In this article, we will discuss the applications that provide more effective thermal management in electronic circuit boards and common methods used to remove excess heat from electronic circuit boards.
Most electronic components emit heat when a current flows through them. This amount of heat depends on the power, device characteristics and circuit design. In addition to the components, the resistance of the electrical connections, copper traces and screws also contribute to heat and power losses.
To avoid malfunctions or circuit failures, the designers should focus on manufacturing electronic circuit boards that work perfectly and stay within safe temperature limits. Some circuits can work without requiring additional cooling, but in some cases, it may be inevitable to add coolers, cooling fans, thermal pastes or a combination of multiple devices.
What Are the Important Considerations to be able to Perform Thermal Management in an Electronic Circuit Successfully?
The main aspects that should be taken into account during the design phase are:
- Performance data and dimensions of components
- Major heat dissipating components
- The size of the electronic circuit board
- Electronic circuit board material, layout and component layout
- Assembly equipment
- The temperature of application environment
- The amount of heat emitted
- Suitable cooling methods, i.e. cooling fans, coolers, thermal pastes etc.
The best practice is to manage the temperature at component and system level and take into account the existing operating environment. Among the factors that should be taken into account when choosing a cooling mechanism are the package properties of the semiconductor, heat dissipation properties etc. This information is generally found on the manufacturer’s data sheet.
Natural convection cooling is sufficient for an electronic circuit board that emits a small amount of heat. However, circuit boards that generate excess heat require coolers, heat pipes, fans, thick copper, or a combination of several cooling techniques and devices.
How to Reduce Thermal Resistance?
First, what is thermal resistance? Thermal resistance is a measure of how difficult it is to conduct heat. Thermal resistance is indicated as the division of the temperature difference between two specific points to the heat flow (the amount of heat transmitted per unit time) between these two points. This translates into that the higher the thermal resistance, the more difficult it is for heat to be transmitted.
A low thermal resistance allows heat transfer to occur faster through the material. This resistance is directly proportional to the length of the thermal path and inversely proportional to the cross-sectional area and thermal conductivity of the thermal path.
Thermal resistance θ = t / (A × K)
Here,
- t: Thickness of the material
- K: Thermal conductivity factor
- A: Cross-sectional area
Designers often reduce thermal resistance in the following ways:
- Using a thinner PCB to reduce the thermal path
- Adding thermal vias for vertical heat transfer
- Using copper foil and thick traces for horizontal heat transfer
How are the Components that Emit Excess Heat Detected?
It is important to understand which components produce the most heat and choose the best heat removal mechanism accordingly. Using the manufacturer’s data sheet, a designer should determine the thermal rating and characteristics of the device. Usually, the manufacturers offer guidance on how to remove excess heat.
Why is the Location, Direction and Organization of the Components Important in Removing
the Heat Generated in the Electronic Circuit?
It is important that the components that consume more power are placed in the areas that offer the best heat removal. However, this should not be on the corners or edges of the PCB unless there is a cooler. Placing the components close to the center provides heat dissipation around the device, but there must be enough space for adequate air circulation.
While it may be difficult to correct (or compensate) the temperature distribution, it is important to avoid installing the high-powered components together or very close to each other. Distributing them evenly prevents formation of hot spots.
Another good practice is to place the sensitive components such as small integrated circuits, transistors and electrolytic capacitors in low temperature areas. In circuits based on convection cooling, arranging the components (for example integrated circuits) horizontally or vertically in a long way helps in thermal management.
How Can We Remove Heat from Electronic Circuit Boards? What are the Products Used to Remove Heat?
Designers can use many techniques to remove heat from the components and electronic circuit boards. Common mechanisms used include coolers, cooling fans, heat pipes and thick coppers. Usually, the circuits that generate more heat require more than one single technology. For example, cooling a laptop processor and display chips requires a cooler, heat pipe and a fan.
Coolers
In electronic devices, a cooler is used to dissipate heat and prevent overheating. It is usually made of a metallic material, such as aluminum or copper. Coolers absorb and transfer heat from the device’s central processing unit (CPU) or other heat-generating components. Coolers work by increasing the available surface area for heat transfer and increasing the passage of air or other cooling medium over the surface through branches or other structures. This helps the device to operate at a safe temperature and prevent damage caused by overheating.
