PCB circuit board cooling skills
Ora pubblicata: 2020-05-29 19:02:33
The heat generated by electronic equipment during operation causes the internal temperature of the equipment to 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 the electronic equipment will decline. Therefore, it is very important to heat dissipate the circuit board.
Factor analysis of temperature rise of printed circuit board
The direct cause of the temperature rise of the printed board is due to the presence of circuit power consumption devices, and electronic devices have power consumption to varying degrees, and the intensity of heat varies with the power consumption.
Two phenomena of temperature rise in printed boards:
(1) Local temperature rise or large area temperature rise;
(2) Short-term temperature rise or long-term temperature rise.
When analyzing PCB thermal power consumption, generally from the following aspects.
1.Electrical power consumption
(1) Analysis of power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB circuit board.
2. The structure of the printed board
(1) The size of the printed board;
(2) Material of printed board.
3. Installation method of printed board
(1) Installation method (such as vertical installation and horizontal installation);
(2) Sealing condition and distance from the casing.
4. Thermal radiation
(1) Emissivity of printed board surface;
(2) The temperature difference between the printed board and the adjacent surface and their absolute temperature;
5. Heat conduction
(1) Install the radiator;
(2) Conduction of other installation structural parts.
6. Thermal convection
(1) Natural convection;
(2) Forced cooling convection.
The analysis of the above factors from the PCB is an effective way to solve the temperature rise of the printed board. Often these factors are interrelated and dependent in a product and system. Most factors should be analyzed according to the actual situation, only for a specific actual situation Only in this way can the parameters such as temperature rise and power consumption be calculated or estimated correctly.
Circuit board cooling method
1. High heat-generating device plus radiator and heat-conducting plate
When a few devices in the PCB generate a lot of heat (less than 3), a heat sink or heat pipe can be added to the heat-generating device. When the temperature cannot drop, a heat sink with a fan can be used to enhance the heat dissipation . When there are more heating devices (more than 3), a large heat sink (board) can be used. It is a special radiator customized according to the position and height of the heating device on the PCB board or in a large flat radiator Cut out the height of different components. Fasten the heat dissipation cover to the component surface, and contact each component to dissipate heat. However, due to the poor consistency of the components during assembly and welding, the heat dissipation effect is not good. Usually a soft thermal phase change thermal pad is added on the component surface to improve the heat dissipation effect.
2. Heat dissipation through the PCB board itself
At present, the widely used PCB plates are copper-clad / epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad plates are used. Although these substrates have excellent electrical performance and processing performance, they have poor heat dissipation. As a heat dissipation route for high heat-generating components, the PCB itself can hardly be expected to conduct heat from the resin of the PCB, but to dissipate heat from the surface of the component to the surrounding air. However, as electronic products have entered the era of miniaturization of components, high-density installation, and high-heat assembly, it is not enough to rely on the surface of components with very small surface area to dissipate heat. At the same time, due to the heavy use of surface-mounted components such as QFP and BGA, the heat generated by the components is transferred to the PCB board in large quantities. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of the PCB itself in direct contact with the heating element. Conduct or emit.
3. Adopt reasonable routing design to achieve heat dissipation
Because the thermal conductivity of the resin in the sheet is poor, and the copper foil lines and holes are good conductors of heat, improving the copper foil residual rate and increasing the thermal conduction holes are the main means of heat dissipation.
To evaluate the heat dissipation capacity of the PCB, it is necessary to calculate the equivalent thermal conductivity (nine eq) of the composite material composed of various materials with different thermal conductivity coefficients—the insulating substrate for PCB.
4. For equipment that uses free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a vertical or horizontal manner.
5. Devices on the same printed board should be arranged according to their heat generation and heat dissipation as much as possible. Devices with small heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) The uppermost part of the cooling airflow (at the entrance), the devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.
6. In the horizontal direction, high-power devices should be placed as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, the high-power devices should be placed as close to the printed board as possible to reduce the temperature of these devices when working Impact.
7. The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heat-generating device. Multiple devices are preferably staggered on the horizontal plane.
8. The heat dissipation of the printed board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be reasonably configured. When the air flows, it tends to flow where the resistance is small, so when configuring devices on the printed circuit board, it is necessary to avoid leaving a large air space in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.
9. Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB as much as possible, and keep the temperature performance of the PCB surface uniform and consistent. It is often difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid areas with too high power density to avoid hot spots that affect the normal operation of the entire circuit. If there are conditions, it is necessary to conduct thermal performance analysis of printed circuits. For example, the thermal efficiency index analysis software module added in some professional PCB design software can help designers optimize circuit design.
10. Place the device with the highest power consumption and the highest heat generation near the best heat dissipation position. Do not place devices with high heat generation at the corners and surrounding edges of the printed board unless a heat dissipation device is arranged near it. When designing the power resistor, choose a larger device as much as possible, and adjust the layout of the printed circuit board to make it have sufficient heat dissipation space.
11. When connecting high heat dissipation devices to the substrate, the thermal resistance between them should be reduced as much as possible. In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as a layer of thermally conductive silica gel) can be used on the bottom surface of the chip, and maintain a certain contact area for the device to dissipate heat.
12. Device and substrate connection:
(1) Try to shorten the length of the device leads;
(2) When selecting high-power consumption devices, the thermal conductivity of the lead material should be considered, and the largest cross-section of the lead should be selected as much as possible;
(3) Select the device with more pins.
13. Device package selection:
(1) When considering thermal design, attention should be paid to the package description of the device and its thermal conductivity;
(2) It should be considered to provide a good heat conduction path between the substrate and the device package;
(3) Air blocking should be avoided in the heat conduction path. If this is the case, thermally conductive materials can be used for filling.Etichetta: PCB