The heat generated during the operation of electronic equipment makes the internal temperature of the equipment rise rapidly. If the heat is not released in time, the equipment will continue to heat up, and the devices will fail due to overheating, and the reliability of electronic equipment will decline. Therefore, it is very important to heat the circuit board.
I. Analysis of temperature rise factors of printed circuit board
The direct cause of PCB temperature rise is the existence of circuit power consumption devices, electronic devices have different degrees of power consumption, heating intensity varies with the power consumption.
Two phenomena of temperature rise in printed boards:
(1) local or large area temperature rise;
(2) short-term temperature rise or long-term temperature rise.
When analyzing PCB thermal power consumption, it is generally analyzed from the following aspects.
1. Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the power distribution on the PCB board.
2. Structure of printed board
(1) the size of the printed board;
(2) printed board materials.
3. Installation method of printed board
(1) Installation method (such as vertical installation, horizontal installation);
(2) sealing condition and distance from the housing.
4. Thermal radiation
(1) the radiation coefficient of the printed board surface;
(2) Temperature differences between the printed board and adjacent surfaces and their absolute temperatures;
5. Heat conduction
(1) Install radiator;
(2) Conduction of other mounting structural parts.
6. Heat convection
(1) natural convection;
(2) forced cooling convection.
The analysis of the above factors from PCB is an effective way to solve the printed board temperature rise, often in a product and system these factors are interrelated and dependent, most of the factors should be analyzed according to the actual situation, only for a specific actual situation to calculate or estimate the temperature rise and power consumption and other parameters.
Two, circuit board heat dissipation mode
1, high heating device with radiator, heat conduction plate
When the heat of a few components in PCB is large (less than 3), a radiator or heat conduction tube can be added to the heating component. When the temperature can not be lowered, a radiator with a fan can be used to enhance the heat dissipation effect. When the amount of heating devices is large (more than 3), you can use a large heat sink (plate), which is customized according to the position and height of the heating devices on the PCB board, or in a large flat radiator cut out different positions of the components. The radiator cover is fastened on the component surface as a whole, and contact with each component to dissipate heat. However, due to the low consistency of components during welding, the heat dissipation effect is not good. Soft thermal phase change pads are usually added on the surface of components to improve the heat dissipation effect.
2, through the PCB board itself heat dissipation
At present, PCB board is widely used in copper/epoxy glass cloth substrate or phenolic resin glass cloth substrate, there is a small amount of paper coated copper board. Although these substrates have excellent electrical properties and processing properties, but poor heat dissipation, as a heat dissipation way of high heating components, almost can not be expected from the PCB resin conduction heat, but from the surface of the components to the surrounding air heat dissipation. However, as electronic products have entered the era of component miniaturization, high-density installation and high thermal assembly, it is not enough to only rely on the surface of components with very small surface area to dissipate heat. At the same time, due to the large number of surface mounted components such as QFP, BGA, the heat generated by components to PCB board, therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of PCB in direct contact with the heating element, through the PCB board conduction out or out.
3, the use of reasonable wire design to achieve heat dissipation
Because the resin in the plate has poor thermal conductivity, and the copper foil line and hole are good conductors of heat, so improving the copper foil residual rate and increasing the heat conduction hole is the main means of heat dissipation.
To evaluate the heat dissipation capacity of PCB, it is necessary to calculate the equivalent thermal conductivity (9EQ) of the insulating substrate for PCB, which is composed of various materials with different thermal conductivity.
4. For the use of free convection air cooling equipment, it is best to arrange integrated circuits (or other devices) by the length of the way, or by the length of the way.
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. Devices with small calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the upper stream of the cooling flow (entrance). Devices with high calorific value or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the downstream of the cooling flow.
6. In the horizontal direction, the high-power devices should be arranged as close to the edge of the printed board as possible, so as to shorten the heat transfer path; In the vertical direction, the high-power components are arranged as close as possible to the top of the printed board, so as to reduce the impact of these components on the temperature of other components.