Heat Sink Archives

Heat Pipe Heat Sink Design

This page compares five heat pipe heat sink designs, evaluating their thermal performance and relative cost. It targets horizontally finned configurations where the two-phase device must route through the fins. Each design is analyzed using CFD data, thermal resistance, and cost scaling to guide engineers in selecting the best solution for their needs.

Application Setup and Design Criteria

To ensure a consistent comparison, the following application parameters were used:

Heat source power: 250 W
Heat source size: 30 × 30 mm
Maximum ambient temperature: 25 °C
Airflow: 40 CFM
Fin block size: 115 × 85 × 65 mm (horizontal fins)

The thermal budget is calculated as the difference between the maximum Tcase and the maximum ambient temperature. This delta must be greater than the heat sink’s temperature rise to ensure proper performance. For example, a 40 °C thermal budget requires the sink to maintain a ΔT below 40 °C. This calculation is important because it drives heat sink choice.

Design 1: Heat Pipe Heat Sink with Aluminum Base

This traditional design uses a solid aluminum base with heat pipes soldered into machined grooves or flattened directly. Thermal grease or a pad is applied between the base and the heat source. Consequently, the heat pipes do not come into direct contact with the heat source.

ΔT: 53.9 °C
Thermal Resistance: 0.2156 °C/W
Relative Cost: 1.0X (baseline)

Although this is the most economical option, its performance is the lowest among the designs evaluated. Given a 25 °C maximum ambient temperature, the thermal budget would need to be above 79 °C (25 + 54).

U-Shaped Heat Pipe Heat Sink with Aluminum or Copper (Design 2) Base Plate

Design 2: Heat Pipe Heat Sink with Copper Base

Replacing the aluminum base with copper increases thermal conductivity and improves performance modestly.

ΔT: 51.6 °C (a 2.3 °C improvement)
Cost Increase: 5% more than aluminum
Relative Cost: 1.05X

This option is ideal when a moderate improvement is needed without a significant cost increase. However, for applications where weight is a concern, this option may not be optimal as copper has 3 times the density of aluminum.

Heat Pipe Heat Sink Cost & Performance of Solid Base Designs

Design 3: Direct Contact Heat Pipe Heat Sink

In this design, the heat source contacts the heat pipes directly. The base plate and solder interface are eliminated, thereby shortening the thermal pathway. However, additional machining is required to flatten the heat pipe surface.

ΔT: 49.3 °C
Thermal Resistance: 0.1972 °C/W
Relative Cost: 1.1X

This configuration offers better performance by removing two thermal junctions, though it incurs a slight cost increase due to machining of the heat pipes for a flatter surface (better heat transfer).

Direct Contact Heat Pipe Heat Sink

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

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

This configuration substitutes multiple heat pipes with a single U-shaped vapor chamber. It allows direct contact with the heat source and requires one-piece vapor chamber manufacturing capabilities.

ΔT: 37.7 °C (21.5% improvement over direct contact heat pipes)
Cost Increase: 4.5%
Relative Cost: 1.15X

Although slightly more expensive than Design 3, this approach provides significant performance gains with only a moderate increase in cost and weight.

U-Shaped Vapor Chamber

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

Design 5: 3D Vapor Chamber Heat Sink

The most advanced design includes a vapor chamber base with eight brazed condenser tubes aligned vertically. Heat spreads across XY and Z axes, making it well-suited for higher power densities or larger base plates.

ΔT: 35.7 °C (best performance)
Relative Cost: 2.17X (117% more than Design 4)

Despite its high cost, the 3D vapor chamber delivers unmatched heat spreading, particularly in space-constrained or high-load applications.

3D Vapor Chamber Heat Sink

Comparative Table: Heat Pipe Heat Sink Design Cost & Performance of All Options

Summary and Recommendation

From aluminum base to 3D vapor chamber, there’s a 17 °C improvement in thermal performance, albeit at over twice the cost. Moderate gains (2–5 °C) are achieved by switching to copper or direct contact configurations, while vapor chamber solutions provide more substantial improvements.

