Many companies we work with prefer to design their own cold plates and have another company do the DFMA review and the manufacturing. ATS’ extensive capability for manufacturing cold plates is demonstrated in the wide variety of tubed cold plates it produces to meet each customer’s specific requirements. Copper, stainless steel, aluminum, 4 or 48 passes, ATS has the manufacturing capability to make nearly any tubed cold plate. Different examples of these custom cold plates, made in the USA, are described on the ATS website at the link: https://www.qats.com/Products/Liquid-Cooling/Custom-Cold-Plates
By Norman Quesnel, Senior Member of Marketing Staff Advanced Thermal Solutions, Inc. (ATS)
Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace spaceconsuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]
One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”
Figure 1. A Liquid Cooling System for a Desktop PC with Two Cold Plates. [2]
For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]
Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.
Basic Cooling Choices
While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.
In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.
Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.
Table 1: Compatibility Match-ups of Common Cold Plate Metals and Cooling Fluids [1]
The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.
Tables 2 and 3. Comparisons of Properties of Typical Electronic Coolants. [4]
An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]
The most commonly used coolants for liquid cooling applications today are:
Water
Deionized Water
Inhibited Glycol and Water Solutions
Dielectric Fluids
Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance. If you determine that your facility water or tap water contains a larger percentage of minerals, salts, or other impurities, you can either filter the water or you can opt to purchase filtered or deionized water. [5,6]
Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]
Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]
Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]
Conclusion
Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.
It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.
Many of today’s electronic devices need the performance of liquid cooling to meet the thermal demands of certain hot components. Liquid cold plates are common cooling systems in high power lasers, fuel cells, battery coolers, motor drives, medical equipment, avionics and other high-power, high-heat flux applications.
Figure 1. A Custom liquid cold plate design by D6 Industries. [1]
Cold plates provide localized cooling by transferring heat from a device to a liquid that flows to a remote heat exchanger and dissipates into either the ambient or to another liquid in a secondary cooling system. Component heat flows by conduction through a thermal interface material and the metal plate to the metal tubing. Then it flows by convection from the internal surface of the fluid path material into the flowing coolant.
A cold plate in electronics cooling is
often an aluminum block with an embedded, coolant-filled metal tube. Another
common cold plate type is made with metal shells that are brazed or friction-welded
together and filled with a liquid coolant. On the inside, the metal shells have integral
cooling fins that are submerged in the coolant.
Tubed
Cold Plates
Embedded tube designs are the simplest
version of cold plate cooling devices. They feature a continuous tube set into
grooves in a metal plate, and are often bonded in place with thermal epoxy. The
flowing coolant moves heat from the component away from the cold plate to a
heat exchanger, where it is cooled before being pumped back into the
plate.
A common example of a tubed cold plate
features an aluminum plate with an exposed copper tube. The tubes can be routed
in different pathways to optimize the thermal performance.
The tubing can be continuous or constructed from straight tubes connected by soldered joints, though joints may increase the potential for leakage.
Figure 2. A Tubed cold plate consists of copper or stainless-steel tubing pressed into a metal plate. [2]
This design can provide a
cost-effective thermal solution for component cooling where the heat load is
low-to-moderate. Tubed cold plates
ensure minimum thermal resistance between the power device and the cold plate
by placing the coolant tube in direct contact with the power device’s base.
Direct contact reduces the number of thermal interfaces between device and
fluid, thus increasing performance for the application.
A variant of this design features a thermal epoxy completely applied over the pressed in tubing and flush with the metal plate surface. These are sometimes called buried tube liquid cold plates. This provides a gap-free thermal interface between the tube and the plate. The epoxy layer protects from any leakage from the metal tube. Another key feature is that that fully buried tube is not exposed to the outside environment.
Figure 3. A buried tube cold plate’s metal tube is covered with a conductive epoxy layer. [3]
The choice of liquid coolant affects thermal performance as well. Choosing the right coolant depends to a great extent on the tube material. Copper tubes are compatible with water and most other common coolants, while stainless steel tubes can be used with deionized water or corrosive fluids.
