PCB thermal control during design, direct impact on PCB manufacturing

2019-08-23 16:15:40

By solving the effective way of temperature rise of printed boards in PCB design, these factors are often related and dependent on one PCB board and system. Most of the factors should be analyzed according to actual conditions, only for a specific actual situation. The situation can correctly calculate or estimate the parameters such as PCB temperature rise and power consumption.

1. PCB proofing production by selecting the heat dissipation of the PCB board itself

Currently widely used PCB sheets are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad sheets are used. Although these substrates have excellent performance and processing properties, they have poor heat dissipation properties. As a heat dissipation path for high-heat-generating components, it is hardly expected to conduct heat from the resin of the PCB itself, but to dissipate heat from the surface of the component to the surrounding air. However, as the product has entered the era of miniaturization, high-density mounting, and high-heat assembly, it is not enough to dissipate heat from the surface of a component with a very small surface area. At the same time, due to the large-scale use of surface mount components such as QFP and BGA, the generated heat is transferred to the PCB in a large amount. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capability of the PCB itself in direct contact with the heat generating component, and conduct it through the PCB board. Or exude it.

2. PCB design high heat-emitting device plus heat sink, heat-conducting plate

When there are a few devices in the PCB that generate a large amount of heat (less than 3), a heat sink or a heat pipe can be added to the heat generating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance heat dissipation. effect. When the amount of heat-generating devices is large (more than 3), a large heat-dissipating cover (plate) can be used, which is a dedicated heat sink customized according to the position and height of the heat-generating device on the PCB or a large flat-plate heat sink. The upper and lower parts of the different components are placed. The heat shield is integrally fastened to the component surface, and is in contact with each component to dissipate heat. However, due to the poor consistency of components during soldering, the heat dissipation effect is not good. A soft thermal phase change thermal pad is usually added to the component surface to improve heat dissipation.

3. For devices that use free convection air cooling, it is best to arrange (or other devices) in a vertically long manner or in a horizontally long manner.

4. Use reasonable wiring design to achieve heat dissipation

Since the resin in the sheet has poor thermal conductivity, and the copper foil line and the hole are good conductors of heat, increasing the copper foil residual ratio and increasing the heat conducting hole are the main means of heat dissipation.

To evaluate the heat dissipation capability of the PCB, it is necessary to calculate the equivalent thermal conductivity (nine eq) of the composite substrate composed of various materials with different thermal conductivity in the PCB design.

5. The devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation. Devices with low heat or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolysis, etc.) should be placed in the cooling. The uppermost flow (in the inlet) of the airflow, devices with high heat or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.

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

7. The heat dissipation of the printed circuit board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area. The same problem should be noted in the configuration of multiple printed circuit boards in the whole machine.

8. Temperature sensitive devices should be placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. Multiple devices are preferably staggered on a horizontal plane.

9. Place the device with the highest power consumption and maximum heat generation near the best position for heat dissipation. Do not place a device with a higher heat on the corners and peripheral edges of the printed board unless a heat sink is placed near it. When designing the power resistor of the PCB, choose a larger device as much as possible, and make it have enough heat dissipation space when adjusting the layout of the printed board.

10. The RF power amplifier or LED PCB uses a metal base substrate.

11. Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB as much as possible, and keep the temperature performance of the PCB surface uniform and consistent. It is often difficult to achieve a strict uniform distribution during the design process, but it is necessary to avoid areas where the power density is too high, so as to avoid the hot spots affecting the normal operation of the entire circuit. If necessary, it is necessary to perform thermal performance analysis of the printed circuit. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize the circuit board design.

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