Microwave Tech - PCB Circuit Board heat dissipation skills

The heat generated when the electronic equipment is working makes the internal temperature of the equipment rise rapidly. If the heat is not dissipated in time, the equipment will continue to rise, 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.

1., Printed circuit board temperature rise factor analysis

The direct cause of the temperature rise of PCB is the existence of circuit power devices, electronic devices have varying degrees of power consumption, heating intensity varies with the power consumption.

Two phenomena of temperature rise in printed board:

(1) local or large area temperature rise;

(2) Short-term temperature rise or long-term temperature rise.

The analysis of PCB thermal power is generally analyzed from the following aspects.

1.Electrical power consumption

(1) Analysis of power consumption per unit area;

(2) Analyze the distribution of power consumption on the PCB board.

2.Structure of printed board

(1) Printed board size;

(2) Printed board materials.

3.Installation method of the printed board

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

(2) the sealing condition and the distance from the shell.

4.Thermal radiation

(1) radiation coefficient of printed board surface;

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

5.Heat conduction

(1) install the radiator;

(2) Conduction of other installed structural parts.

6.Heat convection

(1) natural convection;

(2) Forced cooling convection.

The analysis of the above factors from 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, most of the factors should be analyzed according to the actual situation, only for a specific actual situation can correctly calculate or estimate the temperature rise and power consumption and other parameters.

2, The circuit board heat dissipation mode

1.High heating device with heat sink and heat conduction plate

When there are a few components in the PCB with high heat (less than 3), a heat sink or heat conduction tube 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 sink (plate) can be used. It is a special radiator customized according to the position and height of the heating device on the PCB board or a large flat radiator to cut out different component height positions. The heat dissipation cover is buckled on the component surface as a whole, and the heat dissipation is in contact with each component. However, the heat dissipation effect is not good because of the poor consistency of components. A soft thermal phase change pad is usually added to the surface of the component to improve the heat dissipation effect.

2.Heat dissipation through the PCB board

At present, PCB board widely used are copper-coated/epoxy glass cloth or phenolic resin glass cloth, and a small amount of paper-coated copper board. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a way of heat dissipation for high-heating components, heat can hardly be expected to be transmitted by the RESIN of the PCB itself, but heat dissipation from the surface of the components to the surrounding air. However, as electronic products have entered the era of component miniaturization, high-density installation, and high thermal assembly, it is not enough to dissipate heat only by the surface of components with a very small surface area. At the same time, due to the extensive use of surface-mounted components such as QFP and BGA, a large amount of heat generated by components is transmitted to the PCB board. Therefore, the best way to solve the heat dissipation problem is to improve the heat dissipation capacity of THE PCB directly in contact with the heating element and conduct or emit it through the PCB board.

3.Adopt reasonable wiring design to achieve heat dissipation

Because the resin in the sheet has poor thermal conductivity, and copper foil lines and holes are good heat conductors, so improving the residual rate of copper foil and increasing heat conduction holes are the main means of heat dissipation.

To evaluate the heat dissipation capacity of PCB, it is necessary to calculate the equivalent thermal conductivity coefficient (nine eq) of insulating substrate for PCB, which is composed of various materials with different thermal conductivity.

4.For equipment cooled by free convection air, it is best to arrange integrated circuits (or other devices) in either longitudinal or transverse lengths.

5.The devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. The devices with low calorific value or poor heat resistance (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the top of the cooling airflow (entrance). Devices with high calorific value or good heat resistance (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 should be arranged as close as possible to the edge of the printed board to shorten the heat transfer path; In the vertical direction, high-power devices are arranged as close as possible to the printed board, to reduce the influence of these devices on the temperature of other devices when they work.

7.The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the equipment), do not put it on the heating device is directly above, multiple devices are best-staggered layout on the horizontal plane.

8.The heat dissipation of the printed board in the equipment mainly depends on airflow, so the airflow path should be studied in the design, and the device or printed circuit board should be reasonably configured. Airflow always tends to flow where resistance is small, so when configuring devices on printed circuit boards, avoid having large airspace in a certain area. The configuration of multiple printed circuit boards in the whole machine should pay attention to the same problem.

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 too high power density, so as not to affect the normal operation of the whole 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 professional PCB design software can help designers optimize circuit design.

10.Place the devices with the highest power consumption and heat dissipation near the best position for heat dissipation. Do not place hot components in the corners and edges of the PCB circuit board unless there is a cooling device near it. In the design of the power resistance as large as possible to choose a larger device, and in the adjustment of the printed board layout so that there is enough space for heat dissipation.