The heat generated during the operation of the electronic equipment causes the internal temperature of the equipment to rise rapidly. If the heat is not released in time, the equipment will continue to warm up, and the device will fail due to overheating, and the reliability of the electronic equipment will decline. Therefore, it is very important to heat the circuit board.

One, printed circuit board temperature rise factor analysis

The direct cause of the PCB temperature rise is the existence of circuit power dissipation devices, electronic devices have different degrees of power dissipation, heating intensity with the size of power dissipation changes.

Two phenomena of temperature rise in printed board:

(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) power consumption per unit area is analyzed;

(2) analyze the distribution of power consumption on PCB.

2. PCB structure

(1) size of printed board;

(2) printed board materials.

3. PCB installation method

(1) installation method (such as vertical installation and horizontal installation);

(2) sealing condition and distance from the housing.

4. Thermal radiation

(1) radiation coefficient of printed board surface;

(2) the temperature difference between the printed board and the adjacent surface and their absolute temperature;

5 heat conduction

(1) install the radiator;

(2) conduction of other mounting structures.

6. Thermal convection

(1) natural convection;

(2) forced cooling convection.

The analysis of the above factors of PCB is an effective way to solve the temperature rise of PCB. These factors are often interrelated and dependent in a product and system. Most of the factors should be analyzed according to the actual situation.

Two, circuit board heat dissipation

1. High heating device, radiator and heat conducting plate

When there are a few components in PCB with high heat (less than 3), heat sink or heat conduction pipe can be added to the heating components. When the temperature cannot be lowered, the radiator with fan can be adopted to enhance the heat dissipation effect. When the amount of heating components is large (more than 3), a large cooling cover (board) can be used. It is a special radiator customized according to the position and height of the heating components on the PCB board, or a large flat plate radiator to cut out different component positions. The heat shield is fastened to the surface of the element to dissipate heat in contact with each element. However, due to the poor consistency in the installation and welding of components, the cooling effect is not good. Usually, a soft thermal phase-change thermal pad is added on the surface of components to improve the heat dissipation effect.

2. Dissipate heat through PCB itself

The most widely used PCB boards are copper-clad/epoxy glass cloth or phenolic resin glass cloth, as well as a few paper-based copper sheets. Although these substrates have excellent electrical performance and processing performance, they have poor heat dissipation. As the heat dissipation path of high-heating components, it is almost impossible to expect heat conduction from the resin of PCB itself, but heat dissipation from the surface of components to the surrounding air. However, as electronic products have entered the era of miniaturization of components, high-density installation and high-frequency thermosetting assembly, it is not enough to rely on components with very small surface area to dissipate heat. At the same time, due to the extensive use of surface mounted components such as QFP and BGA, a large number of heat generated by components is transferred to the PCB board. Therefore, the best way to solve the problem of heat dissipation is to improve the heat dissipation capacity of the PCB itself in direct contact with the heating components, and conduct or send out through the PCB board.

3. Reasonable wiring design is adopted to realize heat dissipation

Because the resin in the plate thermal conductivity is poor, and the copper foil line and hole is a good conductor of heat, so increase the copper foil residual rate and increase the thermal hole is the main means of heat dissipation.

To evaluate the heat dissipation capacity of PCB, it is necessary to calculate the equivalent thermal conductivity (9 eq) of composite materials composed of various materials with different thermal conductivity.

4. For equipment cooled by free convection air, it is best to arrange the integrated circuits (or other devices) lengthwise or lengthwise.

5. On the same piece of PCB device should as far as possible according to the calorific value of the size and degree of heat partition, calorific value is small or poor heat resistance devices (such as small signal transistor, small-scale integrated circuit, electrolytic capacitors, etc.) on the cooling airflow of the best () at the entrance, calorific value big or good heat resistance devices (such as power transistor, large scale integrated circuit, etc.) in the most downstream cooling airflow.

6. In the horizontal direction, the high-power devices should be arranged as close as possible to the edge of the PCB to shorten the heat transfer path; In the vertical direction, the high-power devices should be placed as close as possible to the top of the PCB in order to reduce the influence of these devices on the temperature of other devices.

7. Temperature-sensitive devices are best placed in the lowest temperature area (such as the bottom of the device), do not place it directly above the heating device, multiple devices are best laid out horizontally.

8. The heat dissipation of the printed board in the equipment mainly depends 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. Air flow tends to flow where the resistance is small, so in the printed circuit board configuration of the device, to avoid leaving a large area of airspace. The same problem should be paid attention to in the configuration of multiple printed circuit boards.

9. Avoid the concentration of hot spots on PCB, distribute power evenly on PCB board as far as possible, and keep PCB surface temperature performance uniform and consistent. It is often difficult to achieve strict uniform distribution in the design process, but it is necessary to avoid areas with high power density, so as to avoid hot spots that may affect the normal operation of the whole circuit. If possible, it is necessary to analyze the thermal performance of printed circuit. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize the circuit design.

10. Place the devices with the highest power consumption and maximum heat consumption near the optimal heat dissipation position. Do not place devices with high heat at the corners and edges of the PCB, unless a radiator is arranged nearby. Choose larger components when designing power resistors, and adjust the layout of the PCB to allow for adequate cooling.

High heat dissipation devices shall minimize thermal resistance between them when connected to the substrate. In order to better meet the thermal characteristics of the chip, some thermal conductivity materials (such as coating a layer of thermal conductive silica gel) can be used on the bottom of the chip, and maintain a certain contact area for device cooling.

12. Connection between device and substrate:

(1) shorten the lead length of the device as far as possible;

(2) when selecting high-power devices, the thermal conductivity of the lead material should be considered.

(3) select devices with more pins.

13 device packaging selection:

(1) when considering thermal design, attention should be paid to device packaging instructions and its thermal conductivity;

(2) consideration should be given to providing a good heat conduction path between the substrate and the device package;

(3) there should be no air partition on the heat conduction path. If there is such a situation, thermal conductive materials can be used for filling.