Most of the electrical energy consumed by electronic devices during work, such as RF power amplifiers, FPGA chips, and power products, is converted into heat dissipation in addition to useful power. The heat generated by electronic equipment causes the internal temperature to rise rapidly. If this 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. SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and the temperature rise of the equipment seriously affects reliability. Therefore, research on thermal design is very important.
The brothers who engage in radio frequency have firewood, so can the heat be radiated?
The heat dissipation of PCB circuit board is a very important link, then what is the heat dissipation technique of PCB circuit board, let’s discuss it together.
For electronic equipment, a certain amount of heat is generated during work, which causes the internal temperature of the equipment to rise rapidly. If the heat is not released in time, the equipment will continue to heat up and the device will fail due to overheating. The reliability of the electronic equipment Performance will degrade. Therefore, it is very important to perform a good heat dissipation process on the 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.
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 Structure of the printed board
(1) the size of the printed board;
(2) Materials for printed boards.
2.3 Installation method of printed board
(1) Installation method (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from the case.
2.4 Thermal 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
2.5 Thermal conductivity
(1) Install the radiator;
(2) Conduction of other mounting structures.
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. 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.
1 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.
2High heating 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.
3 For equipment using free convection air cooling, it is best to arrange integrated circuits (or other devices) in a lengthwise manner, or in a horizontal lengthwise manner.
4Use 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.
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 cooled. At the top of the airflow (at the entrance), devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed downstream of the cooling airflow.
6 In the horizontal direction, high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices are arranged as close to the top of the printed board as possible to reduce the temperature of these devices on other devices. influences.
7 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.
8 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 heat-generating device. Multiple devices are preferably staggered on a horizontal plane.
9 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.
10 RF power amplifier or LED PCB adopts metal base substrate.
11 Avoid the concentration of hot spots on the PCB, and distribute the power as evenly as possible on the PCB to maintain uniform and consistent temperature performance on the PCB surface. 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.
4.1 Material selection
(1) The temperature rise of the conductors of the printed board due to the passing current plus the specified ambient temperature should not exceed 125 ℃ (the typical value commonly used. It may be different according to the selected board). As the components mounted on the printed board also emit a part of the heat, which affects the operating temperature, these factors should be considered when selecting materials and printed board design. The hot spot temperature should not exceed 125 ℃. Whenever possible, choose a thicker copper foil.
(2) Under special circumstances, aluminum-based, ceramic-based, and other materials with low thermal resistance can be selected.
(3) Adopting multilayer board structure helps PCB thermal design.
4.2 Ensure that the heat dissipation channel is unobstructed
(1) Make full use of technologies such as component layout, copper cover, window opening, and heat dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure that the heat is smoothly exported to the PCB.
(2) Setting of heat-dissipating through-holes Designing some heat-dissipating through-holes and blind holes can effectively increase the heat-dissipating area and reduce thermal resistance, and increase the power density of the circuit board. For example, set up a via hole on the pad of the LCCC device. During the circuit production process, the solder fills it up, which improves the thermal conductivity. The heat generated during the circuit operation can be quickly transferred to the metal heat dissipation layer or copper boluses on the backside through the through holes or blind holes. In some specific cases, a circuit board with a heat dissipation layer is specially designed and used. The heat dissipation material is generally copper / molybdenum and other materials, such as printed boards used in some module power supplies.
(3) Use of thermally conductive materials In order to reduce the thermal resistance of the thermal conduction process, thermally conductive materials are used on the contact surface between the high power consumption device and the substrate to improve the thermal conduction efficiency.
(4) The process method is likely to cause local high temperature in some areas with devices on both sides. In order to improve heat dissipation conditions, a small amount of fine copper can be added to the solder paste. After reflow soldering, there will be a certain amount of solder joints under the device. height. The gap between the device and the printed board is increased, and convection heat dissipation is increased.
4.3 Arrangement requirements of components
(1) Perform software thermal analysis on the PCB and perform design control on the highest internal temperature rise;
(2) Consider designing and installing components with high heat and radiation on a printed board;
(3) The heat capacity of the board is evenly distributed. Be careful not to place large power-consuming devices in a centralized manner. If it is unavoidable, place short components upstream of the airflow and ensure that sufficient cooling air flows through the heat-concentration area;
(4) Make the heat transfer path as short as possible;
(5) Make the heat transfer cross-section as large as possible;
(6) The layout of components should take into account the influence on the heat radiation of surrounding parts. Heat-sensitive components and components (including semiconductor devices) should be kept away from heat sources or isolated;
(7) (liquid medium) capacitors are best kept away from heat sources;
(8) Pay attention to make the direction of forced ventilation consistent with natural ventilation;
(9) The additional daughter board and device air duct are consistent with the ventilation direction;
(10) Adequate distance between intake and exhaust as much as possible;
(11) The heating device should be placed as far as possible above the product, and should be on the air flow channel when conditions permit;
(12) Do not place components with high heat or current at the corners and edges of printed boards. Whenever 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 case or chassis to dissipate heat as much as possible.
4.4 Requirements for wiring
(1) Board 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 junction;
(4) The large current lines should be surfaced as much as possible; if the requirements cannot be met, the use of bus bars can be considered;
(5) Try to reduce the thermal resistance of the contact surface as much as possible. To this end, the heat conduction area should be increased; the contact plane should be flat and smooth, and thermal grease can be coated if necessary;
(6) Thermal stress points consider stress balancing measures and bold lines;
(7) The heat dissipation copper skin needs to adopt the window opening method of heat dissipation stress, and the window is properly opened by heat radiation resistance welding;
(8) Depending on the surface, copper foil with large area may be used;
(9) Use larger pads for the ground mounting holes on the printed board to make full use of the mounting bolts and copper foil on the surface of the printed board for heat dissipation;
(10) Place as many metalized vias as possible, and make the aperture and disk surface as large as possible, relying on the vias to help heat dissipation;
(11) Supplementary means for device heat dissipation;
(12) In the case where the surface large-area copper foil can be used, the method of adding an additional heat sink may not be used for economic reasons;