The heat generated during the operation of electronic equipment causes the internal temperature of the equipment to 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 reliability of the electronic equipment will decline. Therefore, it is important to dissipate heat from the circuit board.
I. Factor analysis of temperature rise of printed circuit board
The direct cause of the temperature rise of the printed board is due to the existence of circuit power consumption devices, electronic devices have power consumption to varying degrees, and the heating intensity changes with the size of power consumption.
Two phenomena of temperature rise in printed boards:
(1) Local temperature rise 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.
Electrical power
(1) Analysis of power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB circuit board.
2. The structure of the printed board
(1) the size of the printed board;
(2) Materials for printed boards.
3.Installation of printed boards
(1) Installation method (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from the case.
4. Heat radiation
(1) the radiation coefficient on the surface of the printed board;
(2) The temperature difference between the printed board and the adjacent surface and their absolute temperature;
5 heat conduction
(1) Install the radiator;
(2) Conduction of other mounting structures.
6. Thermal 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 the printed board. Often these factors are interrelated and dependent in a product and system. Most of the factors should be analyzed according to the actual situation, only for a specific specific Only the actual situation can correctly calculate or estimate parameters such as temperature rise and power consumption.
Circuit board cooling method
1. High heat generating device plus heat sink and heat conduction plate
When there are a few devices in the PCB with a large amount of heat (less than 3), a heat sink or a heat pipe can be added to the heating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance heat dissipation. effect. When there are many heating devices (more than 3), a large heat shield (board) can be used. It is a dedicated heat sink customized according to the position and height of the heating device on the PCB board or a large flat heat sink. Cut out different component height positions. The heat sink is buckled on the surface of the component as a whole, and contacts each component to dissipate heat. However, the heat dissipation effect is not good due to the poor consistency of the components during assembly and welding. Generally, a soft thermal phase change thermal pad is added on the component surface to improve the heat dissipation effect.
2. Heat dissipation through the PCB board itself
At present, the widely used PCB boards are copper-clad / epoxy glass cloth substrate or phenolic resin glass cloth substrate, and there are a few paper-based copper-clad boards used. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation properties. As a heat dissipation method for highly heat-generating components, it is almost impossible to rely on the PCB resin to conduct heat, but to dissipate heat from the surface of the components to the surrounding air. However, as electronic products have entered the era of component miniaturization, high-density mounting, and high-heat assembly, it is not enough to dissipate heat only on the surface of components with very small surface areas. At the same time, due to the large use of surface mount components such as QFP and BGA, the heat generated by the components is transferred to the PCB board. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capability of the PCB itself that is in direct contact with the heating element. Conducted or emitted.
3. Use reasonable wiring design to achieve heat dissipation
Due to the poor thermal conductivity of the resin in the plate, and the copper foil lines and holes are good conductors of heat, increasing the copper foil residual rate and increasing the heat conduction holes 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 (nine eq) of a composite material composed of various materials with different thermal conductivity, one by one, for an insulating substrate for a PCB.
4. For equipment using free convection air cooling, it is best to arrange integrated circuits (or other devices) in a lengthwise manner, or a horizontal lengthwise arrangement.
5. The devices on the same printed board should be arranged as much as possible according to the amount of heat generated and the degree of heat dissipation. Devices with low heat generation 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 airflow (at the entrance), devices (such as power transistors, large-scale integrated circuits, etc.) with high heat generation or good heat resistance are placed downstream of the cooling airflow.
6. In the horizontal direction, the high-power devices should be arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, the high-power devices should be arranged as close to the top of the printed board as possible to reduce the temperature of these devices when they are working. Impact.
7. It is better to place the temperature sensitive device in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. It is better to stagger the multiple devices on the horizontal plane.
8. The heat dissipation of the printed board in the device mainly depends on the air flow, so the air flow path should be researched in the design, and the device or printed circuit board should be reasonably configured. When air flows, it tends to flow in a place with low resistance, so when configuring the device on a printed circuit board, 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. Avoid the concentration of hot spots on the PCB, and distribute the power as evenly as possible on the PCB to keep the PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution during the design process, but it is necessary to avoid areas with too high power density to avoid excessive hot spots that affect the normal operation of the entire circuit. If possible, it is necessary to analyze the thermal efficiency of printed circuits. For example, the thermal efficiency index analysis software module added in some professional PCB design software can help designers optimize circuit design.
10. Place the device with the highest power consumption and heat generation near the best heat dissipation location. Do not place devices with high heat generation at the corners and peripheral edges of the printed board, unless a heat sink is arranged near it. When designing the power resistor, select a larger device as much as possible, and make sure that there is enough space for heat dissipation when adjusting the layout of the printed board.
11. High heat dissipation devices should minimize the thermal resistance between them when connected to the substrate. In order to better meet the thermal characteristics requirements, some thermally conductive materials (such as a layer of thermally conductive silicone) can be used on the bottom surface of the chip, and a certain contact area is maintained for the device to dissipate heat.
12. Device and substrate connection:
(1) Keep the lead length of the device as short as possible;
(2) When selecting high power consumption devices, the thermal conductivity of the lead material should be considered, and if possible, the maximum cross section of the lead should be selected as much as possible;
(3) Select a device with a large number of pins.
13 device package selection:
(1) When considering thermal design, pay attention 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 the device package;
(3) Air interruption should be avoided on the heat conduction path. If this is the case, it can be filled with a thermally conductive material.