With the exception of thermal storage heat sinks, the term heat sink is a misnomer. Standard heat sinks for electronics cooling are actually heat exchangers, taking the heat from the electronics, and transferring it to a fluid, either air or a coolant. Phase Change Material (PCM) heat sinks are the only heat sink that actually acts as a (temporary) sink for heat. If you have been wondering, “How do thermal storage (PCM) heat sinks work?” you first need to understand the different types of phase change materials and how they fit into the PCM heat sink process. What follows is a quick introduction to everything you need to know about phase change material heat sinks. More detail can be found on ACT’s Thermal Storage Resource Page.
What is PCM?
A Phase Change Material (PCM) is a material that absorbs a large amount of energy when undergoing a phase change. These materials have a high latent heat of fusion, which means they will absorb a large amount of energy while maintaining a constant temperature. Typically PCM refers to materials using the solid-liquid phase change to absorb energy.
What is a PCM Heat Sink?
How does PCM fit in with heat sink technology? A PCM heat sink put simply is a hermetically enclosed fin stack with PCM filling the void space. Most PCMs have very low thermal conductivities and require features to enhance heat transport to effectively utilize all of the latent heat potential. This can be achieved easily by incorporating fins or even heat pipes into the PCM to better distribute the heat and evenly melt or freeze the PCM.
How a PCM Heat Sink Works
As heat is applied to the heat sink, the temperature rise is dictated by the sensible heat (specific heat) of the system, until the PCM starts to melt. The temperature stays nearly constant while the PCM is melting. Once all of the PCM material is melted, the system temperature will begin rising again until the heat source is turned off.
What are common applications for PCM Heat Sinks?
Typical applications include:
- Cooling electronics with a known duty cycle
- Dampening of heat loads in pulse mode operation
- Protection from momentary failure
- Storage for one-time use applications
- Thermal energy storage for renewable energy applications
If you are not sure whether a PCM heat sink is appropriate for your situation, please contact Advanced Cooling Technologies right away. One of our cooling technology experts will help you determine the solution you need.
What are the two main reasons to use advanced PCM thermal solutions?
- Smoothing out the thermal energy during pulsed or cyclic operation, allowing the heat removal system to be designed for the average heat load rather than the peak load
- Short-Term Thermal Storage, where a suitable heat sink is not available
- Protection from Failure During Coolant Interruptions, when the cooling system is temporarily unavailable
- Thermal Storage to increase cooling capacity during hot days, using the colder air at night to recharge the thermal storage
Smoothing Out Pulsed/Cyclic Operation
Several applications have pulsed heating, with a period of high heating, followed by a longer period of lower heating. PCM thermal storage devices can provide thermal storage for repeated duty cycle components, allowing the heat rejection system to be sized for the average rather than the peak heating. ACT has demonstrated the benefits of load-leveling in systems that range from individual transistors up to large heat exchangers for Directed Energy Weapon (DEW) systems.
The figure at Left shows how a PCM heat exchanger can smooth out the heat removal rate, leveling the load for a vapor compression system. The PCM melts and absorbs heat during the power-on condition. It then gradually freezes over the rest of the cycle, so that heat can be dissipated over the full duty cycle. By dampening the heat load, the PCM heat exchanger allows the ultimate heat rejection system to be sized for much lower heat loads, as indicated. For applications with extreme heat loads, such as directed energy weapons, PCM can drastically reduce the overall size of the required heat sink. The basic technology is applicable in every type of cooling system:
- Vapor Compression Systems – Reduce condenser/compressor size
- Air cooled system – Reduce fin volume
- Liquid cooled systems – Reduce pump size
- Space Systems – Reduce Radiator Area
ACT is experienced with full thermal system design and component-level PCM packaging challenges.
