The electrical energy consumed by electronic equipment during operation, such as RF power amplifiers, FPGA chips, and power products, is mostly converted into heat dissipation in addition to useful work. The heat generated by the electronic device causes the internal temperature 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. SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and seriously affects the reliability of equipment. Therefore, research on thermal design is very important.
Brothers who engage in radio frequency have firewood, so can heat dissipation?
For the heat dissipation of the PCB circuit board is a very important part, then what is the heat dissipation technology of the PCB circuit board, let's discuss it together.
For electronic equipment, a certain amount of heat is generated during operation, so that the internal temperature of the equipment rises rapidly. If the heat is not released in time, the equipment will continue to heat up, the device will fail due to overheating, and the electronic equipment is reliable. Performance will drop. Therefore, it is very important to perform a good heat dissipation process on the board.
Second, the analysis of temperature rise factors of printed circuit boardsThe direct cause of the temperature rise of the printed board is due to the existence of circuit power consumption devices. The electronic devices have different degrees of power consumption, and the heat generation intensity varies with the 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.
2.1 Electrical power consumption
(1) Analysis of power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB.
2.2 Printed board structure
(1) the size of the printed board;
(2) The material of the printed board.
2.3 How to install the printed board
(1) Installation method (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from the casing.
2.4 Thermal radiation
(1) the emissivity of the surface of the printed board;
(2) The temperature difference between the printed board and the adjacent surface and their absolute temperature
2.5 heat conduction
(1) installing a radiator;
(2) Conduction of other mounting structural members.
2.6 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. 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 In actual conditions, parameters such as temperature rise and power consumption can be calculated or estimated correctly.
Third, some methods of PCB thermal design1 Cooling through the PCB board itself
Currently widely used PCB sheets are 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 properties. 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-heating 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.
2 high heat-generating device plus heat sink, heat-conducting plate
When there are a few devices 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 heat generating device. When the temperature cannot be lowered, a heat sink with a fan can be used to enhance heat dissipation. effect.
When the amount of heat generating devices is large (more than 3), a large heat sink (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 board or a large flat plate radiator. 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 the components during welding, the heat dissipation effect is not good. A soft thermal phase change thermal pad is usually added to the component surface to improve heat dissipation.
3 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.
4 Use reasonable wiring design to achieve heat dissipation
Since the resin in the sheet has poor thermal conductivity, and the copper foil line and the hole are good conductors of heat, increasing the copper foil residual ratio and increasing the heat conducting hole are the main means of heat dissipation.
To evaluate the heat dissipation capability of the PCB, it is necessary to calculate the equivalent thermal conductivity (nine eq) of the composite material composed of various materials having different thermal conductivity.
5 Devices on the same printed board should be arranged 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 in the cooling. The uppermost flow (in the inlet) of the airflow, the device with high heat or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) is placed at the most downstream of the cooling airflow.
6 In the horizontal direction, the high-power devices are placed as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board to reduce the temperature of other devices during operation of these devices. influences.
7 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 during the design, and the device or printed circuit board 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.
8 Temperature sensitive devices should be placed in the lowest temperature area (such as the bottom of the device), do not place it directly above the heating device, and multiple devices are preferably staggered in a horizontal plane.
9 Place the device with the highest power consumption and maximum heat generation near the best heat dissipation position. Do not place higher heat generating components 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.
10 RF power amplifier or LED PCB uses a metal base substrate.
11 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.
4.1 Material selection
(1) The temperature rise caused by the current passing through the conductor of the printed board plus the specified ambient temperature shall not exceed 125 °C (typical values ​​commonly used. Depending on the plate selected may vary). Since the components are mounted on the printed board and also emit a portion of the heat that affects the operating temperature, these factors should be considered when selecting materials and printed board designs. The hot spot temperature should not exceed 125 °C. Choose a thicker copper foil as much as possible.
(2) In special cases, plates with low thermal resistance such as aluminum base and ceramic base can be selected.
(3) The use of a multi-layer board structure contributes to the thermal design of the PCB.
