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Thermodynamics and PCB design

When PCB design is becoming smaller, thinner, and PCB products with a large amount of accumulation ability, we must take the heat factor into consideration. You need to make the PCB good heat dissipation, so that the components can work within the safety regulations. Otherwise it will seriously affect the life of PCBA. On the other hand, high temperature will cause the PCB to expand, and high temperature will increase the pressure on the key structure of the PCB. Especially in some HDI PCB, there are some tiny holes with BGA and high aspect ratio

Two phenomena of PCB heating

1)Local heating or large area heating

2)heating for a short time or for a long time

When analyzing PCB thermal power consumption, it is usually analyzed from the following aspects

1 Electrical power consumption

1)Analyze the power consumption per unit area

2) Analyze the distribution of power consumption on the PCB

2.PCB structure

PCB size

PCB material

3. How to install PCB

Vertical installation, horizontal installation

The sealing condition in the enclosure and the distance from the enclosure

  • Heat radiation

The emissivity of the PCB board surface

The temperature difference between the PCB board and the adjacent surface and their absolute temperature

  • Heat Conduction

Install the radiator

  • Thermal convection

Natural convection

Forced convection

The analysis of the above factors is an effective way to solve the PCB heating up and heat dissipation. In a complete product, these factors are often interrelated and dependent. Most of the time, the analysis should be based on the actual situation.

High heating element plus radiator or heat conducting plate

When there are a few components in the PCB that generate a large amount of heat, it is less than three. A radiator or heat pipe can be added to the heating element. When the temperature still cannot drop, a radiator with a fan can be used to enhance the heat dissipation effect. When there are more than 3 heating elements, a large heat dissipation cover can be used, which is a specially customized heat dissipation cover according to the height of the heating elements on the PCB. However, due to the poor consistency of the components, the heat dissipation effect is not fully utilized.

Heat dissipation through the PCB board itself. At present, the commonly used material is FR4, and a small part of it uses FR1, CEM-1. These materials have good electrical properties and processing properties, but poor heat dissipation. The current components themselves are developing in a smaller and more precise direction, and it is also difficult to rely on the components to dissipate heat. Therefore, the board material and design of the PCB are important ways to solve the heat and heat dissipation.

Use reasonable wiring design to dissipate heat

Due to the poor thermal conductivity of the resin in the sheet, copper foil lines and holes are good conductors of heat. Therefore, improving the residual rate of copper foil and increasing the thermal conduction holes are the main means of heat dissipation.

On the same PCB, the components should be arranged according to the degree of heat generation and heat dissipation capacity of the components.Parts with low heat generation or poor heat resistance (such as electrolytic capacitors, small transistors or ICs)It is placed upstream of the cooling air flow, which is the entrance of the line. Components with high heat generation or high heat resistance (such as large-scale integrated circuits) are placed at the most downstream of the cooling airflow.

In the horizontal direction, high-power components are best arranged on the edge of the PCB. This can shorten the heat transfer path. High-power components should be placed above the PCB as much as possible to reduce the influence of heat on other components.Components that are more sensitive to temperature must be kept away from components with high power and heat. Multiple components should be arranged alternately and separated.

Avoid the concentration of hot spots on the PCB. Try to distribute high-power components evenly on the PCB. Keep the PCB surface temperature uniform and consistent. It is often difficult to achieve complete equality when designing. But be sure to avoid areas with too high power density, otherwise it will affect the work of the entire PCB board.If possible, the thermal efficiency analysis of the PCB board can be carried out. Some PCB design software has the function of thermal efficiency analysis. After testing, suggestions can be made to help designers find and solve problems.

When connecting high heat dissipation devices to the PCB, minimize the thermal resistance between them.In order to better meet the requirements of thermal characteristics, some thermally conductive materials can be used on the chip ground. And keep a certain area for heat dissipation.

Minimize the lead length of components

Consider the thermal conductivity and large cross-section of the lead

Choose components with more pins

When selecting components, confirm the package and thermal conductivity.

Solve the problem by housing design

The PCBA housing should Increase surface roughness and increase heat dissipation area

Choose a material with good thermal conductivity for the shell, such as aluminum alloy

Apply heat sink to the inside of the shell to enhance the shell’s ability to use radiation to dissipate heat

Increase the ventilation holes on the case, and the holes should be unobstructed to avoid short circuit of air flow.

Therefore, heat dissipation must be considered when designing the circuit board. Choose a sheet with a high TG value, considering the copper thickness, line width and line spacing, aperture size and ratio, etc. The performance and thermal conductivity of the components are evenly distributed, and the housing is ventilated and dissipated. These reasons must be taken into account, so that PCBA products can have stable performance and long life.

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