PCB Blog - PCB board heat dissipation design skills

1. The importance of PCB board thermal design
The electrical energy consumed by electronic equipment during operation, such as radio frequency power amplifiers, FPGA chips, and power products, in addition to useful work, is mostly converted into heat for dissipation. The heat generated by the electronic equipment causes the internal temperature 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 decrease. SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and the temperature rise of the equipment seriously affects the reliability. Therefore, the research on thermal design is very important. The heat dissipation of the PCB board is a very important link, so what is the heat dissipation technique of the PCB board 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 dissipated 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 decrease. Therefore, it is very important to conduct a good heat dissipation treatment on the circuit board.

2. Analysis of PCB board temperature rise factors
The direct cause of the temperature rise of the printed board is due to the existence of circuit power consumption devices. Electronic devices all have power consumption to varying degrees, and the heating intensity varies with the size of 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 the thermal power consumption of the PCB board, it is generally analyzed from the following aspects.
2.1 Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB.
2.2 The structure of the printed board
(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) The sealing condition and the distance from the casing.
2.4 Thermal radiation
(1) The emissivity of the printed board surface;
(2) The temperature difference between the printed board and the adjacent surface and their temperature
2.5 Heat conduction
(1) Install the radiator;
(2) Conduction of other installation structures.
2.6 Thermal convection
(1) Natural convection;
(2) Forced cooling convection.
The analysis of the above-mentioned factors from the PCB board is an effective way to solve the temperature rise of the printed board. These factors are often related and dependent on each other in a product and system. Most of the factors should be analyzed according to the actual situation. The specific actual conditions can be more accurately calculated or estimated parameters such as temperature rise and power consumption.

3. Some methods of PCB board thermal design
1. Heat dissipation through the PCB board itself
At present, the widely used PCB boards are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad boards are used. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat dissipation method for high-heating components, it is almost impossible to expect heat from the resin of the PCB itself to conduct heat, but to dissipate heat from the surface of the component to the surrounding air. However, as electronic products have entered the era of miniaturization of components, high-density mounting, and high-heating assembly, it is not enough to rely on the surface of a component with a very small surface area to dissipate heat. 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 in a large amount. Therefore, the solution to heat dissipation is to improve the heat dissipation capacity of the PCB board directly in contact with the heating element, and conduct it through the PCB board. Go out or send out.
2. High heat-generating components plus radiator and heat conduction plate
When a small number of components in the PCB board generate a large amount of heat (less than 3), a radiator or heat pipe can be added to the heating component. When the temperature cannot be lowered, a radiator with a fan can be used to enhance heat radiation. When the number of heating devices is large (more than 3), a large heat dissipation cover (board) can be used, which is a special heat sink customized according to the position and height of the heating device on the PCB or a large flat heat sink Cut out different component height positions. The heat dissipation cover is integrally buckled on the surface of the element, and it is in contact with each element to dissipate heat. However, the heat dissipation effect is not good due to the poor consistency of height during assembly and welding of components. Usually, a soft thermal phase change thermal pad is added on the surface of the component to improve the heat dissipation effect.
3. For equipment that adopts free convection air cooling, the integrated circuits (or other devices) are arranged vertically or horizontally.
4. Use reasonable wiring design to achieve heat dissipation
Because the resin in the plate has poor thermal conductivity, and the copper foil lines and holes are good conductors of heat, increasing the remaining rate of the copper foil and increasing the heat-conducting holes are the main means of heat dissipation. To evaluate the heat dissipation capacity of a PCB board, it is necessary to calculate the equivalent thermal conductivity of a composite material composed of various materials with different thermal conductivity-an insulating substrate for a PCB board.
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 low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed On the upstream (inlet) of the cooling airflow, devices with large heat or 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 placed 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 placed as close as possible to the top of the printed board to reduce the temperature of other devices when these devices work. Impact.
7. The heat dissipation of the printed board in the equipment mainly relies on air flow, so the air flow path should be studied during the design, and the device or printed circuit board should be reasonably configured. When air flows, it always tends to flow in places with low resistance, so when configuring devices 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. The temperature-sensitive device is placed in a temperature area (such as the bottom of the device). Never place it directly above the heating device. Multiple devices are arranged in a staggered horizontal plane.
9. Arrange power dissipation and heat-generating components near the heat dissipation location. Do not place high-heating devices on the corners and peripheral edges of the printed board, unless a heat sink is arranged near it. When designing the power resistor, choose a larger device as much as possible, and make it have enough space for heat dissipation when adjusting the layout of the printed board.
10. The RF power amplifier or LED PCB board adopts a metal base substrate.
11. Avoid the concentration of hot spots on the PCB board, distribute the power evenly on the PCB board as much as possible, and keep the surface temperature performance of the PCB board uniform and consistent. It is often difficult to achieve strict uniform distribution during the design process, but areas with too high power density must be avoided to prevent hot spots from affecting the normal operation of the entire circuit. If possible, it is necessary to analyze the thermal performance of printed circuits. For example, the thermal performance index analysis software module added in some PCB board design software can help designers optimize the circuit design.

