Understanding Heatsinks for Electronic Equipment

All electronic components and equipment generate heat which leads to temperature rise - heatsinks are the main way to remove excess heat and keep temperatures down.

>Heatsinks are used in many pieces of electronics equipment to ensure that heat can be removed from pieces of equipment or particular components within them.

For example with microprocessors running very hot these days, they normally have heatsinks clamped or attached to them

In addition to this fans are also included in PCs to ensure that the components stay cool and operate within their operating temperature ranges.

It is not only PCs that contain heat sinks. Many other pieces of equipment contain them. In fact heatsinks are used wherever there are sources of heat, and this heat needs to be removed.

Heatsinks can take a variety of forms, and they are widely available for electronics applications. If only a small amount of heat needs to be removed, small or simple heatsinks can be used. However if significant amounts of heat need to be removed, then more complicated heat sinks are needed. Using some relatively simple thermal calculations, it is possible to determine which heatsinks may be applicable for a given application. They are specified in a way that makes it possible to determine their performance.

Basics of Heat Transfer

Before delving into the specifics of heatsinks, it's essential to understand the fundamental principles of heat transfer. Heat transfer occurs through three primary mechanisms:

Conduction: Heat transfer through direct contact between two materials.
Convection: Heat transfer through the movement of fluids, such as air or water.
Radiation: Heat transfer through electromagnetic waves.

In the context of electronic devices, heat is generated by the components themselves, primarily due to resistive losses.

To maintain optimal performance and reliability, this heat must be efficiently dissipated.

Key Factors Affecting Heatsink Performance

The performance of a particular heatsink depends upon a variety of factors. In some areas there may be a trade-off between actual performance and other factors such as size and weight.

The main factors affecting performance are summarised:

Thermal Resistance: This measures the ability of a heatsink to conduct heat away from a heat source. A lower thermal resistance indicates better heat dissipation.
Surface Area: A larger surface area allows for more efficient heat transfer.
Material: The thermal conductivity of the heatsink material is crucial. Materials like aluminum and copper are commonly used due to their high thermal conductivity.
Fin Design: The shape and spacing of the fins influence the airflow and heat dissipation.
Mounting Method: Proper mounting is essential to ensure good thermal contact between the heat source and the heatsink.

Types of heatsinks and their specifications

Heatsinks come in a variety of sizes and shapes. Their purpose is to remove heat from the source where it is generated as efficiently as possible.

To achieve this there are several styles and techniques that are used.

Passive Heatsinks:
- Extruded Heatsinks: These are manufactured by extruding aluminum or copper into various shapes, such as fins or plates. They are cost-effective and widely used in a variety of applications.
- Skived Heatsinks: These are made by cutting thin sheets of metal into specific shapes and stacking them together. They offer high thermal performance but can be more expensive.
- Bonded Fin Heatsinks: These consist of a base plate with fins bonded to it. They offer high thermal performance and can be customized to specific requirements.
Active Heatsinks:
- Heatsinks with Fans: Fans are used to force air across the fins of the heatsink, increasing the rate of heat dissipation.
- Liquid Cooling Systems: Liquid cooling systems use a liquid, such as water or a specialized coolant, to transfer heat away from the heat source. They are often used in high-power applications where air cooling is insufficient.

Normally a heat sink is specified in terms of its dissipation for a given temperature rise, i.e. its thermal resistance. The heat dissipation is given as a number of watts, and the temperature rise in terms of degrees Celsius. Thus a particular heat sink may have a rating of 10 watts per degree Celsius. This means that if it dissipates 10 watts of power then its temperature will rise 1 degree. Similarly if it dissipates 100 watts, its temperature will rise 10 degrees.

These figures are essential for any thermal calculations that are needed. By knowing the performance of the heatsink, the correct one can be chosen for the required heat dissipation, and allowable temperature rise.

Designing Effective Heatsink Solutions

When designing a circuit that dissipated a significant amount of heat - sufficient to require a heatsink, it is necessary to consider several factors to ensure that the heatsink matches the requirement and can dissipate sufficient heat.

Heat Dissipation Requirements: Determine the amount of heat that needs to be dissipated by the electronic component.
Thermal Resistance Budget: Allocate a specific thermal resistance budget for each component in the system.
Heatsink Selection: Choose a heatsink with a suitable thermal resistance and mechanical compatibility.
Mounting Method: Select an appropriate mounting method to ensure good thermal contact between the heatsink and the heat source.
Thermal Interface Material (TIM): Use a high-quality TIM to fill any gaps between the heat source and the heatsink, improving thermal conductivity.
Airflow Considerations: Ensure adequate airflow around the heatsink to facilitate efficient heat dissipation.
Environmental Factors: Consider the ambient temperature and humidity, as these factors can affect the performance of the heatsink.

Simple thermal calculations

It is possible to undertake some simple thermal calculations to determine the required performance for a heatsink. While some thermal calculations can become very involved, the thermal calculations needed for choosing heat sinks is very easy and quite straightforward.

The first step in any thermal calculation is to determine the amount of power being dissipated. This is simply done using one of the three equations below:

P = V I = \frac{V^{2}}{R} = I^{2} R

With the power dissipation of the component calculated, the next stage in the thermal calculations can be undertaken. This is to calculate the required thermal resistance of the heatsink.

The thermal resistance of the heatsink = (Max temperature rise / watts dissipated) - (junction to case resistance + case to heatsink resistance)

The above equation includes terms used for semiconductor devices, the items that most commonly need heatsinks. Often it is not possible influence of receive data for the junction to case resistance, or for the case to heatsink resistance. Thus it is safest to leave a small margin to cover these. This means that by leaving a margin the equation simplifies to:

Thermal resistance of the heatsink = Max temperature rise / watts dissipated

Heatsinks are an essential element of many electronics designs. For components that dissipate large amounts of heat, a heatsink is an essential requirement, and often fans may also be needed to assist in the cooling.

The thermal calculations needed to select he require heatsink can be quite straightforward, although some thermal calculations to determine more complex thermal issues can be very involved. Fortunately these are rarely needed for home designs.

By understanding the principles of heat transfer and the various types of heatsinks available, engineers can design effective thermal management solutions that meet the specific needs of their applications.

Written by Ian Poole .
Experienced electronics engineer and author.