The heat generated by the working of electronic equipment rapidly increases the internal temperature of the equipment. If the heat is not released in time, the equipment will continue to heat up, and 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 of printed circuit board
The direct cause of PCB temperature rise is the power consumption of electronic devices, which varies with the power consumption.
Two phenomena of temperature rise in PCB:
(1) local temperature rise or large area temperature rise;
(2) short-term or long-term temperature rise.
When analyzing 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) analyze the power distribution on PCB.
2. PCB structure
(1) the size of the printed board;
(2) printed board materials.
3. PCB installation mode
(1) installation mode (such as vertical installation and horizontal installation);
(2) sealing condition and distance from the housing.
4. Thermal radiation
(1) radiation coefficient of PCB surface;
(2) the temperature difference between the PCB and the adjacent surfaces and their absolute temperature;
5. Heat conduction
(1) install radiators;
(2) conduction of other installation structural parts.
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, and these factors are often correlated and dependent on each other in a product and system. Most of the factors should be analyzed according to the actual situation, and the temperature rise, power consumption and other parameters can only be correctly calculated or estimated according to a specific situation.
Circuit board heat dissipation mode
1. Add heat sink and heat conducting board for high heating devices
When there is a small number of devices in PCB with high heat (less than 3), a radiator or heat conducting tube can be added to the heating device. When the temperature is not falling, a radiator with fan can be used to enhance the heat dissipation effect. When the amount of heating devices is large (more than 3), a large cooling cover (board) can be used. It is a special radiator customized according to the position and height of heating devices on the PCB board, or different positions of components can be cut out on a large flat radiator. The heat sink is integrated on the component surface and contacts with each component to dissipate heat. However, due to the low and high consistency when the components are installed and welded, the heat dissipation effect is not good. Soft thermal phase change heat conduction pads are usually added on the surface of components to improve the heat dissipation effect.
2. Heat dissipation through PCB board itself
At present, PCB is widely used as copper/epoxy glass cloth substrate or phenolic resin glass cloth substrate, and a small amount of paper-based copper plate is used. Although these substrates have excellent electrical properties and processing properties, they are poor in heat dissipation. As the heat dissipation path of high heating elements, it is almost impossible to transfer heat from the resin of PCB itself, but to dissipate heat from the surface of components to the surrounding air. However, as electronic products have entered the era of miniaturization, high-density installation and thermal-heat assembly, it is not enough to only rely on the surface of components with very small surface area to dissipate heat. At the same time, due to the extensive use of surface mounted components such as QFP and BGA, the heat generated by the components is transferred to PCB board in large quantities. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of PCB itself that directly contacts with the heating elements, and conduct out or send out through PCB board.
3. Adopt reasonable wiring design to realize heat dissipation
Due to the poor thermal conductivity of the resin in the plate, and copper foil line and hole is a good conductor of heat, so improve the residual rate of copper foil and increase the thermal conductivity hole is the main means of heat dissipation.
To evaluate the heat dissipation capacity of PCB, it is necessary to calculate the equivalent thermal conductivity (9 eq) of PCB insulation substrates for composite materials composed of various materials with different thermal conductivity coefficients.
4. For equipment with free-convection air cooling, it is best to arrange integrated circuits (or other devices) in vertical or horizontal length.
5. On the same piece of PCB device should as far as possible according to the calorific value of the size and degree of heat partition, calorific value is small or poor heat resistance devices (such as small signal transistor, small-scale integrated circuit, electrolytic capacitors, etc.) on the cooling airflow of the best () at the entrance, calorific value big or good heat resistance devices (such as power transistor, large scale integrated circuit, etc.) in the most downstream cooling airflow.
6. In the horizontal direction, high-power devices should be arranged as close as possible to the edge of the PCB, so as 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, so as to reduce the influence of these devices on the temperature of other devices.
7. Temperature sensitive devices are best placed in the area with the lowest temperature (such as the bottom of the device), never put it directly above the heating device, multiple devices are best in horizontal staggered layout.
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 printed circuit board should be reasonably configured. Air flow always tends to flow where resistance is low, so in the printed circuit board configuration device, to avoid leaving a large 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 PCB, distribute power evenly on PCB board as much as possible, and maintain the uniform and consistent performance of PCB surface temperature. It is often difficult to achieve strict uniform distribution in the design process, but we must avoid areas with too high power density, lest hot spots affect the normal work of the whole circuit. If possible, it is necessary to conduct thermal efficiency 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. Arrange the devices with the highest power consumption and maximum heating near the optimal position of heat dissipation. Do not place high-heating components on the corners and edges of a printed board unless a cooling device is arranged nearby. When designing power resistors, we should choose larger devices as far as possible, and adjust the layout of PCB so that there is enough heat dissipation space.
11. High heat dissipation devices should minimize the thermal resistance between them when connected with the substrate. In order to better meet the requirements of thermal characteristics, some thermal conductivity materials (such as a layer of heat-conducting silica gel) can be used on the underside of the chip, and a certain contact area can be maintained for device heat dissipation.
12. Connection between device and substrate:
(1) minimize the lead length of the device;
(2) when selecting high-power devices, the thermal conductivity of lead materials should be considered, and the largest lead section surface should be selected as far as possible;
(3) select devices with a large number of pins.
13. Packaging selection of devices:
(1) when considering the thermal design, attention should be paid to the packaging description of the device and its thermal conductivity;
(2) consideration should be given to providing a good heat conduction path between the substrate and the device package;
(3) air partition should be avoided in the heat conduction path. If this happens, thermal conductive materials can be used for filling.