Short Term Thermal Storage
The heat storage capacity of PCM is advantageous in many applications including:
- Short duration applications without a suitable heat sink such as missile electronics thermal management
- Large temperature swings on electronics leading to thermal fatigue solder joints such as in satellite payload electronics
- Maintaining critical component temperatures during cooling interruption such as power outages or during transit
- Space and system weight limits do not allow for a bulky thermal solution
PCM heat sinks can be designed for:
- Long Term Storage Before Use
- Large Accelerations
- Moderate Loading Stresses
- Ambient Pressure Changes
PCM heat sinks are exceptionally compact, lightweight, and offer increased reliability due to their passive operation. This is a significant advantage compared to traditional active steady-state solutions. With ACT’s expertise creating capable conduction paths to fully melt the PCM with minimal temperature rise, a solution can be realized for challenging, high heat, one-time use applications. Unlike pulsed operation, one-time use applications use the PCM as the final heat sink, absorbing the heat load during full operation.
What are the Benefits of a PCM Heat Sink?
There are some universal benefits of a PCM heat sink across almost all applications. First, PCM heat sinks are passive, with no moving parts to maintain, and no power required to operate. Second, they offer long life with no maintenance. The vacuum seal prevents liquid losses, and protective coatings can give each device a long-lasting guard against corrosion. A PCM heat sink can also lower costs by lowering the operating temperature, these devices can increase the Mean Time Between Failure (MTBF) for electronic assemblies. In turn, this lowers the maintenance required, and the replacement costs.
Common Questions on PCM Heat Sink Design
Now that you have the basics, we’re sure you have more complex questions. While some answers are specific to your needs and system requirements, these responses to standard questions will give you a better understanding as to how these devices operate.
What is the Appropriate Mass and Volume of my PCM Heat Sink?
ACT has developed a PCM heat sink calculator that provides approximate volume, weight, and temperature profile as a function of time for temperatures ranging from -10°C to 85°C.
The Calculator only uses organic PCMs. Please contact ACT if you have an application using hydrated salt or metallic PCMs, or one that is outside of this temperature range.
What is the Operating Temperature Range?
The operating temperature range depends on the application. Many paraffins are appropriate for electronics applications where the temperature is typically maintained below 100°C; see Table 1 below. For higher temperature applications, such as energy storage for power plants, salts and metals can be used.
What is the Storage Temperature Range?
The storage temperature of a PCM heat sink will depend on the type of PCM selected. Most PCM heat sinks can be stored at any temperature below its maximum operating temperature. Because most PCMs become less dense, expanding, as their temperature increases, PCM heat sinks are filled in an environment above their maximum operating point. If the heat sink were heated beyond its fill temperature it could become over pressurized and deform or open up a leak path. If a PCM heat sink is designed to keep a source below 90°C, it should not be stored in an environment above 90°C. (Note that in this case, 90°C is the maximum allowable design temperature; it is always above the melting point of the PCM).
Will the PCM Heat Sink Operate in Any Orientation?
A PCM heat sink will operate in any orientation, however, in applications that require tight temperature control, the orientation should be considered to minimize temperature variations.
What Constrains the Maximum PCM Heat Sink Size?
The PCM heat sink size will be limited by the ability to effectively move the heat into the PCM. If the volume of PCM required is large and/or the heat flux is very large, the manufacturability of the internal fin structure will restrict the PCM heat sink size and storage capabilities. In certain cases heat pipes can be used to spread the heat to multiple PCM modules.
How is the low thermal conductivity of PCM overcome?
In most cases fins are used to overcome the low thermal conductivity of PCM. Their increased surface area and high conductivity allows the heat to penetrate the PCM, resulting in an even melt front and lower temperature gradient within the PCM. Heat pipes can also be used with or without fins to better distribute heat to the PCM in high heat flux applications.
What Materials are Used for the Envelope and Fins?
A variety of materials can be used for the envelope and fins. Aluminum is the most common material used for PCM heat sinks, but ACT has also worked with copper, steel, and magnesium.
When Should I use Paraffin Wax, Hydrated Salts, or Metallic Phase Change Materials?
There are several factors that need to be considered when selecting a phase change material. An ideal PCM will have high heat of fusion, high thermal conductivity, high specific heat and density, long-term reliability during repeated cycling, and dependable freezing behavior. As shown in Table 1 below, PCM options include paraffin wax, non-paraffin organics, hydrated salts, and metals; green text indicates attractive features of these PCM types, while red indicates challenges. You can also reference our PCM Heat Sink Calculator to find the right material for your application.