From a general perspecive, the best choice based on performance and cost is the U-shaped vapor chamber design. However, in practice, the best heat pipe heat sink design depends on the Max Tcase of the heat source. Choose the design whose total temperature rise (max ambient of 25 °C plus the Delta-T from the chart above) is LOWER than the Max Tcase of the heat source.

Vapor Chamber vs Heat Pipe

In the battle of two-phase devices, vapor chamber vs heat pipe, there’s no clear winner. Each has attributes that make one superior to the other. This article covers differences in two-phase devices and usage rules of thumb. See these heat pipe and vapor chamber links for more information on components parts and working principles.

Heat Transport of Vapor Chamber vs Heat Pipe

When considering which two-phase device best fits an application, it’s best to begin with this generally accurate rule of thumb. Use vapor chambers to spread heat to a local heat sink; use heat pipes to move heat to a remote heat sink. Unlike heat pipes that move heat in a linear fashion, vapor chambers move it in multiple directions away from the heat source.

heat pipes move heat and vapor chambers spread heat

Vapor Chambers Spread Heat to Local Heat Sink | Heat Pipes Move Heat to Remote Heat Sink

It’s important to remember that the thermal conductivity of these two-phase devices change with the distance heat is moved or spread. As the distance is decreased thermal conductivity goes down, almost to the same level as solid copper, which will be a less costly option. For heat sinks using a vapor chamber, t’s generally recognized that you want the area of the vapor chamber to be at least 10 times as large as the area of the heat source. Anything much less and solid copper may be a better alternative. For heat pipe heat sinks, you want the effective heat transport length to be at least 40 -50mm.

In the end, both heat pipes and vapor chambers do an excellent job of transporting heat, it’s just that the application changes slightly. After all, the manufacturing process and working principles are functionally identical for these devices.

Heat Transport Winner: Tie

Design Flexibility of Vapor Chamber vs Heat Pipe

Think of this as the ability of vapor chambers and heat pipes to be used in a myriad of ways, depending on the thermal challenge.  Heat pipes can have multiple bends to avoid components while reaching a remote heat sink, be used alone or in combination, and in different directions.  In short, they are an indispensable thermal option, especially for thermal challenges involving a difficult path from the heat source to the heat sink or when the fin stack is very high, necessitating the heat pipes be run up through the fins.

Compare the design flexibility of heat pipes and vapor chambers

Heat Pipes Offer Slightly More Design Flexibility Than Vapor Chambers

The historical design flexibility of vapor chambers was limited to the X and Y planes, with only small ‘steps’ feasible in the Z-direction. However, because the outer layer of a traditional vapor chamber is made from two stamped copper plates, almost any contiguous shape along the XY axes is possible.

Fortunately, there is another type of vapor chamber with design flexibility in the up and down (Z) direction. Knows as 1-piece vapor chambers because they begin the manufacturing process as a very large tube (20-70mm diameter), they can be bent post-production into L and U-shapes. However, their starting shape is limited to a rectangle or a rectangle with a small portion removed.

Design Flexibility Winner: Heat Pipes (but it’s a close call)

Heat Carrying Capacity of Vapor Chamber vs Heat Pipe

Also known as Qmax, heat carrying capacity is the maximum power input (in watts) that can be applied to a heat pipe or vapor chamber and still have it work properly.

By virtue of its contiguous cross-sectional area, a single vapor chamber designed for electronics cooling can handle power input upwards of 450 watts. By contrast, the largest generally available heat pipe tops out at around 125 watts when used in the horizontal orientation (gravity neutral).

However, heat pipes are often used in combination to divvy up the heat load, whereby increasing total heat carrying capacity. To ensure each heat pipe has a relatively equal heat load, the pipes must be positioned directly above the heat source. Typically, a multiple heat pipe configuration will be close to its Qmax limit in operation while a single vapor chamber will have plenty of room to spare.

