PCB Tech - PCB temperature rise and heat dissipation factors

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, it is very important to conduct heat dissipation treatment on the printed circuit board.

One. Analysis of temperature rise factors of printed circuit boards

The direct cause of the temperature rise of the printed circuit board is due to the existence of circuit power consumption devices, and electronic devices have different levels of power consumption, and the heating intensity varies with the size of the power consumption.

Two phenomena of temperature rise in printed circuit boards:

(1) Local temperature rise or large area temperature rise;

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

When analyzing the thermal power consumption of the PCB circuit board, 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 PCB circuit board.

2. The structure of the printed circuit board

(1) The size of the printed circuit board;

(2) Materials for printed circuit boards.

3. How to install the printed circuit board

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

(2) The sealing condition and the distance from the case.

4. Thermal radiation

(1) The emissivity of the printed circuit board surface;

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

5. Heat conduction

(1) Install a 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 circuit board is an effective way to solve the temperature rise of the printed circuit board. These factors are often related and dependent on each other in a product and system. Most of the factors should be analyzed according to the actual situation. Only a specific actual situation can be more accurately calculated or estimated parameters such as temperature rise and power consumption.

2. PCB circuit board heat dissipation method

1. High heat-generating device plus radiator, heat conducting plate

When a small number of components in the PCB circuit board generate a large amount of heat (less than 3), a radiator or heat pipe can be added to the heating component. When the temperature cannot be lowered, a radiator 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 circuit board or a large flat plate for heat dissipation Cut out different component height positions on the device. The heat dissipation cover is integrally buckled on the surface of the component, and it is in 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 surface of the component to improve the heat dissipation effect.

2, heat dissipation through the PCB circuit board itself

The currently widely used PCB circuit boards are copper clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, as well as a small amount of paper-based copper clad boards. 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 resin of the PCB circuit board 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 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 large-scale use of surface mount components such as QFP and BGA, the heat generated by the components is transferred to the PCB circuit board in a large amount. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capacity of the PCB itself that is in direct contact with the heating element. The circuit board conducts or radiates.

3. Use reasonable wiring design to realize heat dissipation

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

To evaluate the heat dissipation capacity of a PCB circuit board, it is necessary to calculate the equivalent thermal conductivity (nine eq) of a composite material composed of various materials with different thermal conductivity-an insulating substrate for a PCB circuit board.

4. For equipment that adopts free convection air cooling, it is best to arrange integrated circuits (or other devices) vertically or horizontally.

5. The devices on the same printed circuit board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with small calorific value or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed At the uppermost flow (inlet) of the cooling airflow, devices with large heat or heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the furthest downstream of the cooling airflow.

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

7. The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

8. The heat dissipation of the printed circuit board in the equipment mainly relies 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. When air flows, it always tends to flow in places with low resistance, so when configuring devices on a printed circuit board, 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.

9. Avoid the concentration of hot spots on the PCB circuit board, distribute the power evenly on the PCB circuit board as much as possible, and maintain the uniform and consistent surface temperature performance of the PCB circuit board. 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 efficiency of the printed circuit. For example, the thermal efficiency index analysis software module added in some professional PCB circuit board design software can help designers optimize the circuit design.

10. Arrange the devices with the highest power consumption and heat generation near the best position for heat dissipation. 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.

11. When connecting high heat dissipation devices with the substrate, the thermal resistance between them should be reduced as much as possible. 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 can be maintained for the device to dissipate heat.

12. Connection between 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 largest cross section of the lead;

(3) Choose a device with more pins.

13. Package selection of the device:

(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 separation should be avoided in the heat conduction path. If this is the case, thermally conductive materials can be used for filling.