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Edge cooling |
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| Home page.........: | Frigus Primore |
| Calculator...........: | Edge cooled plate with discrete heat sources |
Introduction
Most electronics is currently cooled by air. There are nevertheless other ways to manage the cooling problem. One of them is edge cooling. The idea is simple but it is associated with several difficulties. The purpose of this article is to discuss some of them and to present an idea of what is possible to achieve.
Figure 1
Some arrangements that have been used.
Figure 1 shows some arrangements that have been used. The leftmost is a "pizza box" design. The PCBs are in this case enclosed in metal cassettes that uses interleaved fin arrangements to interface with two external heat sinks. It is a compact design well suited for rough environments. The upper right image is a similar design but with integral heat sinks. This arrangement bypasses one of the interface problems. It is convenient for systems that only have a few units or systems that successively needs to be expanded. The lower right image shows that it is easy to combine liquid and edge cooling.
This small expose makes it apparent that edge cooling is associated with several thermal problems. The heat must be conducted from the components to the PCB edges, be interfaced with a heat sink and finally dissipated. The flow path can be looked at as a chain in which each link transfers heat at the expense of a temperature difference. An essential part of the design problem is therefore to arrange the links so that the total thermal resistance is optimised. It is essentially a cost problem for which the outcome depends on the circumstances.
Figure 2
The temperature profile for a PCB with a uniform heat load. A 200x200 mm 4-layer PCB, thermal conductivity 16.5 W/mK, heat load 5 W.
PCB conduction basics
As a first approach to the PCB conduction problem it is convenient to look at a very simple case, a PCB with a uniform heat load, figure 2. It is a vast oversimplification but it does provide an idea of the performance. The temperature profile is in this case parabolic and the maximum temperature difference is proportional to the square of the PCB width. This is a strong dependence and it is one of the difficulties with edge cooling. Doubling the PCB width while preserving the temperature would for example reduce the allowed heat load with a factor 4. This is contrary to the tendency for air-cooled PCBs, for which the heat load increases when the surface is enlarged.
The total temperature difference available for a telecom application is typically 35 degC. The maximum temperature difference in the example above is approximately 20 degC. About 15 degC is therefore left to push the heat trough the interfaces and the heat sink. Those are reasonable values but the heat load is not impressing, merely 5 W. It is not much for a 200x200 mm PCB, less than can be dissipated by natural convection. It is apparent that edge cooling never can be an attractive solution unless radical measures are taken to overcome this capacity problem.
One obvious improvement possibility is to increase the thermal conductivity of the PCB. Compared with the figure 2 case, it can be doubled by using thicker copper layers. Other options could be to use high performance graphite materials, heat pipes or thermosyphones. Those are however all high cost options and can therefore only be expected to be advantageous for niche applications.
The simplest way to enhance the cooling capacity is to place the major heat sources along the edges of the PCB. It is a measure that is both effective and inexpensive. The drawback is that it imposes such restraints on the electrical layout that it only can be used in special cases. Another enhancement possibility is to place the PCB in a metal box and let the box walls participate in the conduction. It is a very effective mean if there are good thermal bridges between the PCB and the box walls,. If not, air conduction and radiation can still make a significant difference. The latter is apparent in the example that follows.
Figure 3
A numerical calculation for an edge cooled PCB enclosed in a cassette. PCB size 265x300 mm. Heat load 30 W.
Calculation
The calculation problem is simpler than that for air-cooled PCBs. There are nevertheless several critical points that an adequate calculation tool must be able to handle smoothly. There must for example be an option to specify thermal resistances along the PCB edges. Radiation and air conduction from the PCB surface to its environment are important and must be well managed. If the PCB is enclosed in a box there are additional complications. That box will typically consist of two parts. The interface in between them will inevitably have a thermal resistance that must be possible to specify. The specification problem for thermal bridges between the PCB the box walls must also be manageable.
An example of a finite element calculation is shown in figure 3. The PCB is enclosed in a cassette that is cooled by external heat sinks. The interface is an interleaved fin arrangement. The major heat sources are placed along the edges of the PCB.
The temperature gradients are a measure of the direction and the intensity of the heat flow. Most of the heat is apparently conducted towards the edges. Some heat however, takes the opposite direction and flows towards the centre of the PCB. On its flow path it is gradually conducted and radiated to the cassette walls were it takes a U-turn and makes it way back to the cassette sides. This somewhat awkward flow pattern shows that the cassette walls can contribute significantly even if they not are in direct contact with the PCB. The PCB size was 265*300 mm and the total heat dissipation was on the 30 W level. It can be concluded that edge cooling, given the right circumstances, can be fairly effective.
Figure 4
An analytical solution for en edge cooled plate with a discrete heat source.
Figure 4 shows an analytical solution for an edge cooled plate with a discrete heat source. It is a member of the same family of solutions as the one described in the article A Fourier Series solution.... This particular equation version only accounts for conduction and can therefore not be used for accurate calculations. Because it is both simple and fast it is nevertheless convenient for quick and approximate overviews. The equation can be modified for any combination of cooled edges and there is also an option to account for a heat transfer coefficient. The included online calculator is based on this algorithm. A Mathcad-file and a corresponding PDF-file that implements this method and also expands it to single edge cooling can be downloaded here.
Figure 5
Thermal vias that connect the ground layers with the surface.
The interface problem
Besides the conduction problem there are also several interface problems. The first one is to bringing the heat, which essentially is carried by the power and ground layers, up to the surface of the PCB. The power layers have a potential so any metal connection between them and an external heat sink is excluded. The power and ground layers are however only separated by thin layers of an insulator material and can therefore exchange heat intimately, so this is not a major problem.
Figure 5 shows the conventional way to arrange a thermal flow path between a surface layer and one or several ground layers. It is simple and relatively effective. The drawback is that the manufacturing process, a chemical deposition procedure, has a tendency to increase the copper thickness around the holes. It may not be much but it disturbs the possibility to make an efficient surface-to-surface contact. Another way to arrange the same flow path would be to deposit copper on the very edges of the PCB. It is however not clear if this can be managed in the manufacturing process.
Figure 6
Example of a thermal interface arrangements.
The second problem is to interface the PCB edges with heat sinks. Various mechanical devices that squeeze the surfaces together can be used, figure 6. An interface material is sometimes needed. If the PCB is enclosed in a box and cooled by external heat sinks there is an additional interface. Interleaved fins can be used for this purpose. It is a simple solution but it is always associated with a heavy prize in form of a significant thermal resistance.
These thermal interfaces create significant temperature differences. They are furthermore very challenging for thermal engineers because they are almost impossible to predict accurately.
Heat sinks
The heat sinks can be of many different types. It is the fact that they are located outside the PCB that provides this flexibility. A general discussion is therefore difficult. There is nevertheless a significant difference between integral and external heat sinks. If many units share the same heat sink the cost per unit is low, which motivates a high performance heat sink design. The additional interface needed can therefore, in spite of the associated problems, be well worth the effort.
Conclusions
One problem with edge cooling is that the heat must be conducted long distances in the PCB, which limits the cooling capacity. There are nevertheless several improvement possibilities. Placing the major heat sources along the edges of the PCB is the simplest. Another is to enclose the PCB in a box and let the box walls contribute to the conduction.
Edge cooling is also always associated with a several thermal interface problems. The temperature differences needed to overcome these obstacles are significant.
The advantages with edge cooling are that it enables compact designs and that it is relatively easy to enclose the PCB in a protective box.
Ake Malhammar