PCB News - PCB thermal design

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 dissipate heat from the PCB board.

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 all have power consumption to varying degrees, and the heating intensity varies with the size of 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 PCB board.

2. The structure of the printed board

(1) The size of the printed board;

(2) The material of the printed board.

3. How to install the printed board

(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 board and adjacent surfaces and their absolute temperature;

5. Heat conduction

(1) Install the radiator;

(2) Conduction of other installation structural parts.

6. Thermal convection

(1) Natural convection;

(2) Forced cooling convection.

The analysis of the above factors from the PCB board is an effective way to solve the temperature rise of the printed 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, and only for a specific The actual situation can calculate or estimate the parameters such as temperature rise and power consumption more correctly.

2. PCB board heat dissipation method

1. High heat-generating device plus radiator and heat conduction plate

When a small number of components in the PCB generate a large amount of heat (less than 3), a heat sink or heat pipe can be added to the heating device. When the temperature cannot be lowered, a heat sink 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 heat sink customized according to the position and height of the heating device on the PCB or a large flat heat sink Cut out different component height positions. 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 board itself

At present, the widely used PCB boards in PCB manufacturing are copper-clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad boards are used. 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 from the resin of the PCB itself to conduct heat, 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 extensive use of surface mount components such as QFP and BGA, a large amount of heat generated by the 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, which is in direct contact with the heating element, through the PCB board. To be transmitted or emitted.

3. Use reasonable wiring design to achieve heat dissipation

Because the resin in the plate has poor thermal conductivity, and the copper foil lines and holes are good conductors of heat, increasing the residual rate of the copper foil and increasing the thermally conductive holes are the main means of heat dissipation.

To evaluate the heat dissipation capacity of the 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 the PCB.

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 board should be arranged as far as possible according to their 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.) should be placed The uppermost flow of the cooling airflow (at the entrance), and the devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the lowermost part of the cooling airflow.

6. In the horizontal direction, high-power devices are placed as close to the edge of the printed board as possible to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close as possible to the top of the printed board to reduce the temperature of other devices when these devices are working. Impact.

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 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, distribute the power evenly on the PCB board as much as possible, and keep the PCB surface temperature performance uniform and consistent. 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 design software can help designers optimize the circuit design.

10. Arrange the devices with the highest power consumption and the highest 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. High heat dissipation devices should minimize the thermal resistance between them when they are connected to the substrate. 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. The connection between the device and the 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. 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) Pay attention to the package description of the device and its thermal conductivity when considering thermal design;

(2) Consider providing a good heat conduction path between the substrate and the device package;

(3) Air partitions should be avoided in the heat conduction path. If this is the case, heat-conducting materials can be used for filling.