- Paraffin wax is ideal for most electronics applications
- There is a large selection of pure hydrocarbon materials and paraffin blends available, with melting temperatures every few degrees, ranging from roughly -20 to 100°C.
- They have a high heat of fusion per unit weight, provide dependable cycling, are non-corrosive, and are chemically inert. meaning they are chemically compatible with most metals.
- Typical Paraffin Wax PCMs used in Electronics Applications
Examples of Paraffins C36H74 C32H66 C30H62 Densitysolid (kg/m3) 857 809 810 Latent Heat (kJ/kg) 223 261 249 Tmelt (°C) 72 to 76 66 to 70 59 to 66
Temperature ranges of 72-76°C, 66-70°C and 59-66°C are good to ensure safe operation of many electronic devices. Many other PCM choices are also available, with melting points differing by a few degrees from -9°C to 90°C.
Key design considerations according to our in-house PCM experts when designing with paraffin PCM
- Void management is important due to the volume change from solid to liquid.
- Designing sufficient conduction paths is critical as paraffin PCM’s have a low thermal conductivity
- Hydrated salts are generally used for larger energy storage applications since they are much cheaper
- They have a high heat of fusion per unit weight and volume, have a relatively high thermal conductivity for non-metals, and show small volume changes between solid and liquid phases.
- These are not commonly used for electronics heat sinks, since they are corrosive and long-term reliability (thousands of cycles) is uncertain. The most common application is for very large thermal storage applications (e.g., solar heating), where much lower cost is very attractive.
- Key design note: using hydrated salts must account for both corrosion and the limited number of cycles available.
- Metallic PCMs are generally used for high-temperature applications
- They are generally used where no suitable paraffin wax is available, due to the high temperatures.
- Other PCM materials such as non-paraffin organics, and liquid-to-gas phase change materials are available but are not often used for electronics heat sinks.
Table 1. Properties of Typical Phase Change Materials
Property or Characteristic
|Paraffin Wax||Non-Paraffin Organics||Hydrated Salts||Metallics|
Heat of Fusion
|Very Low||Low||High||Very High|
Melt Temperature (°C)
|-20 to 100+||5 to 120+||0 to 100+||150 to 800+|
Latent Heat (kJ/kg)
|200 to 280||90 to 250||60 to 300||25 to 100|
|$$||$$$ to $$$$||$||$$ to $$$|
|Stable||Elevated Temperature Can Cause Decomposition||Unstable over Repeated Cycles||Stable|
How does ACT use a Differential Scanning Calorimeter (DSC) to measure the properties of the PCM material?
Differential Scanning Calorimetry (DSC) is a thermoanalytical instrument in which the difference for heat required to increase temperatures of a sample and a reference is measured as a function of temperature, which is usually programmed to change linearly as a function of time. It is frequently used to measure the latent heat of Phase Change Materials (PCM). In operation, an empty pair of aluminum or copper pan and lid is used as the reference and the sample is placed in another pair of pan and lid with the same material and weight as the reference. The result of a DSC experiment is a curve of heat flux versus temperature or time, and the integration of phase transition peak area on the curve indicates the latent heat of the sample. ACT owns and operates a DSC-600 with the following specifications.
I Have a Question Not Answered Here
If you still have questions about phase change materials, heat sinks or any of the other issues that have been covered here, Advanced Cooling Technologies is happy to help. While some of these concepts are complex, we have experienced professionals standing by that will be able to answer any questions or clarify any issues you may have. Customer service is our top priority, so please don’t hesitate to contact us with your questions today!
Get Your Quote Today
Now that you’ve learned what a PCM heat sink is and how they are used, it’s time to contact ACT for more information and a quote for including a PCM heat sink with your equipment. We will help you decide how best to meet your needs with equipment including:
- Thermal management
- PCM Heat Sinks
- Heat pipe assemblies
- HiK™ plates
- Vapor chamber assemblies
- Single and Two Phase Pumped Loop Cooling
- Cold plates
- And much, much more.
We’ll provide you with everything you need to understand cost and installation of PCM heat sinks, as well as related, single-use options.
Reduce your costs and improve the life of your equipment with a simple conversation designed to make your operations easier and affordable. Contact ACT for more information about how we can help meet all your thermal management and energy storage needs.