The heat produced by the electronic equipment will increase the temperature inside the equipment rapidly. If the heat is not circulated in time, the equipment will keep heating up. The device will overheat and the reliability of the electronic equipment will decline. Therefore, it is very important to deal with the heat dissipation of the circuit board.
Factor analysis of temperature rise of printed circuit board
The direct reason for the temperature rise of printed circuit board is that the power consumption of the electronic device varies in different degrees due to the existence of the circuit power consumption device, and the heating intensity varies with the power consumption.
2 phenomena of temperature rise in printed circuit board:
(1) local temperature rise or large area temperature rise;
(2) short time temperature rise or long time temperature rise. .
It in the analysis of PCB power consuming and general analyses from the following aspects.
1. Electrical power dissipation
(1) analyze the power consumption per unit area;
(2) analyze the distribution of power consumption on PCB circuit board.
2. The structure of printed circuit board
(1) dimension of printed circuit board;
(2) materials for printed boards.
3. The installation method of PCB
(1) installation methods (such as vertical installation, horizontal installation);
(2) the sealing condition and the distance from the casing.
4, thermal radiation
(1) radiation coefficient of printed board surface;
(2) the temperature difference between the PCB and the adjacent surface and their absolute temperature;
5, heat conduction
(1) install radiators;
(2) conduction of other mounting structures.
6. Thermal convection
(1) natural convection;
(2) forced cooling convection.
From the analysis of the above factors. PCB is an effective way to solve the PCB temperature rise, often in a product and system, these factors are interrelated and dependent, most of the factors should be based on the actual situation, only for a specific situation to accurately calculate or estimate the temperature and power consumption the parameters such as.
Two. Circuit board heat dissipation mode
1, high heating device plus radiator, heat conduction plate
I have a fever when the PCB device is large (less than 3), with a radiator or heat on a heating device, when the temperature is not reduced, the radiator with fan, in order to enhance the cooling effect. When the number of the heating device when the quantity (more than 3), adopting a large radiating hood (plate), which is based on the heating device of the PCB board and the position and height of the special radiator custom or in a large flat radiator cutout on different components of high and low position. The radiating cover is integrally buckled on the element surface, and is contacted with each element to radiate heat. However, due to the poor consistency of assembly and welding, the cooling effect is not good. Usually, soft thermal phase change heat conduction 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 board is copper clad / epoxy glass cloth base or phenolic resin glass cloth base material, and a small amount of paper based copper clad sheet. The substrate has good electrical properties and processing performance, but poor heat dissipation, as the way of heating element, can hardly expect itself by PCB resin of heat conduction, but the heat from the component surface to the surrounding air. But with the electronic products have entered the miniaturization of components, high density installation, high fever assembly era, if only the surface area of the surface of the small components to dissipate heat is very inadequate. At the same time, due to the extensive use of QFP, BGA and other surface mounting components, components of the large amount of heat transmitted to the PCB board, therefore, the best way to solve the heat dissipation is to improve the cooling capacity in direct contact with the heating element of the PCB itself, the PCB board is transmitted out or out.
3. Using reasonable route design to achieve heat dissipation
Due to the poor thermal conductivity of the resin in the plate, and copper wire line and hole is a good conductor of heat, so to improve the residual rate of copper foil and increase the heat conduction 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 (nine EQ) of the insulating substrate PCB for the composites composed of various materials with different thermal conductivity.
4, we used for free convection air cooling equipment, the best is the integrated circuit (or other device) according to the longitudinal arrangement, or by transverse arrangement.
5, some devices with a piece of printed board should be according to the size and degree of heat radiating device partition arrangement, small heat or low heat resistance (such as small signal transistor, small scale integrated circuit, such as electrolytic capacitors) on the upper cooling airflow (entrance), heat or good heat resistance devices (such as power transistor, large scale integrated circuit) on the cooling airflow in the downstream.
I, 6 in the horizontal direction, high power devices as close as possible to the printed circuit board edge layout, so as to shorten the heat transfer path; in the vertical direction, high power devices as close as possible to the printed circuit board is arranged, impact on other devices in order to reduce the temperature of these devices work.
7, the temperature sensitive device is best placed in the lowest temperature area (such as the bottom of the device), do not put it on the heating device directly above, many devices are best staggered layout on the horizontal plane.
8. The heat dissipation of printed circuit board mainly depends on air flow, so it is necessary to study the air flow path and configure the device or printed circuit board in the design. When air flows, it tends to flow in places where resistance is small, so it is necessary to avoid larger airspace in a region when configuring devices on printed circuit boards. The same problem should be noticed in the configuration of multi block printed circuit boards in the whole machine.
9. Avoid the concentration of hot spots on the PCB, and distribute the power evenly on the PCB board as much as possible, so as to maintain the uniformity and consistency of the PCB surface temperature performance. It is difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid the region with too high power density, so as to avoid excessive hot spots and affect the normal operation of the whole circuit. If possible, thermal efficiency analysis of printed circuit is necessary, such as thermal efficiency index now increase the number of professional design software PCB analysis software module, can help designers to optimize the circuit design.
10. The device with the highest power consumption and maximum heating is arranged near the optimum heat dissipation position. Do not place the device with higher heating on the corners and edges of the printed circuit board unless there is a heat sink near it. In the design of power resistors, choose the larger device as much as possible, and adjust the layout of the printed board to make it have enough heat dissipation space.
11, high thermal dissipation devices should be able to minimize the thermal resistance between the substrate and the substrate when they are connected to the substrate. In order to meet the requirements of thermal characteristics better, some thermal conductive materials (such as applying a layer of thermal conductive silica gel) can be used on the bottom of the chip, and a certain contact area is maintained for the cooling of the device.
12. Connection between device and substrate:
(1) shorten device lead length as much as possible;
(2) the thermal conductivity of lead materials should be considered when selecting high power devices. If possible, the maximum cross section of the lead wire should be selected;
(3) select a device with more pins.
Package selection of 13:
(1) attention should be paid to the package description and its thermal conductivity when thermal design is taken into account;
(2) a good thermal conduction path should be considered between the substrate and the device package;
(3) air separation should be avoided on the heat conduction path. If this is the case, the thermal conductive material can be used to fill it.