Heat Carrying Capacity Winner: Vapor Chambers

Isothermality of Vapor Chamber vs Heat Pipe

Whether spreading or moving heat, the goal for most higher-performance thermal applications is to minimize the temperature differential (delta-T) in the base of the heat sink and/or to reduce hot spots across the die face.

Minimizing the temperature gradient across the base of a heat sink is critical when the thermal budget is tight. Defined as the difference between the maximum thermal design power (TDP) of the chip minus the maximum ambient operating temperature of the device, this measurement gives us an indication of if a two-phase device should be used (usually thermal budgets less than 40 deg C).

There are two commonly implemented ways heat pipe heat sinks improve isothermality when compared to solid copper, both of which relate to how the heat pipes interface with the heat source.

Indirect Interface – The most common method is a base plate of either aluminum or copper that’s mounted to the heat source which in turn conducts heat to embedded heat pipes.
Direct Interface – The second method is to mount the heat pipes directly to the heat source. This will invariably require the heat pipes to be machined to ensure good direct contact with the heat source. This method, while generally more expensive, performs better as the base plate and additional solder are removed from the heat sink assembly.

Illustrates the two options for mounting heat pipes to the heat source

Options for Mounting Heat Pipes to the Heat Source: Indirect & Direct

As mentioned earlier, vapor chambers have a very large internal cross-sectional area, even when compared – in practice – to multiple heat pipes embedded in the heat sink. Moreover, a vapor chamber can ‘connect’ multiple heat sources to the same heat sink and in the process create a situation where temperature differences between and around the heat sources are minimized.

6 ASICS Remain within 2 Degrees Celsius of Each Other

Lastly, shrinking microprocessor die size has resulted in ever-increasing power density that needs to be dispersed quickly. Heat pipes are typically used for applications with a power density of less than 50 W/cm2, while vapor chambers are almost a certainty when cooling power densities above 50 W/cm2.

Isothermality Winner: Vapor Chambers

Cost of Vapor Chamber vs Heat Pipe

Commercial use of heat pipes began in the 1960s at a time when, relative to today, heat loads and power densities were low. Often a single heat pipe sufficed. A vapor chamber would have been ‘overkill’. Consequently, the volume manufacturing process was refined sooner, and competition increased – driving prices down.

The traditional – two stamped copper plates – method of manufacturing vapor chambers is inherently more costly than the heat pipe method of production. Additionally, demand for vapor chambers only began to dramatically grow at the turn of the millennia due to higher power density devices.

Traditional Vapor Chamber | 1-Piece Bendable Vapor Chamber

The advent of 1-piece vapor chambers, in conjunction with the higher demand, has driven vapor chamber pricing close to parity with multiple heat pipe designs. While a few consumer applications have spawned standard-size vapor chambers, the majority of the designs are custom, lower volume projects.

Regarding relative cost and performance, we have written two blogs that compare heat pipe heat sinks to vapor chamber heat sinks for two different applications.

For additional heat sink design tips please see Heat Pipe Design Guide and Vapor Chamber Cooling Design Guide.

Cost Winner: Heat Pipes

Summary

In the battle of vapor chambers vs heat pipes, we have a tie if we weight the above criteria equally.

Clearly, we have a tie when comparing vapor chambers to heat pipes if all the mentioned criteria are weighted equally. In practice, thermal applications require that design engineers’ weight these differently. Most often, heat pipes prevail – that is why they represent the bulk of two-phase choices. But, when every degree counts and cost becomes slightly less important, vapor chambers win the contest.

Heat Pipes Are the Best Choice If:

Heat needs to be moved to a remote fin stack more than 40-50mm away
The thermal budget (difference between TDP and max ambient operating temperature) is below 40 0C
Nominal power densities are <50 w/cm2
Cost is a key consideration – every penny counts!

