Heat Sink Design Options

Heat Sink Design Options

Summary – This article compares three categories of heat sink design: solid metal base, embedded heat pipe base, and vapor chamber base. Each heat sink design is compared on the basis of performance, weight, and cost.

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 semiconductor).  In these cases, alternative heat sink materials should be considered.

In this article, we’ll take a look at 5 heat sink design options (in 3 categories), each using a different material for the heat sink base: 6mm solid metal base (aluminum & copper), embedded heat pipes in a 6mm thick base (Al & Cu), and a 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 Options Compared

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:

  • Heat Source: 10x10mm generating 100W
  • Max Ambient: 45 oC
  • 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
Direction of airflow across heat sink

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.

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 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 (delta-t) 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 is the most efficient, 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.

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Heat Sink Size Calculator Use Instructions

Heat Sink Size Calculator Use Instructions

Heat Sink Size Calculator Use 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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