4. Summary
4.1 Material selection
(1) The temperature rise of the wires of the PCB board due to the passing current plus the specified ambient temperature should not exceed 125 degree Celsius (commonly used typical value. It may be different depending on the selected board). Since the components installed on the printed board also emit some heat, which affects the operating temperature, these factors should be considered when selecting materials and the design of the printed board, and the hot spot temperature should not exceed 125 degree Celsius. Choose thicker copper clad as much as possible.
(2) In special cases, aluminum-based, ceramic-based, and other plates with low thermal resistance can be selected.
(3) Adopting multilayer board structure is helpful for PCB board thermal design.
4.2 Ensure that the heat dissipation channel is unblocked
(1) Make full use of the components arrangement, copper skin, window opening and heat dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure that the heat is smoothly exported from the PCB board.
(2) The setting of heat dissipation through holes Design some heat dissipation through holes and blind holes, which can effectively increase the heat dissipation area and reduce the thermal resistance, and increase the power density of the circuit board. For example, set up via holes on the pads of LCCC devices. Solder fills it in the circuit production process to improve the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper pad on the back through the through holes or blind holes to be dissipated. 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 on some module power supplies.
(3) The use of thermally conductive materials In order to reduce the thermal resistance in 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 heat conduction efficiency.
(4) The process method is likely to cause local high temperature in some areas where the device is mounted on both sides. In order to improve the heat dissipation conditions, a small amount of small copper can be mixed into the solder paste, and there will be a certain amount of solder joints under the device after flow soldering. high. The gap between the device and the printed board is increased, and the convection heat dissipation is increased.
4.3 Arrangement requirements of components
(1) Perform software thermal analysis on the PCB board, and design and control the internal temperature rise;
(2) It can be considered to specially design and install components with high heat generation and large radiation on a printed circuit board;
(3) The heat capacity of the board is evenly distributed. Be careful not to place high-power components in a concentrated manner. If it is unavoidable, place short components upstream of the airflow and ensure that sufficient cooling air flows through the heat-consumption concentrated 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 of heat radiation on surrounding parts. Heat sensitive parts and components (including semiconductor devices) should be kept away from heat sources or isolated;
(7) (Liquid medium) Keep the capacitor away from the heat source;
(8) Pay attention to the direction of forced ventilation and natural ventilation;
(9) The additional sub-boards and device air ducts are in the same direction as the ventilation;
(10) As far as possible, make the intake and exhaust have a sufficient distance;
(11) The heating device should be placed above the product as much as possible, and should be placed in the air flow channel when conditions permit;
(12) Components with high heat or high current should not be placed on the corners and peripheral edges of the printed board. They should be installed on the radiator as long as possible, and kept away from other components, 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) Board selection (reasonable design of printed board structure);
(2) Wiring rules;
(3) Plan the channel width according to the current density of the device; pay special attention to the channel wiring at the junction;
(4) The high-current lines should be as surface as possible; if the requirements cannot be met, the use of bus bars can be considered;
(5) To minimize the thermal resistance of the contact surface. For this reason, the heat conduction area should be enlarged; the contact surface should be flat and smooth, and thermally conductive silicone grease can be coated if necessary;
(6) Consider stress balance measures for thermal stress points and thicken the lines;
(7) The heat-dissipating copper skin needs to adopt the window method of heat dissipation stress, and use the heat-dissipating solder mask to open the window properly;
(8) If possible, use large-area copper foil on the surface;
(9) Use larger pads for the ground mounting holes on the printed board to make full use of the mounting bolts and the copper foil on the surface of the printed board for heat dissipation;
(10) Place as many metalized vias as possible, and the aperture and disk surface should be as large as possible, relying on vias to help heat dissipation;
(11) Supplementary means for device heat dissipation;
(12) In the case that the large-area copper foil on the surface can be guaranteed, the method of adding a heat sink may not be used for economic considerations;
(13) Calculate the appropriate heat dissipation copper area of the PCB board surface according to the power consumption of the device, the ambient temperature and the allowable junction temperature (guarantee principle tj≤(0.5～0.8)tjmax).