One cold plate OEM offers a proprietary
technology with a tube locking system and pressing techniques that ensure the
tube is flush with the plate surface, providing good thermal contact with the
component being cooled. This manufacturing method eliminates the need for
thermal epoxy between the tube and plate which improves thermal performance. [4]
Submerged
Fin Cold Plates
Another type of cold plate is an all-metal construction with brazed or friction welded internal fin field.
Figure 4. Standard, liquid coolant-containing metal cold plate [5]
The integral, internal fins increase
the surface area that contacts the fluid and enhances heat transfer. Fin shape
and fin density affect the performance of heat exchangers and cold plates. By
their geometry, the fins also create turbulence, which minimizes the fluid
boundary layer and further reduces thermal resistance.
One high-performance version features tightly packed aluminum pin fins that create turbulence with low flow rate values, resulting in high thermal performance with low pressure drop. In this design, the high density of the internal fins increases the heat transfer area without adding bulk to the cold plate assembly. [6]
Figure 5. Close-spaced pin fins with complex geometry create turbulence with low flow rate values inside submerged fin cold plates. [6]
In most high-performance applications, fins are made of copper or aluminum. Aluminum fins are preferred in aircraft electronic liquid cooling applications due to their lighter weight. Copper fins are mostly used in applications where weight is not an important factor, but compatibility with other cooling loop materials is.
For submerged-fin cold plates, many different fin geometries can be tested to find the best improvement in performance. Some of the most commonly used are louvered, lanced offset, straight, and wavy fins.
Figure 6. Fin designs for submerged-fin cold plates. Clockwise from top: louvered, lanced offset, wavy, and straight fins. [7]
With cooling requirements increasing
in many areas of electronics, engineers are turning to liquid cooling to
replace air cooling. Lower cost, safer liquid cooling systems have also spurred
the trend to liquid cooling.
The prime example is the cold plate – relatively simple in design, affordable, available in alternative versions, and extremely customizable. Cold plates should be considered wherever thermal performance above air cooling is needed.
Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. This month’s webinar will be held on Thursday, Sept. 27 from 2-3 p.m. ET and will cover the design and deployment of liquid cold plates in electronics systems. Learn more and register at https://qats.com/Training/Webinars.
ATS cold plates were displayed at the Richardson RFPD booth with Vincotech’s new mid-power VINcoPACK E3, which is a low-profile package for motion control and UPS applications that features a six-pack configuration. (Richardson RFPD)
The showcased solution demonstrated how a high-powered device easily connects with the mounting patterns manufactured on ATS cold plates to meet industry-standard insulated-gate bipolar transistors (IGBT), such as those from Mitsubishi, Vincotech, which was on display at PCIM (pictured above), and other leaders in the power electronics industry.
The flexibility in the ATS design allows for cooling of high-powered devices, such as those made from silicon carbide (SiC) or gallium nitride (GaN), without the need for associated tooling costs, which are commonly found when changing the mounting pattern of liquid cold plates.
The cold plates have an innovative, high aspect ratio fin field that provides 30% better thermal performance than comparable products on the market and are manufactured to be easily customizable for systems with specific thermal or space requirements.
ATS cold plates are the perfect choice for engineers looking for liquid cooling solutions for high-powered systems.
Richardson RFPD has a rich history of providing engineering solutions and distributing components for the global electronics market, with more than 35 locations around the world and specialized knowledge in power electronics. Richardson RFPD is the leader in helping customers design-in the latest products and most innovative technology from the market leaders on its line card.
PCIM Europe 2018, one of the largest power electronics shows on the continent, featured more than 11,000 visitors, more than 500 exhibition, and more than 800 conference attendees. Having ATS cold plates on display, thanks to the relationship with Richardson RFPD, gave ATS a host of potential new customers for its liquid cooling and power electronics cooling solutions.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
