Electronics must be maintained below a specific temperature for mission success. Within the thermal design engineer’s toolbox are passive technologies that are often considered for thermal management of electronic devices and systems.
Passive Thermal Management Solutions for Electronics Cooling
Conduction cooling is the standard cooling method used for less challenging and complex cooling applications. It is often used in certain high-end applications where passive two-phase cooling is not suitable. Conduction cooling methods move heat from electronics to a cooler area over a short distance where it can be removed by liquid or forced air cooling for thermal management.
Thermal Management Technology Options for Conduction Cooling
- Baseline Aluminum Conduction Cooling: Aluminum plates are both the lowest-performing and the cheapest solution for thermal management.
- Encapsulated Conduction Cooling: Used for cooling discrete components by moving heat off the chip to a remote heat sink and therefore managing the thermal load.
Thermal Management Technology Options for Two-Phase Heat Transfer
- Spot Cooling Heat Pipes can provide low thermal resistance paths between critical electronic components and heat sinks.
- HiK™ or high conductivity plates with embedded heat pipes can provide highly effective heat spreading or heat transport at a low mass and price point.
- Vapor chambers can act as flux transformers, spreading the thermal load out in two dimensions. They are used when temperature uniformity is necessary.
The table below summarizes each of the thermal management technologies shown above, evaluating effective densities, spreading, effective thermal conductivity, maximum heat flux, minimum thickness, maximum height, and relative cost. Use this table to determine which cooling method is best suited for your project specifications or contact ACT for help in determining which solution will be best. In general, two-phase heat transfer devices weigh more than aluminum or encapsulated conduction cards, but have higher effective thermal conductivity. As a result, they have higher specific thermal conductivity.
Technology Comparison of Conduction Cooling and Two-Phase Cooling Methods
Table 1. Comparison between Baseline Conduction Cooling, Spot Heat Pipes, HiK™ Plates, Vapor Chambers, and Encapsulated Conduction Cooling.
- Maximum Heat Flux for Heat Pipes, HiK™ Plates, and Vapor Chambers is set by the wick design
- Maximum Heat Flux for Aluminum and Encapsulated Conduction Plates is set by the maximum allowable chip temperature and is lower compared to heat pipe solutions
Conduction Cooling and Two-Phase Cooling Selection Criteria Summary
What is a Baseline Aluminum Plate?
It is simply a piece of aluminum used for thermal management in electronics. They are both the lowest-performing and the cheapest solution for thermal management.
How are Baseline Aluminum Plates different from HiK™ Plates?
Baseline aluminum plates are just that, metal plates. ACT’s HiK™ Plates are typically aluminum plates with heat pipes embedded for increased thermal conductivity.
When to use Baseline Aluminum Plates
- Your system requires low-power
- Low-density requirements (lower when compared to two-phase systems)
More on Baseline Aluminum Plates
What is a Heat Pipe?
A heat pipe is a sealed vessel that is evacuated and backfilled with a working fluid, typically in a small amount. The pipe uses a combination of evaporation and condensation of this working fluid to transfer heat in an extremely efficient way. Learn more here.
What is Spot Cooling?
This term describes how a heat pipe is used. Heat Pipes are used for moving heat from point A to point B, spreading, and isothermalization. Spot cooling refers to cooling discrete components by moving heat off the chip to a remote heat sink. Copper/water heat pipes are made of copper and typically operate in the temperature range of 20 to 150°C. They are cost-effective, their effective thermal conductivity ranges from 10,000 to 100,000 W/m-K, and the heat transfer is one-dimensional.
When to use Spot Cooling for heat transfer and thermal management?
- You need a temperature decrease in order to increase the maximum power output
- To cool individual components by transferring heat to an external sink
- To route heat around other components, heat pipes are able to be bent and manipulated
Figure 1. Spot cooling heat pipes cool discrete components by moving heat off the chip to a remote heat sink.
What is a HiK™ Plate?
HiK™ Plates are (commonly aluminum) plates with strategically embedded heat pipes used to increase thermal conductivity. Their effective thermal conductivity ranges from 600 to 1200 W/m K. The heat transfer is 1.5-D since there is preferential thermal spreading along the heat pipes axes. They are relatively inexpensive, and there are plate materials that allow for the direct bonding of electronics.
How is a HiK™ Plate different from a baseline aluminum plate?
ACT’s patented HiK™ Plates are typically aluminum metal plates with heat pipes strategically embedded for maximum heat transfer.
When do I use a HiK™ Plate?
- When you need enhanced thermal conduction to cold rails
- When more efficient or smaller forced and natural convection heat sinks are needed over aluminum plates
- To enhance card guides and metal chassis assemblies
Figure 2. HiK™ plates provide an inexpensive way to increase the effective thermal conductivity of baseline aluminum plates by two to four times.
What is a Vapor Chamber?
Vapor chamber heat spreaders are planar heat pipes that spread heat from concentrated heat source(s) to a large-area heat sink with effective thermal conductivities greatly exceeding those of copper. Vapor Chambers are generally used for high heat flux applications or when genuine two-dimensional spreading is required. The effective thermal conductivity ranges from 5,000 to 100,000 W/m K, with two-dimensional spreading. Vapor chambers can be designed for High Heat Flux (up to 750 W/cm2) capability, are isothermal to within 1-2°C or less, and allow for direct bonding. Learn more here.
When to use a Vapor Chamber
- When genuine two-dimensional heat spreading is required
- You need to cool a single microprocessor or multiple processors in a single plane
- High heat flux applications
More on Vapor Chamber Assemblies
Figure 3. Vapor Chambers spread heat in two dimensions, with very high effective conductivity.
What is Encapsulated Conduction Cooling?
Encapsulated Conduction Cooling is using a highly conductive, fragile material by encapsulating it, typically with aluminum. They have an effective thermal conductivity of around 550 W/m-K, with two-dimensional spreading. In some encapsulated conduction plates, the effective thermal conductivity can decrease with thermal cycling.
How is Encapsulated Conduction Cooling different from Conduction Cooling?
Encapsulated conduction cooling is much more expensive compared to conduction cooling. It has longer lead times and a lower effective thermal conductivity than HiK™ plates or Vapor Chamber solutions. Because of this, it is normally chosen when the passive two-phase devices are not acceptable due to intrinsic limitations.
When to use Encapsulated Conduction Cooling
- Sustained high accelerations, when a two-phase device (heat pipe) can’t be oriented favorably
- When heat must be transported below 25ºC
- When thin sections are required (the minimum thickness for encapsulated conduction cooling is 1.5 mm (0.060 in.), while the minimum thickness for a HiK™ plate is 1.83 mm (0.072 in.).
- Over distances longer than 50 cm (20 inches) vertically
Figure 4. Temperature profiles on plates with identical dimensions, calculated with a 50 W heat source at the top, and forced air convection cooling on the entire back surface of the plates. The maximum temperature of the aluminum plate was 55°C.