PCB News - PCB heat dissipation skills of multi-layer circuit board factories

PCB heat dissipation skills of multi-layer circuit board factories

When electronic equipment is working, it will generate a lot of heat, causing 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 PCB boards of batch manufacturers of multilayer circuit boards.

One. Analysis of temperature rise factors of printed circuit boards

The direct cause of the temperature rise of the multilayer circuit board is due to the existence of power consumption devices in the circuit, and the 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 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 a PCB multilayer 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 multilayer circuit boards.

2, the structure of the printed circuit board

(1) The size of the printed circuit board;

(2) Printed circuit board materials.

3, thermal convection

(1) Natural convection;

(2) Forced cooling convection.

4, heat conduction

(1) Install a radiator;

(2) Conduction of other installation structures.

5. 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.

6, heat radiation

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

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

The analysis of the above factors from the PCB circuit board proofing manufacturers 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. According to a specific actual situation, parameters such as temperature rise and power consumption can be more accurately calculated or estimated.

2. Heat dissipation method of multilayer circuit board

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 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 thermal 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. 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 multilayer 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 multilayer circuit board.

3, heat dissipation through the PCB circuit board itself

At present, the widely used multi-layer circuit board materials are copper clad/epoxy glass cloth substrate or phenolic resin glass cloth substrate, and a small amount of paper-based copper clad board is 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 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 board conducts or radiates.

4. 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.

5. 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 work. Impact.

6. Place 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.

7. 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.) 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.

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

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

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) Choose a device with more pins;

(3) 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.

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