Heat Pipe Heat Sink Design

Summary: This article compares 5 heat pipe heat sink designs for cost and performance:

  1. Heat sink with U-shaped heat pipes and a solid aluminum base plate
  2. Heat sink with U-shaped heat pipes and a solid copper base plate
  3. Heat sink with U-shaped heat pipes making direct contact with the heat source
  4. Heat sink with U-shaped vapor chamber making direct contact with the heat source
  5. Heat sink using a 3D vapor chamber heat pipe

Heat sink design considerations often include heat pipes as part of a more advanced thermal solution. Today, let’s look at some heat pipe heat sink designs where two-phase devices need to be oriented vertically in order to embed them into the horizontally oriented heat sink fins. Determining the optimal design involves comparing performance and cost to the specific application requirements.

Heat sink performance is determined by its total delta-T using FloTHERM CFD software while cost/price is based on volumes high enough to justify hard tooling. Moreover, heat sink cost is expressed using the low-cost solution as a baseline (1.0X) with other heat pipe heat sink design options being expressed as a multiple of the baseline; a thermal solution that’s 20% more expensive has a cost of 1.2X.

Heat Pipe Heat Sink Design: Thermal Solution Parameters

  • Heat source thermal design power (TDP): 250 watts
  • Heat source size: 30×30 mm
  • Max ambient temp: 25 oC
  • Available airflow: 40 CFM
  • Condenser: zipper fin size 115*85*65 mm with fins running horizontally
  • Max Tcase and/or Tjunction: unspecified (we can hypothetically assign values to see how the thermal budget changes and what effect that has on heat pipe heat sink design choice)

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.

Heat Pipe Heat Sink Designs #1 and #2: Solid Aluminum vs Solid Copper Base

This is the most traditional heat pipe heat sink design. Thermal grease/pad is used between the heat source and the bottom of the solid aluminum base plate. The heat pipes are soldered to the base plate. This example shows slightly flattened pipes, but round heat pipes can be used if groves for each heat pipe are cut into the base plate.

 

U-Shaped Heat Pipe Heat Sink with Solid Aluminum or Solid Copper (not shown) Base

 

CFD of Heat Pipe Heat Sink Design with Aluminum Base

 

The FloTherm model for the heat sink with the aluminum mounting plate solution shows a temperature rise above ambient of 53.9 oC (78.9 – 25oC = Base Maximum – Max Ambient). Dividing the heat sink temperature rise (53.9) by the heat source power (250 watts) tells us the thermal resistance of this heat pipe heat sink: 0.2156 oC/watt.  For these examples, this is the low-cost solution, so its cost baseline is 1.0X.

Heat Pipe Heat Sink Cost & Performance of Solid Base Designs

If 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 51.6 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.

 

Heat Pipe Heat Sink Design #3: Direct Contact Heat Pipes

 

Direct Contact Heat Pipe Heat Sink

 

This thermal solution 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 operation).

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).

Heat Pipe Heat Sink Cost & Performance of Solid and Direct Contact Heat Pipe Bases

 

Heat Pipe Heat Sink Design #4: U-Shaped Direct Contact Vapor Chamber

 

U-Shaped Vapor Chamber

 

Using a U-shaped vapor chamber, this thermal solution design replaces four 6mm heat pipes. 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(s). An important consideration before choosing this thermal solution 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 thermal solution performs 21.5% better (11.6 oC lower heat sink delta-T) while only costing 4.5% more. However, the increased wall thickness of the vapor chamber drives a weight increase of around 75 grams.

Heat Pipe Heat Sink Cost & Performance of Solid and Direct Contact Bases

 

Heat Pipe Heat Sink Design #5: 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 thermal 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.

 

Heat Pipe Heat Sink Design Cost & Performance of All Options

 

The table above 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%.

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 U-shaped vapor chamber cooling solution. Although it’s 15% more expensive than the baseline, performance is increased by 28% (15.2 oC improvement).

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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.