Heat Sink Design Comparison (Pt 2)

Summary - This article compares the performance, weight, and cost trade-off of three different categories of heat sink design: solid metal base, metal base with embedded heat pipes, and a vapor chamber base.

Engineers are regularly tasked with having to make performance, weight, and cost trade-offs when developing a thermal solution. 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 this article, we’ll take a look at three potential heat categories (5 designs) for an application where the heat source and fin stack are in close proximity. In other words, where heat needs to be spread over the heat sink base and into the fins. The five heat sink options we’ll be testing fall into three categories: 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).

Figure 1: Heat Sink Design Options - Solid Metal Base, Embedded Heat Pipe Base, Vapor Chamber Base

Further, heat sinks are 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. Fin thickness = 0.3mm, Fin gap = 1.2mm
• Airflow: 50 CFM

Figure 2: Heat Sink Airflow Direction

Category 1: Heat Sinks Using Solid Metal Base of Either Aluminum or Copper (6mm)

When evaluating a thermal solution, 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, two-phase cooling solutions (heat pipes and vapor chambers) should be considered when the thermal budget is below 40 oC.

Figure 3: Aluminum Base (L) and Copper Base (R) Heat Sinks

Although the aluminum and copper solutions 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 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 thermal module max weight. While not shown it the table, increasing the copper base thickness to 12mm yields a module delta-t of 34.4 oC but weighs in at over 1,800 grams.

Category 2: Heat Sinks Using Embedded Heat Pipes in Aluminum or Copper Base

In this scenario, we’ve added to the 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. 

Figure 4: Embedded Heat Pipes in Al Base (L) and Cu Base (R)

Compared with their solid metal base counterparts, thermal solutions with added heat pipes improve thermal module delta-t by nearly 26 oC for the aluminum version and nearly 8 oC for the copper version. Here we see both solutions 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.

Category 3: Heat Sink Using Vapor Chamber (VC) Base

It should come as little surprise that the vapor chamber solution has the lowest 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 solution 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. 

Figure 5: Vapor Chamber Base (far right) vs Other Options

Summing up our choices, we’ve eliminated both the solid metal base heat sinks as they do not meet thermal requirements but are the least expensive solutions. From a weight and cost perspective, the embedded heat pipe 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.