Summary： We all know that the heat generated by the operation of the electronic device causes the internal temperature of the device to rise rapidly. If the heat is not dissipated in time, the device will continue to heat up, and the device will fail due to overheating, and the reliability of the electronic device will decrease. Therefore, it is very important to dissipate the board. Here are the relevant experiences shared by PCBHERO for you!
The direct factor for the temperature rise of PCB is the existence of power consumption parts and the heat generation intensity varies with the power consumption.
There are two phenomena of temperature rise.
1.Sectional temperature rise or full -area temperature rise;
2. Short-term temperature rise or long-term temperature rise.
For the detailed reasons, they are generally analyzed from the following aspects.
1.Electrical power consumption
(1) Analysis of power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB.
2.The structure of the PCB
(1) The size ;
(2) The material.
3. How to install the PCB
(1) Installation method (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from the casing.
4. Thermal radiation
(1) The emissivity of the surface of PCB;
(2)The temperature difference between PCB and the adjacent surface, and their absolute temperature;
5. Heat conduction
(1) Installing a radiator;
(2) Conduction of other mounting structural members.
6. Heat convection
(1) Natural convection;
(2) Forced cooling convection.
The analysis of the above factors from the PCB is an effective way to solve the temperature rise of PCB. PCBWay believes that these factors are often related and dependent in a product and system. Most of the factors should be analyzed according to the actual situation. Only for a specific actual situation can the parameters such as temperature rise and power consumption be calculated or estimated correctly.
High heat generating device with heat sink and heat conducting board
When there are a few parts 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 device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance the heat dissipation effect. When the amount of parts is large (more than 3), a large heat-dissipating cover (board) 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 pad is usually added to the component surface to improve heat dissipation.
Cooling through the PCB itself
Currently widely used PCB is 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 electrical properties and processing properties, they have poor heat dissipation. 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 electronic products have 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 number of surface mount components such as QFP and BGA, the heat generated by the components 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 components. Conducted out or emitted.
Use reasonable layout design to achieve heat dissipation
Because the resin in the board has poor thermal conductivity, and the copper line and the hole are good conductors of heat, PCBWay believes that increasing the copperresidual ratio and increasing the heat conduction hole are the main means of heat dissipation.
To evaluate the heat dissipation capability of a PCB, it is necessary to calculate the equivalent thermal conductivity of a composite material composed of various materials having different thermal conductivity coefficients.
For devices that use free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a vertically long manner or in a horizontally long manner.
Components on the same PCB should be placed 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, electrolytic capacitors, etc.) should be placed. The uppermost flow (at the inlet) of the cooling airflow, the device that generates a large amount of heat or heat (such as a power transistor, a large-scale integrated circuit, etc.) is placed at the most downstream of the cooling airflow.
In the horizontal direction, the high-power components are placed as close as possible to the edges of PCB to shorten the heat transfer path; in the vertical direction, the high-power components are placed as close as possible to the top of PCB to reduce the temperature of other components while these components are operating.
Temperature sensitive components 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.
The heat dissipation of the PCB 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 PCB 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.
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 printed circuits. For example, the thermal performance index analysis software modules added in some professional PCB design software can help designers optimize circuit design.
Place the components with the highest power consumption and maximum heat generation near the best location for heat dissipation. Do not place a component 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, choose a larger device as much as possible, and have enough space for heat dissipation when adjusting the layout of the printed board.
High heat dissipation devices should minimize the thermal resistance between them when they are connected to the substrate. In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as 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.
Connection of components and substrate
(1) Minimize the length of the components leads;
(2) When selecting a high-power components, the thermal conductivity of the lead material should be considered, and the largest cross-section of the lead should be selected as much as possible;
(3) Select a components with a large number of pins.
Device package selection
(1)When considering thermal design, pay attention to the package description of the components and its thermal conductivity;
(2)Consideration should be given to providing a good thermal conduction path between the substrate and the device package;
(3)Air partitions should be avoided on the heat conduction path. If this is the case, a heat conductive material can be used for filling.