Vapor Chambers Should be Considered If:

Heat needs to be spread quickly to a heat sink base that’s 10X the area of the heat source
The thermal budget (difference between TDP and max ambient operating temperature) is below 30 0C
Multiple heat sources need to be isothermalized
Power densities are high – certainly by the time they hit 50 w/cm2
Performance is a key consideration – every degree counts!

Winner: Every Thermal Engineer

Heat Sink Design Options

Summary – This article compares the performance, weight, and cost of three categories of heat sink design: heat sinks with a solid metal base, heat sinks with embedded heat pipe base, and heat sinks with a vapor chamber base. Heat sink fins for all designs are oriented vertically.

For more comparisons see “Heat Pipe Heat Sink Design”, which compares only heat pipe and vapor chamber designs used when the fin stack is oriented horizontally.

Engineers are regularly tasked with heat sink design optimization, making careful trade-offs between heat sink performance, weight, and cost. Sometimes the decision is easy, such as when the low-cost alternative allows the device to meet or exceed all product requirements. However, the decision is more difficult when the thermal budget is tight and/or when a single heat sink is required for different product configurations (higher power semiconductors). In these cases, alternative heat sink designs should be considered.

In this article, we’ll take a look at 5 heat sink design options (in 3 categories), each using a different configuration for the heat sink base:

Heat Sink Design Category 1: 6mm thick solid metal base (one with aluminum base & one with copper base),
Heat Sink Design Category 2: embedded heat pipes in a 6mm thick base (copper water heat pipes embedded in both a copper and aluminum base),
Heat Sink Design Category 3: 4mm thick copper/water vapor chamber base. For all options, the heat source makes direct contact with the device (no mounting plate).

Heat Sink Design Comparison: Solid Metal Base, Embedded Heat Pipe Base, Vapor Chamber Base

Further, each heat sink design is subject to the following operating parameters and performance targets:

Heat Source: 10x10mm generating 100W
Tcase Max: 80 ^oC
Max Ambient: 45 ^oC
Thermal Budget: 35 ^oC (80-45)
Target heat sink thermal resistance: 0.35 ^oC/W or less (35/100)
TIM: K = 3W/(mK)
Aluminum Fin Pack Dimensions: 150 x 99 x 30mm.
Heat Sink Fin Thickness = 0.3mm, Fin Gap = 1.2mm
Airflow: 50 CFM

Heat Sink Airflow Direction

Heat Sink Design Category #1: Solid 6mm Metal Base of Either Aluminum or Copper

When evaluating any heat sink design, the single most important parameter is the thermal module delta-T relative to the calculated thermal budget (Tcase Max – Max Ambient). We know max ambient is 45 ^oC and if we assumed max Tcase was 80 ^oC, our thermal budget would be 35 ^oC. As a general rule, consider heat sinks designed with heat pipes or vapor chambers when the thermal budget is below 40 ^oC. Heat sinks with a lower delta-T will also have reduced thermal resistance.

Aluminum Base (L) and Copper Base (R) Heat Sinks

Although the aluminum and copper heat sink designs are the most cost-efficient, neither thermal module delta-T falls within the calculated thermal budget of 35 ^oC. If the budget was 5 degrees higher, the copper heat sink base version would meet requirements, but at a hefty weight penalty (500 vs 1,055 grams). This could be problematic as many applications have strict shock & vibration and/or portability requirements that dictate heat sink maximum weight. While not shown it the table, increasing the copper base thickness to 12mm yields a thermal module delta-t of 34.4 ^oC but weighs in at over 1,800 grams.

Heat Sink Design Category #2: Embedded Heat Pipes in Aluminum or Copper Base

In this heat sink design scenario, we’ve added to the heat sink base two 6mm copper/water heat pipes that have been bent and flattened to 3mm. Note that because these are direct contact heat pipes, the surface under the heat source is machined (0.025mm/cm) to ensure good contact between it and the heat source.

