The electrical energy consumed by electronic devices during work, such as rf power amplifier, FPGA chip and power supply products, is mostly converted into heat emission in addition to useful work. The heat generated by the electronic equipment causes the internal temperature 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, and the reliability of the electronic equipment will decline. SMT increases the installation density of electronic equipment, reduces the effective cooling area, and seriously affects the reliability of equipment temperature rise. Therefore, the research on thermal design is very important.
The brother that makes radio frequency has firewood, so come loose heat also ok?
The heat dissipation of PCB is a very important link. Then what is the heat dissipation technique of PCB? Let’s discuss it together.
For electronic equipment, a certain amount of heat will be generated during operation, so that the internal temperature of the equipment will rise rapidly. If the heat is not released in time, the equipment will continue to warm up, the device will be due to overheating and failure, the reliability of electronic equipment will decline. Therefore, the circuit board for a good cooling treatment is very important.
The direct cause of the PCB temperature rise is the existence of circuit power dissipation devices, electronic devices have different degrees of power dissipation, heating intensity with the size of power dissipation changes.
Two phenomena of temperature rise in printed board:
(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) power consumption per unit area is analyzed;
(2) analyze the distribution of power consumption on PCB.
2.2 PCB structure
(1) size of printed board;
(2) printed board materials.
2.3 installation of PCB
(1) installation method (such as vertical installation and horizontal installation);
(2) sealing condition and distance from the housing.
2.4 thermal radiation
(1) radiation coefficient of printed board surface;
(2) the temperature difference between the printed board and the adjacent surface and their absolute temperature
2.5 heat conduction
(1) install the radiator;
(2) conduction of other mounting structures.
2.6 heat convection
(1) natural convection;
(2) forced cooling convection.
The analysis of the above factors of PCB is an effective way to solve the temperature rise of PCB. These factors are often interrelated and dependent in a product and system. Most of the factors should be analyzed according to the actual situation.
Heat dissipation through PCB itself
The most widely used PCB boards are copper-clad/epoxy glass cloth or phenolic resin glass cloth, as well as a few paper-based copper sheets. Although these substrates have excellent electrical performance and processing performance, they have poor heat dissipation. As the heat dissipation path of high-heating components, it is almost impossible to expect heat conduction from the resin of PCB itself, but heat dissipation from the surface of components to the surrounding air. However, as electronic products have entered the era of miniaturization of components, high-density installation and high-frequency thermosetting assembly, it is not enough to rely on components with very small surface area to dissipate heat.
At the same time, due to the extensive use of surface mounted components such as QFP and BGA, a large number of heat generated by components is transferred to the PCB board. Therefore, the best way to solve the problem of heat dissipation is to improve the heat dissipation capacity of the PCB itself in direct contact with the heating components, and conduct or send out through the PCB board.
High heating device plus radiator, heat conducting plate
When there are a few components in PCB with high heat (less than 3), heat sink or heat conduction pipe can be added to the heating components. When the temperature cannot be lowered, the radiator with fan can be adopted to enhance the heat dissipation effect.
When the amount of heating components is large (more than 3), a large cooling cover (board) can be used. It is a special radiator customized according to the position and height of the heating components on the PCB board, or a large flat plate radiator to cut out different component positions.
The heat shield is fastened to the surface of the element to dissipate heat in contact with each element. However, due to the poor consistency in the installation and welding of components, the cooling effect is not good. Usually, a soft thermal phase-change thermal pad is added on the surface of components to improve the heat dissipation effect.
For equipment cooled by free convection air, it is better to arrange the integrated circuit (or other devices) in longitudinal or transverse length.
4. Reasonable wiring design is adopted to realize heat dissipation
Because the resin in the plate thermal conductivity is poor, and the copper foil line and hole is a good conductor of heat, so increase the copper foil residual rate and increase the thermal 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 (9 eq) of composite materials composed of various materials with different thermal conductivity.
Five of the same piece of PCB device should as far as possible according to the calorific value of the size and degree of heat partition, calorific value is small or poor heat resistance devices (such as small signal transistor, small-scale integrated circuit, electrolytic capacitors, etc.) on the cooling airflow of the best () at the entrance, calorific value big or good heat resistance devices (such as power transistor, large scale integrated circuit, etc.) in the most downstream cooling airflow.
6. In the horizontal direction, the high-power devices should be arranged as close as possible to the edge of the PCB, so as to shorten the heat transfer path; In the vertical direction, the high-power devices should be placed as close as possible to the top of the PCB in order to reduce the influence of these devices on the temperature of other devices.
The heat dissipation of the printed board in the equipment mainly depends on the air flow, so it is necessary to study the air flow path during the design, and properly configure the device or printed circuit board. Air flow tends to flow where the resistance is small, so in the printed circuit board configuration of the device, to avoid leaving a large area of airspace. The same problem should be paid attention to in the configuration of multiple printed circuit boards.
Temperature sensitive devices are best 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 best placed in the horizontal staggered layout.
