat pipes. Below is a brief summary of the pros and cons of both types of vapor chambers, but more detail can be found here.