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There are three main sources of heat in the PCB circuit board:
(1) Heating of electronic components;
(2) The heating of the PCB circuit board itself;
(3) Heat from other parts.
Among the three heat sources, electronic components have the highest calorific value and are the main heat source, followed by the heat formed by the PCB circuit board. The external heat depends on the overall thermal design of the system and is not considered for the time being. Then the purpose of thermal design is to select appropriate measures and methods to greatly reduce the temperature of the electronic components and the temperature of the PCB circuit board, so that the system can work normally at a suitable temperature.
It can be considered from the following aspects:
1. Dissipate heat through the PCB circuit board itself. At present, the widely used PCB circuit board materials are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and there are also a small number of paper-based copper-clad plates used. Although this type of substrate has excellent electrical properties and processing properties, it has poor heat dissipation. As a heat dissipation path for high-heating electronic components, it is basically impossible to hope that the heat is conducted by the resin of the PCB circuit board itself, but from the surface of the electronic component. Dissipate heat to the surrounding air.
However, as time goes by, smart electronic products have entered the era of miniaturization of components, high-density mounting, and high-heating assembly. It is not enough to rely on the surface of electronic components with a very small surface area to dissipate heat. At the same time, due to the large selection of surface-mounted electronic components such as QFP and BGA, the heat formed by electronic components is transferred to the PCB board in a large amount. For this reason, the most suitable way to relieve heat dissipation is to increase the PCB circuit board that is in direct contact with the heating electronic components. Its own heat dissipation capacity is conducted or emitted through the PCB circuit board.
2. High heat-generating electronic components plus radiator and heat conduction board. When a small number of electronic components in the PCB circuit board have a large heat value (less than 3), a heat sink or heat pipe can be added to the heating electronic components. When the temperature cannot be lowered, a fan can be selected. Radiator to enhance the heat dissipation effect. When the amount of heating electronic components is large (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 electronic components on the PCB circuit board or Cut out different electronic components on a large flat radiator.
The heat dissipation cover is integrally buckled on the surface of the electronic component, and it is in contact with each electronic component to dissipate heat. However, due to the poor consistency of height during assembly and welding of electronic components, the heat dissipation effect is not very good. Generally speaking, a soft thermal phase change thermal pad is added on the surface of the electronic component to improve the heat dissipation effect.
3. Choose a suitable wiring design to achieve heat dissipation. Because the resin in the plate has poor thermal conductivity, and the copper foil lines and holes are good heat conductors, increasing the remaining rate of copper foil and increasing the heat conduction holes are the main means of heat dissipation.
4. High heat dissipation electronic components should minimize the thermal resistance between them when they are connected to the substrate. In order to better meet the requirements of thermal characteristics, some thermal conductive materials (such as a layer of thermally conductive silica gel) can be used on the bottom surface of the chip, and a certain contact area should be maintained for electronic components to dissipate heat.
5. In the horizontal direction, the high-power devices are arranged as close to the edge of the printed board as possible, which can shorten the heat transfer path; in the vertical direction, the high-power devices are arranged as close to the top of the printed board as possible, which can reduce the work of such electronic components. The influence of time on the temperature of other electronic components.
6. The heat dissipation of the printed board in the equipment mainly relies on air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be reasonably configured. When air flows, it always tends to flow in places with low resistance, so when configuring devices on a printed circuit board, avoid leaving a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should also pay attention to the same problem.
7. Devices that are more sensitive to temperature are best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.
8. Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB board as much as possible, and keep the PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution during the design process, but areas with too high power density must be avoided to prevent hot spots from affecting the normal operation of the entire circuit.
