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).
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
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.
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.
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.
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.