4.2 Ensure that the heat dissipation channel is unblocked
(1) Make full use of components, copper, window and heat dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure the smooth export of heat to the PCB.
(2) Setting of heat dissipation through holes Some heat dissipation through holes and blind holes are designed to effectively improve the heat dissipation area and reduce the thermal resistance, and improve the power density of the circuit board. For example, a via hole is formed on the pad of the LCCC device. During the production process of the circuit, the solder fills it, so that the thermal conductivity is improved, and the heat generated during the operation of the circuit can be quickly transmitted to the metal heat dissipation layer or the copper provided on the back surface through the through hole or the blind hole. In some specific cases, a circuit board with a heat dissipation layer is specially designed and used, and the heat dissipation material is generally a material such as copper/molybdenum, such as a printed board used in some module power supplies.
(3) Use of heat conductive material In order to reduce the thermal resistance of the heat conduction process, a heat conductive material is used on the contact surface of the high power consumption device and the substrate to improve heat transfer efficiency.
(4) The process method is likely to cause local high temperature in some areas with double-sided devices. In order to improve the heat dissipation condition, a small amount of fine copper material may be mixed in the solder paste, and after soldering, there is a certain solder joint under the device. height. Increases the gap between the device and the printed board, increasing convection heat dissipation.
4.3 component layout requirements
(1) Perform software thermal analysis on the PCB to design and control the internal maximum temperature rise;
(2) It can be considered to design and install components with high heat and high radiation on a printed board;
(3) The heat capacity of the board surface is evenly distributed. Be careful not to place the large power consumption devices in a centralized manner. If it is unavoidable, place the short components upstream of the airflow and ensure that sufficient cooling airflow flows through the heat dissipation concentration area;
(4) making the heat transfer path as short as possible;
(5) making the heat transfer cross section as large as possible;
(6) The layout of components should take into account the effects of heat radiation on surrounding components. Heat sensitive parts and components (including semiconductor devices) should be kept away from heat sources or isolated;
(7) (liquid medium) capacitors are preferably kept away from heat sources;
(8) Pay attention to the direction of forced ventilation and natural ventilation;
(9) The additional sub-board and device air duct are in the same direction as the ventilation;
(10) Make the intake air and exhaust gas have a sufficient distance as much as possible;
(11) The heat-generating device should be placed as far as possible above the product, and should be in the airflow path if conditions permit;
(12) Components with large heat or large current should not be placed on the corners and surrounding edges of the printed board. If possible, they should be installed on the heat sink and away from other devices, and ensure that the heat dissipation channel is unobstructed;
(13) (small signal amplifier peripheral devices) try to use devices with small temperature drift;
(14) Use metal chassis or chassis to dissipate heat as much as possible.
4.4 Requirements for wiring
(1) Plate selection (reasonable design of printed board structure);
(2) wiring rules;
(3) Plan the minimum channel width according to the device current density; pay special attention to the channel wiring at the joint;
(4) The high current lines should be surfaced as much as possible; if the requirements are not met, the bus bar can be considered;
(5) Try to reduce the thermal resistance of the contact surface. To this end, the heat conduction area should be increased; the contact plane should be smooth and smooth, and if necessary, coated with thermal grease;
(6) Thermal stress points consider stress balance measures and bold lines;
(7) The heat-dissipating copper skin needs to adopt the window-opening method of adiabatic stress, and the window is opened by heat-dissipating soldering;
(8) It is possible to use a large area of ​​copper foil on the surface;
(9) Use a larger pad on the grounding mounting hole on the printed board to fully utilize the mounting bolts and the copper foil on the surface of the printed board for heat dissipation;
(10) Place as many metallized vias as possible, and the aperture and disk surface should be as large as possible, and rely on the via holes to help dissipate heat;
(11) means for heat dissipation of the device;
(12) In the case where large-area copper foil on the surface can be used, the method of attaching a heat sink may be omitted for economic reasons;
(13) Calculate the appropriate surface heat dissipation copper foil area according to device power consumption, ambient temperature and maximum junction temperature (guaranteed principle tj ≤ (0.5 ~ 0.8) tjmax).
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