Designing Custom Liquid Cold Plates

Liquid-cooled cold plates offer superior cooling for high power electronic devices and can be purchased as standard products or can be custom designed. A custom cold plate is needed when there is a special shape or interface requirement or an extreme performance requirement. An extreme performance requirement occurs when the specified performance cannot be uniformly applied across the entire cold plate or the pressure drop and/or cost of a compliant cold plate would be too high. The thermal map, or distribution of heat loads, may have one or several areas with high heat loads. If there are pressure drop requirements, cold plate surface temperature uniformity requirements, special shape or interface requirements, or cost limitations that eliminate a standard cold plate design, then a custom cold plate is the solution. Understanding cold plate technologies, thermal specifications, and the steps involved in the design process will help to optimize the custom cold plate design so it provides the best value possible.

Performance requirements generally dictate choice of cold plate technology and design, and cold plate technology will drive cold plate cost. Generally, cold plate cost will increase with improving performance. Cold plate technologies include Press-Lock™ tubed, Hi-Contact ™, gun-drilled with or without expanded tubes, channeled, and brazed with internal fin.

These technologies are listed in order of what is typically increasing cold plate efficiency and cost:

In addition to four types of cold plate technologies, there are also four scenarios of thermal requirements, which are listed below:

• Uniform Heat Flux, Fixed Flow Rate, 1 Maximum Pressure Drop, 1 Maximum Surface Temperature - With thermal scenario one, there is uniform input heat flux, a fixed flow rate, one specified maximum pressure drop that is limited at a fixed flow rate, and one specified maximum surface temperature where the surface temperature does not need to be uniform.
• Same as 1, but Non-Uniform Heat Flux - Thermal scenario two has the same specifications as scenario one, but heat loads vary instead of being uniform. The heat loads are concentrated in several locations under components or under specific areas.
• Same as 1, but Surface Temperature Maximum Varies - Thermal scenario three also has the same specifications as scenario one, but thermal scenario three has specified maximum surface temperatures that vary across the cold plate, usually at the individual components.
• Same as 1, 2, or 3, but Surface Temperature Uniformity Required - With thermal scenario four, the thermal specifications may be the same as with thermal scenarios one, two, or three, but with the additional requirement that the maximum surface temperature must be uniform across the entire cold plate or under specific components. For example, if there are two types of components mounted on the cold plate, each component type may require temperature uniformity of the common components, but the two types may have different maximum surface temperatures.

Cold plate scenarios 2 and 3 are the ones most commonly encountered in custom cold plate design. Scenarios 1 through 4 are listed in order of increasing complexity and cost.

When designing custom cold plates to any specification, the logical steps most thermal experts take are defining the thermal map, generating the liquid circuiting concept, calculating temperature rise and pressure drop, and rerouting the liquid circuit if necessary.

With several possible thermal scenarios, step one in custom cold plate design is to define the thermal map in detail. To create a thermal map, an engineer needs the dimensions, locations, and heat loads of the components to be cooled. The maximum allowable cold plate surface temperature(s); the coolant composition, its flow rate, and inlet temperature; and available pressure drop are needed as well. Also, heat flux must be calculated for each component which includes thermal spreading, if necessary.

The next step is to generate the first iteration on a liquid circuit concept. The liquid circuit must provide the required performance to cool the component with the highest heat flux and each component after it on the liquid circuit. In addition, it must do so with the specified flow rate and with an acceptable pressure drop. Sometimes techniques such as uneven widths of liquid series passes, different fin densities under individual components, and varying fin heights and types can be used to satisfy the competing requirements of performance and pressure drop. The fin's geometry and height determine the "fin efficiency", or how well it transfers heat to the liquid.

Sometimes the shape of high heat flux components (e.g. - a large round footprint) requires a change from the natural uniform flow distribution over the pass width to force non-uniformity, which can be achieved by using different lengths of fin or different fin densities over the pass width. Before the next component, some liquid equalizing pools (i.e. - mixing pools) should be designed in. Another fluid distribution challenge is the need for islands in the fluid path to accommodate component mounting. Any complication mentioned above can increase the cost of the cold plate due to the additional number of fin pieces, multiple depths in a cavity, multiple fin-forming equipment set-ups, and EDM cutting needed.

After the liquid circuit concept is outlined, the thermal map should be verified by calculating the maximum surface temperature under each component and calculating the total pressure drop. All the critical thermal areas must be modeled. If any one of the requirements is not met, the liquid circuits must be reworked and the calculations rerun.

If the cold plate requires a varying maximum surface temperature (as in thermal scenario three) and normal liquid circuiting does not meet the specifications, the liquid circuit should be rerouted to deliver the coolest liquid to critical devices first or to by-pass part of the liquid directly to these components.

If the cold plate requirements specify maximum surface temperatures and temperature uniformity as in thermal scenario four, the design process is even more complex. The simplest solution to provide uniformity of maximum surface temperatures of identical components is to position the components on similar points of similar parallel liquid passages. The result should be a circuit that delivers liquid with a common temperature at sufficient flow rates to these components. Another technique that is used to provide a more uniform surface temperature across the entire cold plate is to use a counterflow arrangement (Figure 2). In a number of parallel channels, on a surface or on both sides of the plate, each second channel has flow in the opposite direction. For a one-side loaded or very thin cold plate, such an approach may significantly reduce surface temperature gradient. A similar effect may be delivered by organizing two separate layers of liquid.

Certain thermal or mechanical requirements may force an illogical pass of the liquid circuit, resulting in greater complexity and a higher cost cold plate. For example, applications frequently have predetermined mounting hole locations that the liquid circuit must navigate around and/or components and fluid inlet and outlet locations that are fixed, significantly limiting the options for the liquid circuit. Generally, the more flexible the design is, the easier the cold plate will be to engineer and the more savings you'll realize. By working closely with a printed circuit board designer or electrical engineer, the thermal engineer can provide input on the spacing and positioning of components to ensure they are designed with electrical as well as thermal requirements in mind. This may significantly simplify the cold plate design and reduce cost. For more information on cold plate costs please see our application note "Cold Plate Manufacturing Cost Drivers".

It's important to understand the various design techniques that allow a custom cold plate solution to meet the most challenging thermal and mechanical requirements. With thousands of permutations for a custom cold plate design, skilled engineering is key. Flexibility with the location of inlets and outlets, proper fluid circuit routing, and the use of fin or channels can help to create a thermal solution that provides the best value for the application. As heat loads become more and more concentrated and the space allocated for cooling becomes smaller and smaller, custom cold plates will be used more and more to meet applications' unique liquid cooling needs. Aavid, Thermal Division of Boyd Corporation has decades of experience designing and manufacturing custom cold plates for printed circuit boards and other electronics and ensuring their high thermal performance requirements and cost limits are met or exceeded.

Learn more about our different Liquid Cold Plate solutions in our Liquid Cold Plate Section.