PCBA processing lecture hall: the common heat dissipation method of circuit board

When we use electronic equipment, we often find that the electronic equipment will generate a certain amount of heat after continuous use for a period of time, so that the internal temperature of the equipment will rise rapidly, if the heat is not dissipated in time, the equipment will continue to heat up, the device will fail due to overheating, and the reliable performance of the electronic equipment will decline. Therefore, it is very important to perform a good heat dissipation treatment on the circuit board. The heat dissipation of PCB circuit board is a very important link, then how does the circuit board heat dissipation, and the common heat dissipation method of the road board1, high heating device plus radiator, heat conduction plate.

When there are a few (less than 3) devices in the PCB heat, you can add a radiator or heat conduction tube on the heating device, if the temperature can not come down, we can use a radiator with a fan to enhance the heat dissipation effect.

When the amount of heating devices is large (more than 3), a large heat shield (plate) can be used, which is a special radiator customized according to the position and height of the heating device on the circuit board, or a large flat radiator to cut out the height of different components. The heat shield is fastened on the component surface as a whole, and the heat is dissipated by contact with each component. However, because the components are installed and welded at different levels, the heat dissipation effect is not good. Usually add a soft thermal phase change thermal pad on the surface of the component to improve the heat dissipation effect.

2, heat dissipation through the circuit board itself

At present, the widely used circuit board substrate is copper-coated/epoxy glass cloth substrate or phenolic resin glass cloth substrate, and there is a small amount of paper-based copper-coated sheet.

Although these substrates have excellent electrical properties and processing properties, but the heat dissipation is relatively poor, as a heat dissipation way of high heating components, almost can not be expected to be conducted by the PCB itself resin heat, but from the surface of the component to the surrounding air heat dissipation.

However, as electronic products have become more and more prone to component miniaturization, high-density installation, and high-heat assembly, it is far from enough to only rely on the surface surface of a very small component to dissipate heat, and due to the large use of surface mounted components such as QFP and BGA, the heat generated by components is transmitted to the circuit board in large quantities. The best way to solve the heat dissipation is to improve the heat dissipation capacity of the PCB itself in direct contact with the heating element, which is transmitted or emitted through the circuit board.

3, the use of reasonable routing design to achieve heat dissipation function

Since the thermal conductivity of the resin in the plate is poor, and the copper foil line and hole are excellent conductors of heat, improving the residual rate of copper foil and increasing the thermal conductivity hole are the main means of heat dissipation.

If you want to evaluate the heat dissipation capacity of a PCB, it is necessary to calculate the equivalent thermal conductivity (9 eq) of the PCB insulating substrate composed of various materials with different thermal conductivity.

4, for the use of free convection air cooling equipment, it is best to arrange the integrated circuit (or other devices) according to the longitudinal length, or according to the transverse length.

5, the device on the same printed circuit board should be arranged as far as possible according to the size of its heat and the degree of heat dissipation, small heat or poor heat resistance devices (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) placed in the most upstream of the cooling air flow (entrance), Devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the downstream of the cooling stream.

6, in the horizontal direction, the high-power device is arranged near the edge of the printed circuit board as far as possible, in order to shorten the path of heat transfer; In the vertical direction, the high-power devices are arranged near the top of the printed circuit board as far as possible, which can reduce the temperature that affects other devices when these devices work.

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

8, the heat dissipation of the printed circuit board in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably configured. When the air flows, it always tends to flow where the resistance is low, so when configuring the device on the printed circuit board, it is necessary to 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.

9, without the concentration of hot spots on the PCB board, the power is evenly distributed on the circuit board as far as possible, and the surface temperature performance of the PCB board is 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 to prevent hot spots from affecting the normal operation of the entire circuit. If conditions exist, it is necessary to carry out thermal performance analysis of printed circuits, such as the thermal performance index analysis software module added to some professional PCB design software, which can help designers improve circuit design.

10. Arrange the components with the highest power consumption and maximum heat dissipation near the best location for heat dissipation. Do not place high heating components on the corners and edges of the printed circuit board, unless a cooling device is arranged near it. When designing the power resistance, select as large components as possible, and adjust the layout of the printed circuit board so that there is enough space for heat dissipation.

11, high heat dissipation components should be as much as possible to reduce the thermal resistance between them when connected with the substrate. In order to better meet the requirements of thermal characteristics, some thermal conductive materials (such as applying a layer of thermal silicone) can be used on the bottom surface of the chip, and a certain contact area is maintained for heat dissipation of the components.

12, the connection of components and substrate: 

1) shorten the length of the component lead as much as possible; 

2) When selecting high-power components, the thermal conductivity of the lead material should be taken into account, and if possible, the largest lead transverse surface should be selected as far as possible;

3) Select components with more pins.

13, the package selection of components: 

(1) In consideration of thermal design should pay attention to the package description of components and its thermal conductivity; 

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

(3) There should be no air partition on the heat conduction path, and if there is such a case, thermal conductive materials can be used to fill.

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