Summary: This article compares the cost and performance trade-off among 5 different heat pipe heat sink designs: U-shaped heat pipes with base plate (Cu & Al), direct contact heat pipes, U-shaped vapor chamber, and 3D vapor chamber.

Regardless of industry or application, engineering decisions regarding the best heat sink solution are likely to hinge on cost vs performance. In theory it seems easy – get the lowest cost product to meet the performance requirements. In practice, sticker shock often necessitates changes to performance specifications (different chip) or performance throttling (how that chip is de-rated depending upon condition). However, there are always instances where performance cannot be sacrificed, often necessitating a more costly thermal solution.

This article compares several different heat pipe / vapor chamber based heat sinks vis-a-vis these two metrics.

  • Performance: computed as the overall heat sink delta-t using FloTHERM CFD package.
  • Cost: We’ve assumed volumes are high enough to invest in hard tooling so these aren’t reflected in the price. Moreover, prices are relative to the lowest cost solution (1X, 1.1X, etc) as volume increases will lower per unit nominal cost.

Heat Sink Design Parameters

  • Heat source dispersed power: 250 watts
  • Heat source size 30*30 mm
  • Max ambient temp: 25 oC
  • Available air flow: 40 CFM
  • Condenser: Zipper fin size 115*85*65 mm

Note that we’ve left Tcase/Tjunction max unspecified as the purpose of this article is not to select a particular solution, simply to show differences given some basic inputs. For those mechanical engineers less familiar with Tcase importance, here’s a quick explanation. Max Tcase temperature minus max ambient give us the “thermal budget”. This figure must be above the heat sink delta-t calculation to determine go/no-go performance. For instance, a thermal budget of 40 oC requires that the max heat sink delta-T from case TIM to fin-to-air temperature rise be below 40 oC.

Competing Heat Pipe Heat Sink Designs

Let’s start with the most basic design and progress to the most complex, seeing how performance and cost are affected.

Heat Pipes with Aluminum or Copper Base Plate

 

U-Shaped Heat Pipes with Aluminum Base Plate

 

This is the most traditional heat pipe heat sink design in terms of the interface of the heat pipe to the heat source. Four U-shaped heat pipes are soldered to an aluminum or copper mounting plate which in turn is mated to the heat source. Consequently, heat must travel through the plate before it reaches the heat pipes.

Other than bending, no other secondary operation is required to the four 6mm heat pipes, although this example has them slightly flattened where they make contact with the base plate.

CFD of Heat Pipe Heat Sink with Aluminum Base

 

The FloTherm model for the AL mounting plate solution shows a temperature rise above ambient of 53.9 oC (78.9 – 25oC = Base: Maximum – Max Ambient) and we’ll use this as Baseline with a Cost Baseline of 1X.

If a bit more performance is required, a copper base plate can replace the aluminum one. With thermal conductivity twice that of aluminum, the copper base improves performance by 2.3 oC to 53.9 oC. This design is 5% more expensive than the baseline and increases weight slightly due to the added weight of the copper slug (vs aluminum) under the heat pipes.

 

Direct Contact Heat Pipes

 

Direct Contact Heat Pipe Heat Sink

 

This design allows the heat source to make direct contact with the heat pipes, eliminating the base plate (in terms of conduction) and an interface (the solder used to secure the heat pipes to the base plate). However, in order to achieve the necessary surface flatness, the heat pipes must be machined (secondary operations).

CFD Direct Contact Heat Pipes

 

Because the thermal pathway is reduced by one base plate and one interface, the performance of this design improved to 49.3 oC; a 4.6 degree improvement over the baseline and a 2.3 degree improvement over the design using the copper base plate. However, the additional machining of the base plate (room for HP to stick through) and machining of the heat pipes yields a cost of 1.1X that of the Baseline design (10% more expensive).

 

U-Shaped Vapor Chamber

 

U-Shaped Vapor Chamber

 

Here, a single U-shaped vapor chamber replaces the four 6mm heat pipes. In design, it’s most similar to the direct contact heat pipe solution as both designs allow the heat source to make direct contact with the two-phase device. An important consideration before choosing this design is whether the heat sink supplier is able to manufacture one-piece vapor chambers because the traditional two-piece design cannot be bent into a U-shape.

CFD U-Shaped Vapor Chamber Heat Sink

 

Versus the direct contact heat pipe solution, the vapor chamber solution performs 21.5% better (11.6 degrees better) while only costing 4.55% more. However, the increased wall thickness of the vapor chamber drives a weight increase of around 75 grams.

 

3D Vapor Chamber

 

3D Vapor Chamber Heat Sink

 

The base plate is a vapor chamber with vertical condenser tubes sharing the same vapor space. It’s produced by brazing 8 open-ended condenser tubes to a vapor chamber with corresponding holes in it. The vapor chamber makes direct contact with the heat source spreading heat evenly along the XY planes and vertically through the condenser tubes.

 

CFD 3D Vapor Chamber

 

This design has the best performance, but at a considerable cost premium. Versus its next closest competitor, the vapor chamber design, this one is almost 2 degrees cooler (a 4.9% gain) but costs over twice as much (117% more).

It should be noted that this application does not fully highlight the potential benefits of a 3D vapor chamber design. As the size of the required base plate increases, the performance difference between this solution and a vapor chamber one increases as well.

 

Summary Points

The table below shows that fairly substantial performance gains are realized as materials or two-phase devices are changed. From the baseline aluminum base heat sink to the 3D vapor chamber solution, there’s a 17 oC performance improvement but cost increases by 150%.

 

Heat Pipe Heat Sink Comparison – Performance & Cost

 

Modest performance gains and cost adders, vs baseline, of around 7-15% are achieved by changing the base plate to a more thermally conductive copper material or by allowing the heat pipes to make direct contact with the heat source.

Given the application parameters, the best overall value is probably the vapor chamber solution. Although it’s 15% more expensive than the baseline, performance is increased by 28% (15.2 oC improvement).

Celsia is a custom heat sink manufacturer using vapor chamber and heat pipe technology. We specialize in: heat sink design, heat sink proto, heat sink manufacturing, heat pipes, vapor chambers, custom heat pipe wick materials.