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Heat dissipation skills of PCB
Addtime: 2020-11-11

The heat generated by the working of electronic equipment makes the internal temperature of the equipment rise rapidly. If the heat is not dissipated in time, the equipment will continue to heat up, the device will fail due to overheating, and the reliability of electronic equipment will decline. Therefore, it is very important to heat the circuit board.

Analysis of temperature rise factors of printed circuit board

The direct cause of PCB temperature rise is due to the existence of circuit power consumption devices, and the power consumption of electronic devices varies with the power consumption.

There are two phenomena of temperature rise in PCB

(1) Local temperature rise or large area temperature rise;

(2) Short time temperature rise or long time temperature rise.

In the analysis of PCB thermal power consumption, generally from the following aspects.

1. Electrical power consumption

(1) The power consumption per unit area is analyzed;

(2) The distribution of power consumption on PCB is analyzed.

2. Structure of printed circuit board

(1) Size of printed circuit board;

(2) The material of printed circuit board.

3. Installation method of printed circuit board

(1) Installation mode (such as vertical installation and horizontal installation);

(2) Sealing condition and distance from the casing.

4. Thermal radiation

(1) Radiation coefficient of PCB surface;

(2) The temperature difference between PCB and adjacent surface and their absolute / relative temperature;

5. Heat conduction

(1) Install the radiator;

(2) Conduct of other mounting structures.

6. Thermal convection

(1) Natural convection;

(2) Forced cooling convection.

The analysis of the above factors from PCB is an effective way to solve the temperature rise of PCB. These factors are often interrelated and dependent in a product and system. Most of the factors should be analyzed according to the actual situation. Only in view of a specific actual situation can the parameters such as temperature rise and power consumption be calculated or estimated correctly.

Heat dissipation mode of circuit board

1. High heating device with radiator and heat conducting plate

When there are a few devices in PCB with large heating capacity (less than 3), heat sink or heat conduction pipe can be added to the heating device. When the temperature can not be lowered, heat sink with fan can be used to enhance the heat dissipation effect. When there are more than 3 heating elements, a large radiator cover (board) can be used, which is a special radiator customized according to the position and height of the heating elements on the PCB board, or a large flat radiator with different elements in high and low positions. The heat dissipation cover is buckled on the surface of the component as a whole and contacts with each component to dissipate heat. However, due to the poor consistency of component assembly and welding, the heat dissipation effect is not good. Soft thermal phase change thermal pad is usually added on the surface of components to improve the heat dissipation effect.

2. Heat dissipation through PCB board itself

At present, the widely used PCB board is copper-clad / epoxy glass cloth or phenolic resin glass cloth, and a small amount of paper-based copper-clad board. Although these substrates have excellent electrical performance and processing performance, they have poor heat dissipation. As the heat dissipation way of high heating elements, it is almost impossible to rely on PCB resin to conduct heat, but to dissipate heat from the surface of the elements to the surrounding air. However, as electronic products have entered the era of miniaturization, high-density installation and high thermal assembly, it is not enough to rely on the surface of components with very small surface area for heat dissipation. At the same time, due to the extensive use of QFP, BGA and other surface mounted components, the heat generated by the components is largely transferred to the PCB board. Therefore, a good way to solve the heat dissipation problem is to improve the heat dissipation capacity of the PCB itself, which is in direct contact with the heating elements, and transmit or emit it through the PCB board.

3. Reasonable wiring design is adopted to realize heat dissipation

Due to the poor thermal conductivity of the resin in the plate, and the copper foil lines and holes are good conductors of heat, the main means of heat dissipation are to improve the residual rate of copper foil and increase the heat conduction holes.

In order to evaluate the heat dissipation capability of PCB, it is necessary to calculate the equivalent thermal conductivity (9 EQ) of the insulating substrate for PCB, which is composed of various materials with different thermal conductivity.

4. For the equipment cooled by free convection air, the integrated circuits (or other devices) should be arranged in longitudinal or transverse way.

5. The devices on the same printed circuit board should be arranged according to their calorific value and heat dissipation degree. The devices with small calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the top (entrance) of the cooling airflow, while the devices with large calorific value or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the cooling airflow The airflow is the most downstream.

6. In the horizontal direction, the high-power devices are arranged as close as possible to the edge of the printed circuit board, so as to shorten the heat transfer path; in the vertical direction, the high-power devices are arranged as close as possible to the top of the printed circuit board, so as to reduce the influence of these devices on the temperature of other devices.

7. The device sensitive to temperature should be placed in the area with low temperature (such as the bottom of the equipment). Never place it directly above the heating device. Multiple devices are staggered on the horizontal plane.

8. The heat dissipation of PCB in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the devices or PCB should be reasonably configured. When the air flows, it always tends to flow where the resistance is small, so when the devices are configured on the printed circuit board, it is necessary to avoid leaving large airspace in a certain area. The same problem should be paid attention to in the configuration of multiple printed circuit boards in the whole machine.

9. Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB as far as possible, and keep the uniformity and consistency of the PCB surface temperature performance. It is often difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid the area with too high power density, so as to avoid hot spots affecting the normal operation of the whole circuit. If there are conditions, it is necessary to analyze the thermal efficiency of printed circuit. For example, the thermal efficiency index analysis software module added in some professional PCB design software can help designers optimize the circuit design.

10. The device with the highest power consumption and maximum heating is arranged near the heat dissipation position. Do not place the components with high heat on the corners and edges of the printed circuit board, unless there is a heat sink nearby. In the design of power resistance, larger devices should be selected as much as possible, and there should be enough heat dissipation space when adjusting the layout of printed circuit board.

11. When high thermal dissipation devices are connected to the substrate, the thermal resistance between them should be reduced as much as possible. In order to better meet the requirements of thermal characteristics, some thermal conductive materials (such as coating a layer of thermal conductive silica gel) can be used on the bottom of the chip, and a certain contact area can be maintained for the device heat dissipation.

12. Connection between device and substrate:

(1) Shorten the lead length as much as possible;

(2) When selecting high-power devices, the thermal conductivity of the lead material should be considered. If possible, the transverse section of the lead should be the largest;

(3) Select the device with more pins.

13. Package selection:

(1) When considering the thermal design, attention should be paid to the package description and thermal conductivity of the device;

(2) It should be considered to provide a good heat conduction path between the substrate and the device package;

(3) The air partition should be avoided in the heat conduction path. If there is such a situation, the thermal conductive material can be used for filling.

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