PCB Tech - What are the cooling modes of the circuit board

1. High heating device with heat sink and heat conduction plate

When there are a few components in the PCB with high heat (less than 3), heat sink or heat conduction tube can be added to the heating device. When the temperature cannot be lowered, heat sink with fan can be used to enhance the heat dissipation effect. When the number of heating devices is large (more than 3), a large heat sink (plate) can be used. It is a special radiator customized according to the position and height of the heating device on the PCB board or a large flat radiator to cut out different component height position. The heat dissipation cover is buckled on the component surface as a whole, and the heat dissipation is in contact with each component. However, the heat dissipation effect is not good because of the poor consistency of components. Soft thermal phase change pad is usually added on the surface of the component to improve the heat dissipation effect.

2. Heat dissipation through the PCB board

At present, PCB boards widely used are copper-coated/epoxy glass cloth or phenolic resin glass cloth, and a small amount of paper-coated copper board. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a way of heat dissipation for high-heating components, heat can hardly be expected to be transmitted by the RESIN of the PCB itself, but heat dissipation from the surface of the components to the surrounding air. However, as electronic products have entered the era of component miniaturization, high-density installation and high thermal assembly, it is not enough to dissipate heat only by the surface of components with very small surface area. At the same time, due to the extensive use of surface mounted components such as QFP and BGA, a large amount of heat generated by components is transmitted to PCB board. Therefore, the best way to solve the heat dissipation problem is to improve the heat dissipation capacity of THE PCB directly in contact with the heating element, and conduct or emit it through PCB board.

3. Adopt reasonable wiring design to achieve heat dissipation

Because the resin in the sheet has poor thermal conductivity, and copper foil lines and holes are good heat conductors, so improving the residual rate of copper foil and increasing heat conduction holes are the main means of heat dissipation.

To evaluate the heat dissipation capacity of PCB, it is necessary to calculate the equivalent thermal conductivity coefficient (nine eq) of insulating substrate for PCB, which is composed of various materials with different thermal conductivity.

4. For equipment cooled by free convection air, it is best to arrange integrated circuits (or other devices) in either longitudinal or transverse lengths.

5. The devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. The devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the top of the cooling air flow (entrance). Devices with high calorific value 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 should be 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 printed board, so as to reduce the influence of these devices on the temperature of other devices when they work.

7. The temperature sensitive device is best placed in the lowest temperature area (such as the bottom of the equipment), do not put it on the heating device is directly above, multiple devices are best staggered layout on the horizontal plane.

8. The heat dissipation of the printed board in the equipment mainly depends on 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 small, so when configuring devices on printed circuit boards, avoid having a large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should pay attention to the same problem.

9. Avoid concentration of hot spots on PCB, distribute power evenly on PCB board as far as possible, and keep PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid areas with too high power density, so as not to affect the normal operation of the whole circuit. If possible, it is necessary to analyze the thermal performance of printed circuit. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize circuit design.

10. Place the devices with the highest power consumption and heat dissipation near the best position for heat dissipation. Do not place hot components in the corners and edges of the printed board unless there is a cooling device near it. In the design of the power resistance as large as possible to choose a larger device, and in the adjustment of the printed board layout so that there is enough space for heat dissipation.

11. High heat dissipation devices should be connected to the substrate to minimize the thermal resistance between them. To better meet the requirements of thermal characteristics, some thermal conductivity materials (such as a layer of thermal conductivity silica gel) can be used on the bottom of the chip, and a certain contact area can be maintained for heat dissipation of the device.

12. Connection between device and substrate:

(1) Shorten the lead length of the device as far as possible;

(2) When selecting high-power devices, the thermal conductivity of the lead material should be considered, and the maximum cross section of the lead should be selected as far as possible;

(3) Select devices with more pins.

13. Device packaging selection:

(1) When considering thermal design, attention should be paid to the package description of the device and its thermal conductivity;

(2) consideration should be given to providing a good heat conduction path between the substrate and device package;

(3) In the heat conduction path should avoid air partition, if this situation can be filled with thermal conductive materials.