In many applications, the device can be an electronic component (e.g. CPU, GPU, ASIC, FET, etc.) and the surrounding liquid may be air. The device transfers heat to the coolers by convection method. While the main mechanism of heat transfer from the cooler is convection, radiation also has a small effect.
There are two types of convection:
Natural convection: The movement of fluid particles is ensured by local density changes due to heat transfer from a solid surface to liquid particles.
Forced convection: The movement of fluid particles is ensured by an additional device, such as a fan or a blower.
The coolers are designed for significantly increasing the contact surface area between solid and fluid, which increases the potential for heat transfer. The surface area of a typical ASIC in contact with air may be only 1600mm2. The surface area of a typical cooler used to cool this device can be, on the other hand, 10 or 20 times this value.
How Do Coolers Work?
The electronic components generate heat while running, as an electric current flows through them. This heat must be dissipated to prevent overheating and malfunction of the components. A cooler works by absorbing this heat and spreading it over a larger surface area, allowing it to dissipate more efficiently.
The cooler is usually connected to the electronic component using a thermal interface material. This material is usually a thermal interface material such as thermal paste or pad. This material helps to transfer heat from the component to the cooler.
When the heat is transferred to the cooler, it is dissipated into the surrounding air through convection. The larger the surface area of the heater, the more effective it is at dissipating heat.
Cooling Fans
A cooler and fan combination is an active cooling solution used in computer systems to cool integrated circuits, usually the central processing unit (CPU). As the name suggests, it includes a passive cooling unit (Cooler) and a fan. The cooler is usually made of a high-temperature conductive material such as aluminum and copper, and the fan is a DC brushless fan, which is the standard used for computer systems.
Thermal Paste
Thermal paste is a substance applied between the processor and the heat sink.
How to Use Thermal Paste?
Since the processors can get very hot, it is crucial to remove the heat as soon as possible. Unfortunately, air is a substance that conducts heat poorly, so there needs to be as little air as possible for optimal transfer between the CPU and the cooler.
If you check a computer case, you will see that the cooler is firmly mounted onto the CPU. This sealing may seem good enough to prevent air from entering, but it is not enough to keep air out.
There are many small grooves and gaps on the surface of the CPU and the contact plate of the cooler. If these are not properly sealed, they allow air to pass between the processor and the cooler, reducing the heat transfer efficiency between these two.
This is where the thermal paste comes into play. Thermal paste is not only a substance that conducts heat well, but also can penetrate into small gaps and grooves on the surfaces of the hardware. This creates an airtight seal and increases the heat transfer rate.
Over time, the thermal paste becomes old and dries out. Dryness reduces its effectiveness and causes the CPU to get too hot. This is why the people recommend reapplying the thermal paste to prevent a computer from overheating.
Heat Pipes
Heat pipes are suitable for compact devices with limited space. These pipes offer a reliable and cost-effective passive heat transfer. Their benefits include a vibration-free operation, good thermal conductivity, low maintenance and quiet operation (since they have no moving parts).
A typical pipe contains a small amount of nitrogen, water, acetone or ammonia. These liquids absorb heat and form steam, and this steam travels along the pipe. The pipe has a capacitor, the vapor condenses as it passes through this capacitor, returns to its liquid form, and the cycle restarts.
Thermal Via Arrays
Thermal vias reduce thermal resistance by increasing the mass and area of the copper and help in conducting heat from critical components. Therefore, a better performance is achieved when the vias are placed in a closer position to the heat source.
In some applications, the heat coming from a device (such as a thermally optimized IC) can be transmitted by a combination of a thermal via array and a pad. This conducts heat better through the PCB without the need for a cooler.
Thick Copper Traces
Using more copper provides a larger surface area, which increases heat distribution and dissipation. Such PCBs are suitable for high power applications.
Result
The PCB thermal management techniques depend on many factors, such as the amount of heat emitted by the components and circuit, the environment, the overall design, the housing etc. If the heat generation is low, the circuit can operate without requiring additional cooling. However, if the circuit generates more heat there must be a cooling mechanism to remove the heat. In order to design thermally-optimized electronic circuit boards, the designers must consider anything that effects temperature, starting from the concept stage throughout the design and manufacturing stages.