Embedded Heat Pipes in Aluminum Base (L) and Copper Base (R)

Compared with their solid metal base counterparts, adding heat pipes improves heat sink performance (lower delta-t and thermal resistance) by nearly 26 ^oC for the aluminum version and nearly 8 ^oC for the copper version. Here we see both heat sinks easily beating our thermal budget of 35 ^oC. Like our solid metal solutions, weight is roughly doubled for the copper version along with the same numeric increase in price.

Heat Sink Design Category #3: Vapor Chamber (VC) Base

It should come as little surprise that the vapor chamber heat sink design has the lowest thermal resistance, having a delta-T at 26.0 ^oC – over 5 degrees cooler than the closest alternative. Moreover, the 4mm vapor chamber reduces the overall height of the heat sink by 2mm. If the designer doesn’t need the extra space, it can be added back to the fin area, further decreasing heat sink thermal resistance.

Vapor Chamber Base (far right) Compared with Alternatives

Summing up our choices, we’ve eliminated heat sink designs using a solid metal base as they do not meet thermal requirements but are the least expensive solutions. From a weight and cost perspective, the embedded heat pipe design with an aluminum base is the clear winner unless other factors are taken into account. For instance, if more powerful heat sources are slated for the same form factor and we want to maximize economies of scale for the heat sink (use the same sink across multiple product configurations), then we should calculate maximum power handling capacity without violating our thermal budget.

With a 35 ^oC thermal budget, we can calculate the following max heat source power input into each of the remaining options.

Heat Pipe with Aluminum Base: 106 watts (35 ^oC /0.327 thermal resistance)
Heat Pipe with Copper Base: 112 watts
Vapor Chamber Base: 135 watts

Of course, in doing this calculation we need to ensure the two-phase devices themselves can handle the additional power before wick dry-out occurs. In this case, both the heat pipes and the vapor chamber can do so.

Related Links

Heat Sink Thermal Resistance Calculator Instructions

This guide provides step-by-step instructions for using Celsia’s heat sink thermal resistance calculator. By inputting variables such as power dissipation, material properties, and airflow conditions, engineers can estimate temperature rise across the base, thermal interface material (TIM), and fin section. The tool also facilitates comparisons between solid metal and vapor chamber base designs, aiding in the optimization of thermal performance.

Here’s a link to the online calculator.

The first thing you should calculate is your thermal budget which is simply Tcase max (or Tjunction max if bare die) that will be supplied by the chip manufacturer MINUS Max Ambient temperature at which the device is designed to operate. Thermal budgets above 40 oC usually only require an aluminum or copper base whereas tighter budgets may need to add a two-phase device such as a vapor chamber.

Heat Source Power (Q) – This is the thermal design power (TDP) which is the maximum amount of heat, in watts, generated by the chip. It should be provided by the chip manufacturer or ASIC engineer if done in-house.

Heat Source Length, Width, and Location – Self-explanatory especially with the provided diagram. Remember that chip location will greatly affect base delta-t for solid metal while its effect on the vapor chamber delta-t is negligible. A general rule of thumb is that vapor chambers become attractive as the ratio of base area to heat source area surpasses 10:1.

Base Information – The length and width dimensions you enter for the base of the heat sink will also be used as the dimensions for the fin pack. It’s important to note that LENGTH (of base and fin pack) will go in the direction of the AIR FLOW. Note: if you are in the early stages of heat sink design and need an estimate of the required heat sink size, refer to this online calculator.

Fin Pack Information – While fin height can vary considerably by application, we typically see figures in the 10-35 mm range. Fin thickness for fin packs (zipper fins) range from 0.2 to 0.6 mm while fin spacing/gap should be at least 2X the fin thickness. Next, input the fin pack material and the max operating temperature at which the device is designed to operate. You can use the ‘Fin Pack Temperature Rise & Pressure Drop” chart as a starting point for selecting the appropriate fan. Conversely, if you know the fan to be used, alter the fin and air flow variables to achieve acceptable pressure drop numbers. Lastly. choose the type of TIM to be used from the drop-down menu.

