What is Heat dissipation?
Heat is a type of energy. The thermal energy of high temperature is usually high. The heat dissipation is used to transfer thermal energy to other places, and reduces the temperature so that the whole object can reach a balance with the temperature of the surrounding environment.
Three heat dissipation methods
Enhance Heat Transfer and Temperature Control-Conduction
Design for improving heat transfer to decrease component and housing temperature to meet thermal specifications. Ex.: Thermal design for electronic devices, etc.
What is Thermal Conductivity and how to calculate thermal conductivity?
According to Fourier's Law, describe the direction of heat conduction. It is to move from high temperature to low temperature, let the whole environment or object, until it reaches a state of thermal equilibrium.
The calculating thermal conductivity is as follows:
Heat conduction is mainly affected by four factors:
Material/Heat dissipation path/Contact area/Temperature distribution
Q is heat conduction.
k is the thermal conductivity constant. Different materials will affect the k value.
A is the contact area. The larger the area, the better the heat conduction effect.
𝑻𝟐−𝑻𝟏 is the temperature change. The heat will go from high temperature to low temperature, so the heat dissipation path can be seen.
d is the thickness of the material. This affects the speed of heat transfer.
Calculating thermal conductivity is influenced by these four factors.
Heat Insulation Design
Design for decreasing the heat transfer to enhance the insulation purpose.
What are thermal insulation examples?
In daily life, there are many examples of thermal insulation. For example, the handle of the pot is often made of wood, so that when cooking food, the hand can be held for a long time without burning.
As mentioned above, the thermal conductivity k will affect the thermal conductivity. The thermal conductivity of wood is approximately 0.17 (W/m.k). Pure iron is about 73 (W/m.K). The thermal conductivity is about 400 times different
| Thermal conductivity of different materials |
| Material |
Conductivity，k (W/moC) |
| Rubber |
0.13 |
| Glass |
0.78 |
| Water |
0.556 |
| Air |
0.026 |
| Dry wood |
0.17 |
Thermal Interface Materials - Conduction information
Due to roughness and flatness, there are many small gap and space between contacting surfaces. They fill the air and cause high thermal resistance since air is very bad in conduction (kair=0.026, kcu=386). The thermal interface materials (TIMs) is to replace air and reduce the thermal resistance at contacting surface.
Common Thermal Interface Materials
| Material |
Gap Pad |
Grease |
PCM |
Adhesive |
| Thermal performance |
Medium |
High |
High |
Low |
| Re-use |
Yes |
No |
No |
No |
| Cost |
Expensive |
Normal |
Normal |
Cheap |
| Application |
Low power, multi heat source with on heatsink or heat spreader, contact to housing, gap filler. |
CPU, GPU, high power heat source |
CPU, GPU, high power heat source |
small chip with low power (<1W) |
What is Convection?
Transmission of heat from one part of a liquid or gas to another by the movement of heated substances. Convection can only take place in the fluid. There are natural and force convection depending on the air flow conditions.
Calculate the heat dissipation based on the following formula:
Convection is mainly affected by three factors:
Air flow, heat exchange area, and temperature distribution.
Q is heat energy.
h is called the heat transfer coefficient and it is related to fluid conductivity, specific heat, density, viscosity, and velocity.
A is the heat exchange area. The larger the heat exchange area, the better the cooling effect.
𝑇𝑊 is the surface temperature；𝑇∞ is the ambient temperature. Here can see the temperature distribution.
What is Radiation?
Physics is the emission of energy as electromagnetic waves or as moving subatomic particles. In natural convection system, the radiation has to be considered in thermal design.
Thermal Improvement
Different heat dissipation components use different heat dissipation methods to transfer heat energy to other places to achieve the purpose of heat dissipation.
|
Component Level |
PCB Level |
System Level |
| Increase area for convection |
- Use heat sink
- Use Heat spreader
|
|
- Increase housing dimension
- Use copper foil, graphite sheet on housing inner side
|
| Improve conduction |
- Use high conductivity material, heat pipe, Vapor Chamber
|
|
- Change housing material
- PCB contact to housing
|
| Increase cooling air |
|
|
- Venting hole design
- Use fan
- Use air duct
|
| Increase radiation |
- Use anodized heatsink or spreader
|
|
- Housing surface treatment
- Use anodized heat spreader
|
| others |
|