We all know that 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, the cool-off design for PCB is of high importance.
The direct cause of the temperature rise of the printed board is the existence of circuit power consumption devices, and electronic devices 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.
Analysis of PCB thermal power consumption is generally done from the following aspects:
Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB circuit board.
The structure of the printed board
(1) The size of the printed board;
(2) The material of the printed board.
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.
(1) The emissivity of the printed board surface;
(2) The temperature difference between the printed board and adjacent surfaces and their temperature;
(1) Install the radiator;
(2) Conduction of other installation structural parts.
(1) Natural convection;
(2) Forced cooling convection.
Analyzing above factors is an effective way to find out root cause for the temperature rise of the printed circuit board.
When a small number of components in the PCB generate a large amount of heat (less than 3), a heat sink or 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 the heat dissipation effect .
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 buckled on the surface of the components as a whole, and contact with each component 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 component surface to improve the heat dissipation effect.
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 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 extensive 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 itself that is in direct contact with the heating element, and conduct it through the PCB board. Or send it out.
Due to the poor thermal conductivity of the resin in the plate, and the copper foil lines and holes are good conductors of heat, the editor believes that increasing the remaining rate of copper foil and increasing the thermal 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 (nine eq) of the composite material composed of various materials with different thermal conductivity-the insulating substrate for the PCB.
Devices with low calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) are placed on the upper flow (entry) of the cooling air flow, and devices with large calories or good heat resistance (such as power transistors, Large-scale integrated circuits, etc.) are placed downstream of the cooling airflow.
Distribute the power evenly on the PCB board as much as possible to keep the PCB surface temperature performance 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 the printed circuit. For example, the thermal performance index analysis software module added in some PCB design software can help designers optimize the circuit design.
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.
In order to better meet the thermal characteristics requirements, some thermal conductive materials (such as a layer of thermally conductive silica gel) can be used on the bottom surface of the chip, and a certain contact area is maintained for the device to dissipate heat.
(1) Try to shorten the lead length of the device;
(2) When selecting high-power devices, the thermal conductivity of the lead material should be considered, and the cross section of the lead should be selected as far as possible;
(3) Choose a device with a larger number of pins.
(1) When considering thermal design, pay attention to the package description of the device and its thermal conductivity;
(2) Consider providing a good heat conduction path between the substrate and the device package;
(3) Air partitions should be avoided in the heat conduction path. If this is the case, heat conduction materials can be used for filling.
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