PCB News - PCB production does not pay attention to thermal design, which is disrespect for product performance

There are three main sources of heat in PCB production: (1) the heating of electronic components; (2) the heating of the PCB itself; (3) the heat transferred from other parts.

Among the three heat sources, the components generate the largest amount of heat and are the main heat source, followed by the heat generated by the PCB board. The heat transferred from the outside 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 take appropriate measures and methods to reduce the temperature of the components and the temperature of the PCB board, so that the system can work normally at a suitable temperature. It can be considered from the following aspects:

PCB production

1. Heat dissipation through the PCB board itself. At present, the widely used PCB boards are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad boards are used. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat dissipation path for high-heating components, it is almost impossible to expect heat from the resin of the PCB itself to conduct heat, 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 mounting, and high-heating assembly, it is not enough to rely on the surface of a component with a very small surface area to dissipate heat. At the same time, due to the extensive use of surface mount components such as QFP and BGA, a large amount of heat generated by the components is transferred to the PCB board. Therefore, the best way to solve the problem of heat dissipation is to improve the heat dissipation capacity of the PCB itself, which is in direct contact with the heating element, through the PCB board. To be transmitted or emitted.

2. High heat-generating device plus radiator and heat conduction board. When a small number of components in the PCB generate a large amount of heat (less than 3), a heat sink or heat pipe can be added to the heating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance heat dissipation Effect. When the number of heating devices is large (more than 3), a large heat dissipation cover (board) can be used, which is a special heat sink customized according to the position and height of the heating device on the PCB or a large flat heat sink Cut out different component height positions.

The heat dissipation cover is integrally buckled on the surface of the component, and it is in contact with each component to dissipate heat. However, the heat dissipation effect is not good due to the poor consistency of height during assembly and welding of components. Usually, a soft thermal phase change thermal pad is added on the surface of the component to improve the heat dissipation effect.

3. Use reasonable 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 conductors of heat, increasing the remaining rate of copper foil and increasing the heat conduction holes are the main means of heat dissipation.

4. When connecting high heat dissipation devices with the substrate, the thermal resistance between them should be reduced as much as possible. In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as applying a layer of thermal silica gel) can be used on the bottom surface of the chip, and a certain contact area is maintained for the device to dissipate heat.

5. In the horizontal direction, high-power devices are arranged as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices are arranged as close as possible to the top of the printed board to reduce the temperature of other devices when these devices work. Impact.

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.