Heat Sink: Definition, How It Works, Components, Types, and Applications

A heat sink is a device used to transfer heat from a heat source to the surrounding environment. Heat transfer is achieved through a combination of conduction, convection, and to a lesser extent, radiation. A heat sink typically has a base with multiple fins on top that are designed to increase the surface area for heat transfer to the surrounding fluid. They can be passively cooled via natural convection or actively cooled by forced convection. 

This article will describe what a heat sink is, how it works, and the various components that make up a heat sink, as well as some common applications of heat sinks. 

What Is a Heat Sink?

A heat sink is a type of heat exchanger used to transfer heat from a heat-producing device or heat source into a surrounding fluid. This fluid is commonly air, but can also be water or some other non-conductive thermal transfer fluid. Heat sinks can be passively cooled via natural convection, or actively cooled by forced convection, using a fan. Heat sinks are commonly made from copper or aluminum.

What Does a Heat Sink Look Like?

A common type of heat sink is shown in Figure 1 below: 

This one has multiple fins which have been cut from a solid block of copper, an excellent heat transfer material. This is an example of a  passive heat sink since there is no fan attached to it. 

How Does a Heat Sink Work?

A heat sink makes use of the principles of conductive, convective, and radiative heat transfer to move heat from a hotter source to a lower-temperature fluid. Heat is conducted from this source into the sink. Heat sinks are produced from materials with a large heat capacity, i.e. they can store more heat per gram of material. This heat then transfers from the sink into the surrounding fluid via convection and radiation. The heat transfer rate is increased by having a large surface area in contact with the heat exchange fluid. Surface area can be dramatically increased by cutting fins into the heat sink base material. 

A heat sink may be passive or active. An active heat sink makes use of the forced convection created by a fan or pump to rapidly transfer heat from the device, while a passive heat sink makes use of natural convection. 

What Is the Purpose of a Heat Sink?

A heat sink is designed to dissipate waste heat caused by the operation of electrical or mechanical devices. This waste heat can build up and cause failures in a device or reduce its overall performance if it is not moved away from the component. 

What Are the Key Components of a Heat Sink?

A heat sink is a relatively simple device. Listed below are the five components that make up a typical heat sink:

1. Base

A heat sink base is typically a flat block or sheet of material with excellent thermal conductivity. The base typically has a consistent cross-sectional thickness, but it can also be designed to have a cross-sectional profile that optimizes heat transfer for the specific geometry of the heat source. The base is typically mounted to the heat source with mounting hardware and thermal paste. 

2. Fins

Fins protruding from the heat sink base are responsible for the transfer of heat to the surrounding fluid. These fins are designed to optimize the surface area that the heat sink presents to the fluid. The larger the surface area, the faster the heat transfer rate. 

The fins can either form an integral part of the base or can be attached separately using various techniques, for example, via a compression process. The shape and arrangement of the fins can dramatically improve the heat transfer rate.

3. Heat Pipes

A heat pipe is designed to transfer heat along its axis. Heat pipes can be incorporated inside standard heat sinks and heat spreaders through press fitting, soldering, and thermally conductive epoxy to improve their heat transfer efficiency. They work by transferring heat via a phase change mechanism that causes fluid to vaporize at the heat source, then travel along the axis of the heat pipe to the point where it cools down and changes back into a liquid via condensation. A typical example of this is shown in Figure 3 below:

4. Thermal Interface Material

Thermal interface materials, or thermal pastes, are used to significantly improve heat transfer between the heat source and the base of the heat sink by filling any air voids between the heat source and the heat sink. Air is a poor conductor of heat, so filling air gaps with a more thermally conductive material enhances the cooling efficiency of a heat sink. Thermal pastes can be metal, ceramic, or silicone-based, with metal-based thermal paste being the most effective. Figure 4 is an example of a thermal paste:

5. Mounting Hardware

Heat sinks can be securely fixed to their target heat sources using a number of different mounting methods. For smaller heat sinks, an adhesive with high thermal conductivity is used to directly stick the heat sink onto a heat source. This method is typically used on smaller PCB components. For larger heat sinks, normal screws can be used, or alternatively, spring-loaded push pins are used to optimize the contact pressure between the heat source and the heat sink, as shown in Figure 5 below:

What Materials Are Heat Sinks Made Of?

Heat sinks are made from materials with high thermal conductivity. The most common of these are listed below.

1. Aluminum: Aluminum is a lightweight, low-cost material that has good thermal conductivity. It is commonly used in heat sinks for electronic devices, such as computers and LED lights.
2. Copper: Copper has excellent thermal conductivity and can be used on more sensitive components like computer CPUs.
3. Aluminum Alloys: Pure aluminum can be difficult to work with as it is too soft, aluminum alloys like 1050 have increased strength without significantly affecting heat transfer while 6 series alloys are even stronger but sacrifice thermal conductivity. 
4. Graphite: Graphite has conductivity approaching that of copper but is significantly lighter.
5. Diamond: Diamond has significantly better heat conductivity than copper, however its cost makes it impractical in most applications, it is typically used in semiconductor applications. 

What Are the Types of Heat Sinks?

There are three basic types of heat sinks. They are described in more detail below:

1. Passive Heat Sinks

A passive heat sink is the simplest type of heat sink. It is simply a base with fins. Heat is transferred primarily through natural convection. As the air around the fins heats up through conduction, the hot air will rise, which will then cause cooler air to replace the hot air. This is a continuous process. These types of heat sinks are not the most effective. 

2. Hybrid Heat Sinks

A hybrid heat sink makes use of a control system to decide when to employ passive or active behavior. When the heat source produces low levels of heat, the fan or pump is not turned on, because natural convection is sufficient to transfer the required amount of heat away from the heat source. When natural convection is not adequate, the fan is activated, and forced convection helps to increase the amount of heat transferred away from the source.