9. The devices with the highest power consumption and maximum heat consumption are arranged near the optimal heat dissipation position. Do not place devices with high heat at the corners and edges of the PCB, unless a radiator is arranged nearby. Choose larger components when designing power resistors, and adjust the layout of the PCB to allow for adequate cooling.
10 rf power amplifier or LED PCB with metal base plate.
Avoid hot spots on PCB, distribute power evenly on PCB as far as possible, and keep PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid areas with high power density, so as to avoid hot spots that may affect the normal operation of the whole circuit. If possible, it is necessary to analyze the thermal performance of printed circuit. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize the circuit design.
4.1 select material
(1) the temperature rise of the PCB wire due to the current plus the specified ambient temperature shall not exceed 125 ℃(the typical value in common use). May be different according to the selected plate). Since the components installed on the PCB also emit part of the heat, affecting the operating temperature, the selection of materials and PCB design should take these factors into account, the hot spot temperature should not exceed 125 ℃. If possible, choose thicker copper foil.
(2) under special circumstances can choose aluminum base, ceramic base and other thermal resistance of small plate.
(3) multi-layer board structure is helpful for PCB thermal design.
4.2 ensure the cooling channel is smooth
(1) make full use of the technology of component arrangement, copper skin, window opening and cooling hole to establish a reasonable and effective low thermal resistance channel to ensure the smooth heat export of PCB.
(2) the design of some heat dissipation through hole and blind hole, can effectively improve the heat dissipation area and reduce the thermal resistance, improve the power density of the circuit board. For example, a guide hole is set on the welding pad of LCCC device. In the process of circuit production, the solder will be filled to improve the heat conduction capacity. The heat generated during the circuit operation can be quickly transferred to the metal heat dissipation layer through the through hole or blind hole or the copper on the back side. In some specific cases, specially designed and used with a heat dissipation layer of the circuit board, heat dissipation materials are generally copper/molybdenum materials, such as some modules used on the power supply PCB.
(3) use of heat-conducting materials in order to reduce the thermal resistance in the heat conduction process, heat-conducting materials are used on the contact surface between high-power devices and the substrate to improve the heat conduction efficiency.
(4) the process method is easy to cause local high temperature in some areas where devices are installed on both sides. In order to improve the heat dissipation condition, a small amount of fine copper can be mixed into the welding paste, and the welding spot below the device will have a certain height after the flow welding. The gap between the device and the PCB increases, which increases the convection cooling.
4.3 configuration requirements of components
(1) conduct software thermal analysis on PCB and design and control the internal maximum temperature rise;
(2) the components with high heating and high radiation can be considered to be specially designed for mounting on a printed board;
(3) the heat capacity of the plate surface is evenly distributed, so pay attention not to distribute the devices with high power consumption centrally. If it is unavoidable, the low components should be placed in the upstream of the airflow, and sufficient cooling air should be guaranteed to flow through the heat consumption 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 shall take into account the influence of thermal radiation on surrounding parts. Heat-sensitive components and components (including semiconductor devices) shall be kept away from or isolated from heat sources;
(7)(liquid medium) the best capacitor away from the heat source;
(8) pay attention to the alignment between forced ventilation and natural ventilation;
(9) the air duct of the additional sub-board and device shall be in the same ventilation direction;
(10) make the intake and exhaust as far as possible to have enough distance;
(11) the heating device shall be placed above the product as far as possible, and shall be in the gas channel when conditions permit;
(12) components with large heat or current shall not be placed in the corners and surrounding edges of the PCB. As long as possible, they shall be installed on the radiator and away from other devices to ensure smooth heat dissipation channel;
(13)(peripheral devices of small signal amplifier) use devices with small temperature drift as far as possible;
(14) use metal chassis or chassis to dissipate heat as much as possible.
4.4 wiring requirements
(1) plate selection (reasonable design of printed board structure);
(2) wiring rules;
(3) plan the minimum channel width according to the current density of the device; Pay special attention to channel wiring at junction points;
(4) large current lines should be as superficial as possible; In the case that the requirements can not be met, bus may be considered;
(5) the thermal resistance of the contact surface shall be minimized. Therefore, the area of heat conduction should be increased; The contact surface should be smooth and smooth, and thermal conductive silicone grease can be coated when necessary.
(6) consider stress balance measures at thermal stress points and add bold lines;
(7) the heat dissipation copper skin shall adopt the method of reducing thermal stress to open the window, and the heat dissipation resistance welding shall be used to open the window properly;
(8) copper foil with large surface area as possible;
(9) a larger welding pad is used for grounding mounting holes on the PCB to make full use of mounting bolts and copper foil on the PCB surface for heat dissipation;
(10) place as many metallized through holes as possible, with the aperture and disk surface as large as possible, and rely on the through hole to help heat dissipation;
(11) device cooling supplementary means;
(12) if the use of copper foil with large surface area can be guaranteed, the method of additional radiator is not adopted for economic considerations;