How does PCB board Dissipate Heat?
The heat generated during the operation of electronic equipment makes the internal temperature of the equipment rise rapidly. If the heat is not released 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 the circuit board.
I. Factor Analysis of Printed Circuit Board Temperature Rise
The direct cause of PCB temperature rise is the existence of circuit power dissipation devices, and the power consumption of electronic devices varies in varying degrees. The heating intensity varies with the power consumption.
Two phenomena of temperature rise in PCB:
(1) Local temperature rise or large area temperature rise;
(2) Temperature rise for a short time or for a long time.
In the analysis of PCB thermal power consumption, it is generally analyzed from the following aspects.
1. Electrical power consumption
(1) Analysis of power consumption per unit area;
(2) The power distribution on PCB board is analyzed.
2. Structure of PCB
(1) Size of PCB;
(2) Material for PCB.
3. Installation of PCB
(1) Installation mode (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from casing.
4. Thermal radiation
(1) Radiation coefficient of PCB surface;
(2) Temperature difference and absolute temperature between PCB and adjacent surfaces;
5 heat conduction
(1) Installing radiators;
(2) Conduction of other installation structures.
6. Thermal convection
(1) Natural convection;
(2) forced cooling convection.
The analysis of the above factors of 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 them should be analyzed according to the actual situation. Only according to a specific actual situation can the parameters such as temperature rise and power consumption be calculated or estimated correctly.
2. Circuit Board Heat Dissipation
1. High-heating devices plus radiators and heat conducting plates
When the heat of a few devices in PCB is larger (less than 3), a radiator or a heat conducting tube can be added to the heating device. When the temperature can not be lowered, a fan radiator can be used to enhance the heat dissipation effect. When the number of heating devices is large (more than 3), a large radiator can be used. It is a special radiator made according to the position and height of heating devices on the PCB board or cuts out different positions of components on a large flat radiator. The radiator cover is integrally fastened on the surface of the components and contacted with each component to dissipate heat. However, the heat dissipation effect is not good due to the poor consistency of the components during assembly and welding. Soft thermal phase change heat conduction pads are usually added to the surface of components to improve the heat dissipation effect.
2. Heat dissipation through PCB board itself
At present, the widely used PCB sheets are copper-clad/epoxy glass cloth or phenolic resin glass cloth, and a small amount of paper-based copper-clad sheets are also used. Although these substrates have excellent electrical and processing properties, they have poor heat dissipation. As a way of heat dissipation for high-heating components, they can hardly be expected to transmit heat by the resin of PCB itself, but heat dissipation from the surface of the components to the surrounding air. However, as electronic products have entered the era of miniaturization, high density installation and high heating assembly, it is not enough to rely only 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 mounting elements, the heat generated by components is transmitted to PCB board in large quantities. Therefore, the best way to solve the problem of heat dissipation is to improve the heat dissipation ability of PCB itself, which is in direct contact with the heating elements, and transmit or emit through PCB board.
3. Using reasonable route design to realize heat dissipation
Due to the poor thermal conductivity of the resin in the sheet, and the copper foil lines and holes are good conductors of heat, improving the residual rate of copper foil and increasing the thermal conductivity holes are the main means of heat dissipation.
To evaluate the heat dissipation capability of PCB, it is necessary to calculate the equivalent thermal conductivity (9eq) of PCB insulating substrates, a composite material composed of various materials with different thermal conductivity.
4. For equipment cooled by free convection air, it is better to arrange integrated circuits (or other devices) in a longitudinal or horizontal way.
5. Devices on the same printed circuit board should be arranged according to their calorific value and heat dissipation degree as far as possible. 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 of the cooling air flow (entrance), and devices with high calorific value or good heat resistance (such as power transistors, electrolytic capacitors, etc.). Large Scale Integrated Circuits, etc.) are placed downstream of the cooling air stream.
6. In the horizontal direction, high-power devices are arranged as close as possible to the edge of the PCB to shorten the heat transfer path; in the vertical direction, high-power devices are arranged as close as possible to the top of the PCB to reduce the influence of these devices on the temperature of other devices.
7. Temperature-sensitive devices are best placed in the lowest temperature area (such as the bottom of the device). Never place them directly above the heating device. Multiple devices are best laid out staggered on the horizontal plane.
8. The heat dissipation of PCB mainly depends on air flow, so it is necessary to study the air flow path in the design and rationally configure the devices or PCB. Air flow always tends to flow in places with low resistance, so when configuring devices on printed circuit boards, it is necessary to avoid leaving a larger space in a certain area. The same problem should be noticed in the configuration of multiple PCBs in the whole machine.
9. Avoid the concentration of hot spots on PCB, distribute power evenly on PCB board as far as possible, and keep the surface temperature performance of PCB 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 in order to avoid excessive hot spots affecting the normal operation of the whole circuit. If conditions permit, 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 circuit design.
10. Arrange the devices with the highest power consumption and the highest heat output near the optimum heat dissipation location. Do not place the heater in the corner and around the edge of the PCB unless there is a heat sink near it. When designing the power resistor, choose the larger device as far as possible, and make it have enough heat dissipation space when adjusting the layout of PCB.
11. High thermal dissipative devices should try to reduce the thermal resistance between them when they are connected to the substrate. In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as coated with 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 heat dissipation of the device.
12. Connection between devices and substrates:
(1) To shorten the lead length of the device as far as possible;
(2) When choosing high power devices, the thermal conductivity of lead materials should be considered. If possible, the maximum cross section of lead should be chosen.
(3) Choose devices with more pins.
13. Packaging Selection of Devices
(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 partition should be avoided on the heat conduction path. If this is the case, thermal conductive materials can be used to fill it.