3. Active Heat Sinks

An active heat sink makes use of forced convection to transfer heat. When a fan or pump causes fluid flow over the heat sink, this constant flow keeps replacing the hot fluid around the heat sink with cooler fluid. The higher the flow rate, the higher the heat transfer rate. Active heat sinks are more effective than passive heat sinks.

What Are the Applications of Heat Sinks?

Heat sinks are used wherever waste heat has the potential for damaging equipment. Some examples are listed below:

1. Computer Processors

Computer processors (CPUs) produce a large amount of waste heat during operation. They often employ copper heat sinks with an active cooling fan. Cool CPUs can operate more effectively.

2. LED Lighting

LED lights don’t produce heat in the same way an incandescent bulb does. However, the electronics used to make an LED work produce a lot of waste heat that must be transferred away. Small LEDs often use passive heat sinks. 

3. Power Electronics

Power supplies convert AC power to DC power for consumer electronics. This conversion process is inefficient and produces some waste heat that can reduce the life of the power supply unit. Heat sinks on power electronics sometimes employ hybrid cooling and make use of aluminum heat sinks to reduce cost. 

4. Automotive Industry

Apart from the heat sinks used on the control circuitry of vehicles, heat sinks are also used to keep electric motors cool during operation as well as to cool onboard chargers for electric vehicles. 

5. Aerospace Industry

Heat sinks can be found on control circuitry used in aerospace applications. They are also used on spacecraft to transfer heat to the vacuum of space. However, these heat sinks transfer heat purely through radiation as there is no heat transfer fluid in space. 

6. Consumer Electronics

Consumer electronics make extensive use of heat sinks to keep devices cool and operating efficiently. Typical examples include the heat sinks in computers and cell phones.

What Devices in a Computer Use Heat Sinks?

A large proportion of the devices within a computer make use of heat sinks. For example, the CPU (central processing unit), GPU (graphical processing unit), RAM (random access memory), and PSU (power supply unit) all make use of heat sinks to improve their efficiency and operating life. Individual SMDs (surface-mounted devices) on the motherboard also make use of small heat sinks to keep them cool.  

How Do I Choose the Right Heat Sink for My Application?

In order to select the correct heat sink for your application, it is important to understand how much heat your device will be producing, as well as the environment in which it will operate. Once these are known, the heat sink can be designed by calculating the heat transfer rate required to keep your device at the optimal temperature and then designing a heat sink configuration to achieve these temperatures. 

What Are the Benefits of Heat Sinks?

Listed below are some common benefits of using heat sinks:

1. Improved Reliability: Heat sinks help maintain a consistent operating temperature, which helps improve the reliability of a device.
2. Extended Lifespan: Heat sinks remove waste heat from a device that would otherwise reduce its lifespan.
3. Improved Performance: Devices like CPUs, for example, operate most effectively when cool. An effective heat sink can improve the performance of a device. 
4. Reduced Noise: If a passive heat sink can be used, then a cooling fan may not be necessary. This will ultimately reduce the noise of the device. 
5. Cost Savings: A heat sink allows the use of cheaper components to do the same job, resulting in an overall lower production cost and price to consumers.

What Are the Limitations of Heat Sinks?

Listed below are some common limitations of using heat sinks:

1. Limited Cooling Capacity: Because of space and material limitations, a heat sink can only remove heat at a certain rate. Trying to make heat sinks that will remove even more heat even faster becomes impractical and uneconomical.
2. Space Constraints: In some cases, the space needed for a suitable heat sink can be larger than the space available.
3. Maintenance Requirements: Heat sinks will often accumulate dust, especially inside a PC. These must be regularly cleaned to prevent a drop in the heat transfer rate. 
4. Noise: Active heat sinks may require a noisy fan in order to operate effectively. Even silent fans will add a degree of noise.
5. Cost: Copper heat sinks are effective but expensive and may not be economically feasible for the device. Cheaper materials like aluminum are available but might not have the same performance as copper.

What Are the Factors That Affect a Heat Sink’s Performance?

A heat sink's performance can depend on a number of factors, as explained below:

1. Thermal Conductivity: The thermal conductivity of the heat sink material is one of the most important factors affecting performance. Materials with higher thermal conductivity, such as copper or diamond, can transfer heat away from the electronic component more efficiently.
2. Fin Design: More fins generally mean a larger surface area for heat transfer, and thus, improved performance.
3. Airflow: Heat is removed from the heat sink by the action of natural or forced convection. The higher the rate of airflow around the fins of the heat sink, the higher the rate of heat transfer. 
4. Thermal Resistance: Resistance to heat transfer at the interface between a heat source and its heat sink can be caused by the existence of air gaps between components. The use of a thermal paste to bridge these gaps can significantly improve heat transfer rate from source to sink.
5. Ambient Temperature: A higher ambient temperature will result in a smaller temperature gradient between the heat source and the surrounding fluid. This will reduce the performance of the heat sink.

Do Heat Sinks Cause Heat Creep in 3D Printing?

No, a heat sink is designed to eliminate heat creep in 3D printing. It transfers heat from the hot end to the surrounding air and prevents it from creeping up the extruder assembly. 

What Is the Difference Between a Heat Sink and a Heat Spreader?

A heat sink consists of a base with fins and is sometimes cooled with forced convection or natural convection, whereas a heat spreader is a large block of material that is designed to connect to the device housing or a heat pipe for example, then transfer the heat via conduction to a larger metal surface or secondary heat exchanger.

Summary

This article presented heat sinks, explained what they are, and discussed how they work. To learn more about heat sinks, contact a Xometry representative.

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