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Posted on 10 July 2019

Cold Plates for Water Cooling Electronic Components

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The thermal and engineering solutions

Cooling electronic components, especially power electronics, is a field where many skills converge: thermal design of the cooling unit, familiarity with the components and with all the requirements pertaining to the power diagram, and mechanical design and construction of the cooling unit.

By L Dubois, JL Dubelloy, and Ferraz Shawmut, Thermal Management, La Mure, France

 

Since the introduction of the IGBT, the concept of a cold plate to cool and serve as a mechanical base for that subassembly has imposed itself as a reference for many reasons:

- excellent cooling performances thanks to forced circulation of a fluid (water or water + antifreeze);
- use of a fluid with little environmental impact;
- the small footprint of the cold plate;
- simplicity of maintenance, since the component can be dismounted with no loss of cooling fluid;
- reasonable cost, weight and robustness;
- the prospect of standardizing the cooling units used, since the power electrician can use the same cold plate for many electrical solutions and diagrams;
- an interface between the product and the need that matches the industrial model and the skills of the three players involved: the designer of the power electronics function, the maker of the power electronics component, and the cold plate manufacturer.

Our goal here is to describe some cooling solutions (shape) and cold plate concepts, and explain the reasons for choosing Ferraz Shawmut products. A cold plate (Picture 1 gives an example) is basically the component's mechanical base, with heat exchange surfaces and circulation of a cooling fluid. To improve performances, larger heat exchange surfaces are provided on the base supporting the component (on the same principle as fins in air cooling devices). Another important "detail" is the cover, which serves to seal the product.

Aluminium is the best material available, because of its good thermal conductivity, its density and its cost, and because it is easy to machine and form into complex geometrical shapes.

Heat extraction cross section

As a young engineer, once I had estimated the thermal requirements (heat exchange surface area, etc.) for my first cooling unit, I was stumped by another question: how would I be able to seal my cold plate effectively with a cover? Contemporary engineering being well versed in automotive applications, a simple "cylinder head gasket" seemed an easy solution. But power electronics products have specific requirements, like outstanding durability and the total exclusion of any leakage (of water!) in a high voltage environment. So, back to the drawing board.

Vacuum brazing of aluminium is a technology Ferraz Shawmut mastered more than 15 years ago and is a simple, reliable solution. Brazing is similar to soldering but without melting the base part. When aluminium is brazed, an aluminium-silicon eutectic alloy is heated to the melting point between the parts. This guarantees both durability and an excellent contact between the cover and the base plate over the entire surface of the cold plate.

Principle of vacuum brazing

Vacuum brazing furnace

Example of a cold plate

There are other technologies available, but brazing with flux cannot guarantee the parts will remain as clean, and electron beam or laser welding, despite the cost aspects, is best used for straight welds and is unsuitable for large flat areas.

With a perfected technology to make a closed, watertight cold plate, we can go on to optimize it thermally. The "improved" heat exchange surfaces we were familiar with in the field of air cooling led us to offer our customers some optimized cooling shapes to meet requirements such as:

- affordable cost;
- a reasonably "open" shape to avoid any risk of plugging in the circuit;
- surefire solutions that guarantee watertightness and high performance for mass produced products with a long life span.

Machining those shapes offers a control of geometry that is not possible with a technology involving the attachment of fins. So Ferraz Shawmut proposes cooling shapes machined directly in the cold plate for optimal product quality. Indeed, brazing filler (the material that is melted) is generally only about 100 microns thick, and any lack of sufficient thickness when a fin is attached can degrade cooling of the component's chip in places (over a few square centimeters) if the contact at that point is not good enough.

Optimizing heat exchanges led us to develop cooling shapes in either pin or wave forms. Heat exchange performances are summarized in the following chart for a typical application (pure water) with these 3 types of shape. The extra heat exchange provided by pin or wave shapes cuts thermal resistance in half compared to conventional straight fins (marked "straight" in the chart).

Thermal performance

In recent developments proposed by FSTM, the concept of a counter current, combined with straight or wavy fins, offers a further step forward. How the cooling fluid heats as it circulates is directly related to fluid flow and power dissipation. For instance, a flow of 30 l/min in a cold plate cooling 8 components, each with power of 2500 W, means the fluid itself (water + 40% EG) will heat by 11°C. So the last component on the cold plate is cooled by fluid at a much higher temperature. That effect can be limited by a dual design with one hot and one cold channel for the cooling fluid, restricting heating to 5.5°C and guaranteeing a uniform temperature under each component, which is an important factor in the electrical balance between components.

Conclusion

FSTM's efforts at optimization in the field of cooling units for power electronics have constantly driven us to improve our cooling topologies, in order to offer the best possible performances and high quality, durable products. New shapes are being tested to further improve both performance and manufacturing costs, so that we can continue to offer our customers the best products on the market. Ferraz Shawmut Thermal Management is one of the foremost players on that market.

 

 

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