For electronic equipment, the internal temperature will rise rapidly during operation. 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, heat dissipation is very important.


1. The Importance of Thermal Design


The electrical energy consumed by electronic equipment during operation, such as RF power amplifiers, FPGA chips, and power products, is mostly converted into heat dissipation in addition to useful work. The heat generated by the electronic device causes the internal temperature to rise rapidly. If the heat is not dissipated in time, the device will continue to heat up, and the device will fail due to overheating, and the reliability of the electronic device will decrease. SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and seriously affects the reliability of equipment. Therefore, research on thermal design is very important.


heat dissipation


For electronic equipment, the internal temperature will rise rapidly during operation. 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, heat dissipation is very important.


2. Analysis of Temperature Rise Factors of Printed Circuit Boards


(1) Electric power consumption;


(2) Size, material;


(3) Installation method ( vertical and horizontal);


(4) Emissivity;


(5) Heat Conduction(installing a radiator);


(6) Hydrothermal convection.


analysis of temperature rise factors


3. Some Methods of PCB Thermal Design


(1) Heat dissipation trough PCB. Currently widely used PCB sheets are copper/epoxy glass cloth substrates or phenolic resin glass cloth substrates. Although these substrates have excellent electrical properties and processability, they have poor heat dissipation properties. As electronic products have entered the era of component miniaturization, high-density mounting, and high-heat assembly, it is not enough to install only surface components.


At the same time, due to the large number of surface mount components such as QFP and BGA, the heat generated by the components is transferred to the PCB at the same time. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capability of the PCB itself in direct contact with the heat generating components. The plate is conducted out or emitted;


(2) High heat generating device with heat sink and heat conducting plate;


(3) For devices employing free convection air cooling;


(4) Use reasonable wiring design to achieve heat dissipation;


(5) Devices on the same printed board should be arranged as far as possible according to their heat generation and 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 placed in the cooling airflow. The uppermost (inlet), heat-generating or heat-resistant devices (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow;


(6) In the horizontal direction, the high-power devices are placed as close as possible to the edges of the printed board; in the vertical direction, the high-power devices are placed as close as possible to the top of the printed board;


(7) The heat dissipation of the printed circuit board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area;


(8).Temperature sensitive devices should be placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heat sink. Multiple devices are preferably staggered on a horizontal plane;


(9).Place the device with the highest power consumption and maximum heat generation near the best heat dissipation position. Do not place higher heat generating components on the corners and peripheral edges of the printed board unless a heat sink is placed near it. When designing the power resistor, choose a larger device as much as possible, and have enough space for heat dissipation when adjusting the layout of the printed board.


methods of pcb thermal design


4. Conclusion


Material


The temperature rise of the conductors of the printed board due to the current flow plus the specified ambient temperature does not exceed 125 °C (typical temperature. Also Depending on the board selected). Thick copper foil is a good choice.In special cases, aluminum-based, ceramic-based and other heat-resistant plates can be selected.

Heat Dissipation Through Hole Design


Some heat dissipation through holes and blind holes can effectively improve the heat dissipation area and reduce the thermal resistance and increase the power density of the circuit board. For example, a via hole is formed on the pad of the LCCC device. During the production process of the circuit, the solder fills it, so that the thermal conductivity is improved, and the heat generated during the operation of the circuit can be quickly transmitted to the metal heat dissipation layer or the copper provided on the back surface through the through hole or the blind hole.

 

In some specific cases, a circuit board with a heat dissipation layer is specially designed and used, and the heat dissipation material is generally a material such as copper/molybdenum, such as a printed board used in some module power supplies. In order to reduce the thermal resistance of the heat conduction process, the heat conductive material can improve the heat conduction efficiency.


Component Layout Requirements


(1) Board surface heat capacity needs to be evenly distributed;


(2) Board surface heat capacity needs to be evenly distributed;


(3) Making the heat transfer cross section as large as possible;


(4) Heat sensitive components and components (including semiconductor devices) should be kept away from heat sources;


(5) (liquid medium) capacitors are preferably kept away from heat source;


(6) The additional sub-board and device air duct are in the same direction as the ventilation;


(7) The heating device should be placed as much as possible above the product;


(8) Do not place components with large heat or large current on the corners and peripheral edges of the printed board.

Wiring


(1) Material;


(2) Plan the minimum channel width according to the device current density; pay special attention to the channel wiring at the joint;


(3) The high current lines should be surfaced as much as possible; if the requirements are not met, the bus bar can be considered;


(4) Try to reduce the thermal resistance of the contact surface. To this end, the heat conduction area should be increased; the contact plane should be smooth and smooth, and if necessary, coated with thermal grease;


(5) Use a large area of copper foil on the surface;


(6) Use a larger pad on the grounding mounting hole on the printed board to fully utilize the mounting bolts and the copper foil on the surface of the printed board for heat dissipation;


(7) Rely on the via holes to help dissipate heat.


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Statement: This post is only the personal view of the author and does not represent the opinions of ALLPCB.com.