PCB heat dissipation tips
The heat generated by electronic equipment during operation 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 decrease. Therefore, it is very important to dissipate the heat of the circuit board.
1.Analysis of factors causing temperature rise of printed circuit boards
The direct cause of temperature rise of printed circuit boards is the existence of circuit power consumption devices. Electronic devices all have power consumption to varying degrees, and the intensity of heat generation varies with the size of power consumption.
2.phenomena of temperature rise in printed circuit 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, the following aspects are generally analyzed.
3.Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB circuit board.
4.Structure of printed circuit boards
(1) Size of printed circuit boards;
(2) Material of printed circuit boards.
5.Installation method of printed circuit boards
(1) Installation method (such as vertical installation, horizontal installation);
(2) Sealing condition and distance from the casing.
6.Heat radiation
(1) Emissivity of the printed circuit board surface;
(2) Temperature difference between the printed circuit board and adjacent surfaces and their absolute temperature;
7.Heat conduction
(1) Installing a heat sink;
(2) Conduction by other mounting structures.
8.Heat convection
(1) Natural convection;
(2) Forced cooling convection.
Analysis of the above factors from the PCB is an effective way to solve the temperature rise of the printed circuit board. These factors are often interrelated and dependent in a product and system. Most factors should be analyzed based on actual conditions. Only for a specific actual situation can the temperature rise and power consumption parameters be calculated or estimated more accurately.
9.Circuit board heat dissipation method
High heat generation device plus heat sink, heat conduction plate
When there are a few devices in the PCB that generate a lot of heat (less than 3), a heat sink or 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 the heat dissipation effect. When there are many heating components (more than 3), a large heat sink (plate) can be used. It is a special heat sink customized according to the position and height of the heating components on the PCB board, or different component height positions are cut out on a large flat heat sink. The heat sink is buckled on the component surface as a whole, and it contacts each component to dissipate heat. However, due to the poor height consistency of components during soldering, the heat dissipation effect is not good. Usually, a soft thermal phase change thermal pad is added to the component surface to improve the heat dissipation effect.
10.Heat dissipation through the PCB board itself
The widely used PCB board materials at present 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 path for high-heat components, it is almost impossible to expect the PCB resin 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 installation, and high-heat assembly, it is very insufficient to rely solely on the surface of components with very small surface areas to dissipate heat. At the same time, due to the large-scale use of surface-mounted components such as QFP and BGA, the heat generated by the components is transferred to the PCB board in large quantities. Therefore, the best way to solve the heat dissipation problem is to improve the heat dissipation capacity of the PCB itself that is in direct contact with the heat-generating components, and conduct or dissipate it through the PCB board.
11.Use reasonable routing design to achieve heat dissipation
Since the resin in the board has poor thermal conductivity, and the copper foil lines and holes are good conductors of heat, increasing the copper foil surplus rate and increasing the thermal conductive holes are the main means of heat dissipation.
To evaluate the heat dissipation capacity of the PCB, it is necessary to calculate the equivalent thermal conductivity (9eq) of the insulating substrate for PCB, a composite material composed of various materials with different thermal conductivities.
12.For equipment using free convection air cooling
it is best to arrange the integrated circuits (or other devices) in a longitudinal or transverse manner.
13.The devices on the same printed circuit board should be arranged according to their heat generation and heat dissipation as much as possible.
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 uppermost (inlet) of the cooling airflow, and devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the downstream of the cooling airflow.
14.In the horizontal direction,
high-power devices should be arranged as close to the edge of the printed circuit board as possible to shorten the heat transfer path; in the vertical direction, high-power devices should be arranged as close to the top of the printed circuit board as possible to reduce the impact of these devices on the temperature of other devices when they are working.
15.Devices that are sensitive to temperature are best placed in the lowest temperature area (such as the bottom of the equipment).
Never place it directly above the heating device. Multiple devices are best arranged in a staggered layout on the horizontal plane.
16.The heat dissipation of the printed circuit board in the equipment mainly depends on air flow,
so when designing, the air flow path should be studied and the devices or printed circuit boards should be reasonably configured. When air flows, it always tends to flow to places with less resistance, so when configuring devices on a printed circuit board, avoid leaving a large airspace in a certain area. The same problem should also be noted when configuring multiple printed circuit boards in a whole machine.
17.Avoid the concentration of hot spots on the PCB,
distribute the power evenly on the PCB board as much as possible, and 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 overheating and affecting the normal operation of the entire circuit. If conditions permit, it is necessary to conduct thermal efficiency analysis of printed circuits. For example, the thermal efficiency index analysis software module added to some professional PCB design software can help designers optimize circuit design.
18.Arrange the devices with the highest power consumption and the highest heat generation near the best heat dissipation position.
Do not place devices with higher heat generation in the corners and edges of the printed circuit board unless there is a heat dissipation device near it. When designing power resistors, choose larger devices as much as possible, and make sure there is enough heat dissipation space when adjusting the layout of the printed circuit board.
19.When high heat dissipation devices are connected to the substrate,
the thermal resistance between them should be reduced as much as possible. In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as applying a layer of thermal conductive silicone) can be used on the bottom surface of the chip, and a certain contact area should be maintained for the device to dissipate heat.
20.Connection between the device and the substrate:
(1) Try to shorten the length of the device lead;
(2) When selecting high power consumption devices, the thermal
conductivity of the lead material should be considered. If possible, try to select the device with the largest cross-section of the lead;
(3) Select a device with a large number of pins.
21. Device package selection:
(1) When considering thermal design, pay attention to the device package instructions and its thermal conductivity;
(2) Consider providing a good thermal conduction path between the substrate and the device package;
(3) Avoid air partitions in the thermal conduction path. If this is the case, fill it with thermal conductive materials.
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