The Results – The first set of delta-ts are common to both the solid metal and vapor chamber base. In this example, we have a total fin pack delta-t of 27.3 oC, and a total TIM and Base to fin delta-t of 2.5 oC.

To this total of we need to add the base delta-T.

If your thermal budget (Tcase max – Tambient max) was initially calculated at 40 oC, we see that an aluminum base, with a total heat sink delta-t of 51.4 oC will not be acceptable. While changing both the fin and solid base material to copper (not shown) will get you below the thermal budget, it comes at a large weight penalty. In this case, a vapor chamber solution is clearly the better option.

← Heat Sink Size Calculator Instructions Heat Pipe Calculator Use Instructions →

Heat Sink Size Calculator Instructions

How to use the online heat sink size calculator used in the early stages of heat sink design. With the exception of choosing the correct “volumetric thermal resistance”, this is probably our most straight forward calculator.

Here’s the link to the calculator.

Heat Source Power (Q) – This is the thermal design power (TDP) which is the maximum amount of heat, in watts, generated by the chip without exceeding its thermal envelope. It should be provided by the chip manufacturer or ASIC engineer if done in-house.

Tcase Max – the maximum temperature of the chip case. For most chip designs this will be provided by the manufacturer. For bare die chips, the max Tjunction temperature will be given. In this case, use the Tjunction spec in place of Tcase max.

Max Ambient – the maximum ambient temperature at which the device is intended to operate.

Thermal Budget – Tcase Max minus Max Ambient. The sum total of all delta-ts in the network, from Tcase to Air temperature rise cannot exceed this limit.

Note – thermal budgets below 40 degrees Celsius are generally good candidates for two-phase cooling using heat pipes or vapor chambers.

Volumetric Thermal Resistance (Rv) – This equation and subsequent guidelines have been shown to closely estimate heat sink volume: V=(Q*Rv)/Delta T. The first step in using the chart below is to know the available airflow across the heat sink. As you know, the higher the airflow the smaller the heat sink. The first challenge you’ll probably face is that the fan manufacturer has given you the ‘bulk airflow’, usually in cubic feet per minute, but not the air velocity. It’s easy to determine the velocity if you know the size of the heat sink but since we don’t it’s a catch 22. Here are some rough guidelines:

Open air natural convection develops about 40 LFM which is about just enough to blow out a match.
1 m/s or 200 LFM you can feel the flow but not hear it.
2.5 m/s or 500 LFM is a good flow that will blow out a whole bunch of candles and you can begin to hear the noise, especially in a quiet environment.
5 m/s or 1000 LFM is going to be noisy. Not to be used in any noise-sensitive environment.

Once you’ve selected the appropriate airflow, the next step is choosing from a range of Rv. In the case of moderate air (2.5 m/s) the range is 80-150. The published rule is as follows:

For heat sinks smaller than 300 cm3 you use the lower limit, in this case 80.
For large heat sinks, greater than 1,000 cm3 use the upper limit.

However, like the CFM to LFM calculation we have a bit of a chicken and egg scenario. My suggestion – use something in the middle to gauge the rough size of the heat sink and adjust from there. For example, if you initially use 115 Rv value and the estimated volume of the heat sink is less than 300 cm3, change the Rv to 80, which is the lower end of the Rv value for moderate airflow.

When designing devices that must work at altitude, it’s important to de-rate the Rv. A solid rule of thumb is 10% for every mile of altitude. For example, at one mile high we’d divide 80 Rv by 0.9 to end up with a de-rated Rv of roughly 89.

Once you’ve determined the volume of the heat sink, the last step is to assign some length, width and height dimensions. Initially, this is generally driven by space constraints so enter some figures and see if you come close to matching the required volume. One note here, we have an online heat sink performance calculator that you might play with to more finely tune heat sink base and fin height.

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