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 dissipate heat to 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 heat intensity changes 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.
Electrical power consumption
(1) Analyze 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. These factors are often related 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 situation can the parameters of temperature rise and power consumption be calculated or estimated correctly.
Circuit board cooling method
1. High heat-generating device plus heat sink and heat conduction plate
When a few devices in the PCB generate a large amount of heat (less than 3), a heat sink or heat pipe can be added to the heat-generating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance the heat dissipation effect . When there are more heating devices (more than 3), a large heat dissipation cover (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) vertically or horizontally.
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.) are placed in The uppermost stream of the cooling airflow (at the entrance), 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, the high-power devices should be placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices should be placed as close as possible to the top of the printed board to reduce the temperature of these devices when working on other devices Impact.
7. The temperature-sensitive device is best placed in the area with the lowest temperature (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 equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the device or the printed circuit board should be reasonably configured. When the air flows, it always 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 conditions permit, thermal efficiency analysis of printed circuits is necessary. For example, thermal efficiency index analysis software modules added in some professional PCB design software can help designers optimize circuit design.