Heat pipes are incredibly efficient thermal management devices used in the construction of most of the CPU coolers Frostytech has reviewed. Though simple in appearance, these thin copper tubes bent into gentle curves literally determine the fate of your computer hardware!

So what goes into an effective heat pipe based CPU cooler?

In two words, thermal conductivity. In fact, the thermal conductivity of a heatpipe surpasses that of the base copper metal, thanks to good old physics. Put simply, a heat pipe operates by leveraging the physics of phase change and capillary action to efficiently transfer heat energy along its length.

What that means, is when heat is applied to the evaporator section of the heat pipe (the hot end), the working fluid inside the pipe absorbs this heat, pressure increases and it evaporates, undergoing a phase change from liquid to vapour. The vapour can absorb a significant amount of latent heat, which it then carries with it as it travels through the hollow core of the heat pipe towards the cooler condenser section (the cold end).

Once the vapour reaches the condenser section, the heat it absorbed is released back into the surrounding metal (ie. cooling fins of the heatsink) and it condenses back into liquid form. The freshly condensed working liquid is then transported back to the evaporator section through an internal wick structure lining the inside of the pipe, driven by capillary action. This silent cycle repeats endlessly; evaporation, vapour transport, condensation and liquid return.

This is the process which allows a copper heat pipe to transfer heat efficiently from a busy CPU crunching numbers to the rest of the heat sink with minimal temperature gradient. Take away the wick structure or use the wrong kind of wick structure and the process would halt in its tracks and your CPU would overheat in minutes.

Typical heat pipe based CPU cooler

So what is the most effective wick structure for a heatpipe CPU cooler?

The performance of any heatsink is significantly influenced by the type of wick structure inside the heat pipe used in its manufacture. A wick is essentially a microscopic structure on the inside of the heat pipe whose sole function is to return working fluid by capillary action.

Rip a corner off some paper towel and dip the edge into a cup of water, the water that gets sucked up is drawn up by capillary forces.

The wick directly impacts the heat transfer efficiency of the whole CPU cooler and there are four main kinds of wick structures encountered in CPU heat pipes: metal sintered powder wick, grooved wick, metal mesh wick, and the hybrid sintered metal powder/grooved wick combo.

Metal Sintered Powder Wick Heat Pipe

Structure: Made from powdered copper particles that are sintered to form a porous structure.
Capillary Action: High capillary pressure due to fine pore sizes, which is beneficial for efficient liquid return to the evaporator.
Thermal Conductivity: Generally high due to the nature of copper, but slightly lower than solid metal wicks because of the pores.
Heat Transfer Performance: Excellent for high heat flux applications due to effective liquid transport.
Operating Orientation: This type of wick performs well in various orientations, including against gravity, due to strong capillary forces.
Complexity and Cost: Manufacturing is complex and relatively expensive due to the copper powder sintering process.

Grooved Wick Heat Pipe

Structure: Features very fine grooves or channels machined into the inner surface of the copper pipe.
Capillary Action: Lower capillary pressure compared to sintered wicks because of larger groove dimensions.
Thermal Conductivity: High, as the wick is an integral part of the copper wall.
Heat Transfer Performance: Effective in applications with lower to moderate heat fluxes.
Operating Orientation: Performance can degrade significantly in orientations where gravity opposes liquid return.
Complexity and Cost: Easier and cheaper to manufacture compared to sintered metal powder wicks.

Metal Mesh Wick Heat Pipe

Structure: Made from woven copper mesh, which is inserted into the heat pipe.
Capillary Action: Moderate capillary pressure; higher than grooves but lower than sintered copper powder wicks.
Thermal Conductivity: Relatively high, though there may be slight thermal resistance at the mesh interfaces.
Heat Transfer Performance: Suitable for moderate heat flux applications.
Operating Orientation: Better than grooves but not as versatile as sintered copper powder wicks in all orientations.
Complexity and Cost: Simple and cost-effective to manufacture.

Sintered Metal Powder/Grooved Wick Heat Pipe (Hybrid)

Structure: Combines sintered powder and grooved wick structures within the same heat pipe.
Capillary Action: High capillary pressure from the sintered copper powder, with enhanced liquid distribution from the grooves.
Thermal Conductivity: High, leveraging the benefits of both sintered and grooved structures.
Heat Transfer Performance: Excellent for a wide range of heat fluxes; very efficient due to the hybrid approach.
Operating Orientation: Highly versatile, performing well in any orientation due to the combined capillary action and liquid distribution.
Complexity and Cost: More complex and expensive to manufacture than single-structure wicks, but offers superior performance.

 

Hybrid heat pipe wick - sintered metal powder and groove

Of the four types of wick structures encountered in CPU cooler heatpipes, hybrid wicks (sintered metal powder/grooved) typically outperform the rest due to their combined advantages. These are the most common type in performance CPU coolers as they perform well in any orientation and offer the highest capillary action. Budget heatsinks of dubious thermal performance heritage may opt for the less expensive grooved or mesh type heat pipe.

So a high performance heatsink has the best wick structure and should be a winning combination then? No, not always.

The devil is in the details and CPU cooler manufacturers don't always integrate heat pipes into a heatsink properly, or worse degrade the heatpipe's thermal conductivity with poor design choices.

So what degrades the thermal performance of a heat pipe in a CPU cooler?

