Electronic Design - Printed circuit boards cooling skills

It is very important to dissipate heat from the printed circuit boards. The heat generated by the electronic equipment makes the internal temperature of the equipment 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.
1. Analysis of temperature rise factors of printed circuit boards
The direct cause of the temperature rise of the printed board is due to the existence of circuit power consumption devices, electronic devices have power consumption to varying degrees, and the heating intensity varies with 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 PCB thermal power consumption, it is generally analyzed from the following aspects.

1. Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the distribution of power consumption on the printed circuit boards.

2. The structure of the printed circuit boards
(1) The size of the printed circuit boards;
(2) The material of the printed circuit boards.

3. Installation method of printed circuit boards(1) Installation method (such as vertical installation, horizontal installation);
(2) The sealing condition and the distance from the casing.

4. Thermal radiation
(1) The emissivity of the printed board surface;
(2) The temperature difference between the printed circuit boards and the adjacent surfaces and their temperature;

5. Heat conduction
(1) Install the radiator;
(2) Conduction of other installation structures.

6. Thermal convection
(1) Natural convection;
(2) Forced cooling convection.
The analysis of the above factors from the PCB is an effective way to solve the temperature rise of the printed board. Often in a product and system, these factors are interrelated and dependent on each other. Most factors should be analyzed according to the actual situation, and only for a specific Only in the actual situation can parameters such as temperature rise and power consumption be calculated or estimated correctly.

Second, the heat dissipation method of the printed circuit boards

2.1 High heat-generating device plus radiator and heat-conducting plate
When there are a few devices in the PCB that generate a large amount of heat (less than 3), a radiator or heat pipe can be added to the heating device. When the temperature cannot be lowered, a radiator with a fan can be used to enhance 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 radiator customized according to the position and height of the heating device on the PCB or a large flat radiator. Cut out the high and low positions of different components. Fasten the heat dissipation cover on the component surface 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 the components during assembly and welding. Usually, a soft thermal phase change thermal pad is added to the component surface to improve the heat dissipation effect.

2.2 Heat dissipation through the PCB itself
At present, the widely used PCB sheets are copper clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and there are also a small amount of paper-based copper clad sheets. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat dissipation path for high-heating components, it is almost impossible to expect heat to be conducted by the resin of the PCB itself, 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-generation assembly, it is not enough to rely on the surface of components with very small surface area 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 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 distributed.

2.3 Use reasonable wiring design to achieve heat dissipation
Since the resin in the sheet has poor thermal conductivity, and copper foil lines and holes are good conductors of heat, improving the residual rate of copper foil and increasing thermal vias are the main means of heat dissipation. To evaluate the heat dissipation capacity of a 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 PCB.

2.4 For equipment cooled by free convection air, the integrated circuits (or other devices) are arranged vertically or horizontally.

2.5 Devices on the same printed board should be arranged as far as possible according to t

heir 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.) Upstream of the airflow (at the inlet), devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed downstream of the cooling airflow.

2.6 In the horizontal direction, the high-power devices are arranged as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, the high-power devices are arranged as close to the top of the printed board as possible to reduce the temperature impact of these devices on other devices. influence.

2.7 Devices that are sensitive to temperature should be placed in a temperature area (such as the bottom of the device). Never place it directly above the heat-generating device. Multiple devices are staggered on the horizontal plane.

2.8 The heat dissipation of the printed board in the equipment mainly depends on the air flow, so the air flow path should be studied in the design, and the device or printed circuit board should be reasonably arranged. When air flows, it always tends to flow where the resistance is small, so when configuring components on a printed circuit board, it is necessary to 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.

2.9 Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB as much as possible, and keep the temperature performance of the PCB surface uniform and consistent. It is often difficult to achieve a strict uniform distribution in the design process, but it is necessary to avoid areas with too high power density, so as to avoid hot spots affecting the normal operation of the entire circuit. If possible, it is necessary to conduct thermal performance analysis of printed circuits. For example, the thermal performance index analysis software module added in some PCB design software can help designers optimize circuit design.

2.10 Arrange power dissipation and heat generating devices near the heat dissipation location. Do not place high-heat components on the corners and 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 adjust the layout of the printed board so that there is enough space for heat dissipation.

2.11 Devices with high heat dissipation should be connected to the substrate with as little thermal resistance as possible between them. In order to better meet the requirements of thermal characteristics, some thermally 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 can be maintained for the device to dissipate heat.

2.12 Connection of device and substrate:
(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 if possible, try to choose the cross section of the lead;
(3) Select a device with a larger number of pins.

2.13 Package selection of the device:
(1) When considering thermal design, attention should be paid to the package description of the device and its thermal conductivity;
(2) It should be considered to provide a good thermal conduction path between the substrate and the device package;
(3) Air partitions should be avoided on the heat conduction path. If this is the case, thermal conductive materials can be used for filling on PCB board.