The efficiency of heat transfer from a CPU processor to cooling fins depends on the wick structure’s ability to manage high heat flux and operate effectively in various orientations.

Several factors can degrade the thermal conductivity of a heat pipe when it's integrated with a heatsink, impacting the overall efficiency of the thermal management system. These can include poor thermal interface, mechanical stresses, wick degradation, working fluid issues and deformation among others.

Poor Thermal Interfaces

Imperfect contact surfaces between the heat pipe and the heatsink, or heatpipe and CPU, invariably leads to increased thermal resistance. Metal surfaces need to be smooth and flat to ensure optimal heat transfer.

The quality and application of thermal interface materials between the heat pipe and the heatsink is also crucial. Insufficient or uneven application of thermal interface materials leads to air gaps and reduces thermal conductivity between the heat pipe and heatsink body.

Mechanical swaging, adhesive bonding, good old fashioned reflow oven soldering and thermal interface compounds are typically used to reduce joint resistance.

Mechanical swagging involves tightly fitting the heat pipe into pre-formed holes in an aluminum fin (for example) and crimping a small collar in tightly without cracking the thin heat pipe wall. Swagging ensures good thermal contact through direct metal-to-metal contact but can loosen over many thermal cycles. Reflow oven soldering is also commonly used as it provides a thermally conductive bond, but requires careful control of temperatures and application of solder paste so as not to create voids. Epoxy-based thermal adhesives are another option, but again voids in the application of the adhesive is a real problem.

Mechanical Stresses

Excessive or insufficient mounting pressure can deform a heat pipe, compromising its contact area and thermal conductivity.

Longer term, mechanical vibrations, shocks and thermal cycling during operation can cause micro-gaps to form at interfaces, leading to increased thermal resistance.

Material Degradation

Oxidation or corrosion of the heat pipe's surface is a sure fire way to degrade thermal conductivity. Copper, while resistant, is not immune to long-term degradation driven by oxidation under damp environmental conditions.

Contaminants such as dust, dirt, or dried out thermal paste residues can interfere with the thermal interface, reducing the effectiveness of heat transfer as well.

Wick Structure Degradation

Over time, the wick inside the heat pipe can become clogged with impurities or degraded material, reducing its ability to transport the working fluid effectively and impairing the heat pipe’s performance.

More commonly seen however, is dry out. If a heat pipe is subjected to excessive heat loads it can easily exceed its capacity to return liquid to the evaporator section. The heat pipe quite literally dries out in one section, leading to a significant and immediate drop in thermal conductivity and spike in temperatures along its length.

Working Fluid Issues

Any leakage of the working fluid will drastically reduce a heat pipe's effectiveness. Ensuring a hermetic seal is crucial for maintaining performance. Typically, cracks or punctures with heatpipe direct touch type heatsinks will see failures of this type.

If the chemistry of the working fluid is off, over time, it can degrade or react with the wick material, reducing its ability to efficiently transfer heat.

Heat Pipe Deformation

Improper handling or installation can bend or kink the heat pipe, disrupting the internal wick structure and flow of the working fluid, which can severely impact thermal performance.

The geometry of the groove a heat pipes is swagged into at the base of a heatsink can also significantly degrade efficiency. Swaging is the process used to enhance mechanical bonding and improve thermal contact with external parts of a heatsink by pressing.

If the grooves a heat pipe is swagged into are improperly designed, they can disrupt the uniformity of the internal wick structure, particularly with the sintered metal powder type of heatpipe; the sintered metal powder type relies on a consistent and uninterrupted wick structure. Grooves with an oval, oblong oval, flat oval or perfectly circular cross section all play a different role in mechanically retaining the heatpipe in position (first and foremost) and attenuating thermal conductivity efficiency.

Inadequate Design Considerations

Differences in thermal expansion coefficients between a copper heat pipe and the base heatsink materials (aluminum, copper, brass, silver, etc.) can cause stress and deformation over many thermal cycles, leading to increased thermal resistance.

Frostytech has seen heat pipes shift, twist and receded from intimate contact with a processor at the base plate due to poor retention design choices. At the cold end, repeated thermal cycling can lead to swagged cooling fins loosening and adhesively bonds cracking.

Poorly designed heatsinks suffer at high TDP (thermal design power) heat loads from incorrect positioning or insufficient contact area between heat pipes and the heatsink. The heat pipe's effectiveness in spreading and dissipating heat is restricted and dry out usually results.

So what makes the best heat pipe heatsink?

There are many factors that go into making an effective heat pipe based CPU cooler... what Frostytech has covered here only scratches one small corner of one surface. The short answer is that as a PC enthusiast you can really only rely on established heatsink manufacturers who have proven - over countless CPU cooler generations - that they know how to build heatsinks.

Frostytech has tested hundreds of heatsinks over the years, travelled to Computex Taipei and spoken directly with nearly every thermal solutions manufacturer you've ever heard of, countless thermal engineers and gleaned hard fought experience by testing CPU coolers to their failing points on our synthetic thermal test platform. All this insight is crammed into each and every one of our heatsink reviews. If you're not sure where to start, check out Frostytech's heatsink reviews - sorted by Heatsink Brand right here.

In the mean time, the next time consider a humble CPU air cooler, remember all the factors that have to be addressed through careful design, proper materials selection, and meticulous assembly processes to ensure efficient thermal management of your computer processor!