PCB News - The factors of PCB temperature rise and their solutions

Abstract: As we all know, the heat generated during the operation of electronic equipment will cause the internal temperature of the equipment to rise rapidly. If the heat is not dissipated in time, the equipment will continue to heat up, and the equipment will fail due to overheating, and the reliability of the electronic equipment will be reduced. Therefore, the dissipation board is very important.

The direct factor of PCB temperature rise is the existence of power-consuming components, and the heating intensity changes with the change of power consumption.

Several factors of the temperature rise of printed circuit boards and their solutions

There are two phenomena of temperature rise.

1. Local temperature rise or whole area temperature rise;

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

For detailed reasons, they are usually analyzed from the following aspects.

1. Power consumption

(1) Analysis of power consumption per unit area;

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

2. PCB structure

(1) Size;

(2) Materials.

3. How to install PCB

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

(2) Sealing conditions and the distance from the casing.

4. Thermal radiation

(1) The emissivity of the PCB surface;

(2) The temperature difference between the PCB and the adjacent surface and its absolute temperature;

5. Heat conduction

(1) Install the radiator;

(2) The conduction of other installation structural components.

6. Thermal convection

(1) Natural convection;

(2) Forced cooling convection.

Analyzing the above factors from the PCB is an effective method to solve the temperature rise of the PCB. Runze Wuzhou believes that these factors are usually related to products and systems and depend on each other. Most factors should be analyzed based on actual conditions. Only in accordance with specific actual conditions, parameters such as temperature rise and power consumption can be correctly calculated or estimated.

solution

High heat generating device with radiator and heat conducting plate

When a small number of parts in the PCB generate a lot of heat (less than 3), a heat sink or heat pipe can be added to the device. When the temperature cannot be lowered, a radiator with a fan can be used to enhance the heat dissipation effect. When the number of parts is large (more than 3), a larger 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 PC. Large flat radiator. Place the upper and lower parts of the different components. The heat shield is integrally fixed on the surface of the components, and is in contact with each component to radiate heat. However, due to the poor consistency of the components during the welding process, the heat dissipation effect is not good.

Cooling through the PCB itself

The currently widely used PCBs are copper clad/epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper clad laminates are used. Although these substrates have excellent electrical properties and processing properties, their heat dissipation properties are poor. As a heat dissipation path for high heat-generating components, it is difficult to expect heat to be conducted from the resin of the PCB itself, but to radiate heat from the surface of the component to the surrounding air. However, as electronic products enter the era of miniaturization, high-density mounting, and high-heat assembly, it is not enough to radiate heat from the surface of a component with a small surface area. At the same time, due to a large number of surface mount components (such as QFP and BGA), a large amount of heat generated by the components is transferred to the PCB. Therefore, the best way to solve the problem of heat dissipation is to increase the heat dissipation capacity of the PCB itself that is in direct contact with the heating element. Conduction or emission.

Use reasonable layout design to achieve heat dissipation

Because the resin on the circuit board has poor thermal conductivity, and copper wires and holes are good thermal conductors, PCB moisturizing believes that increasing the residual ratio of copper and increasing the thermal holes are the main means of heat dissipation.

In order to evaluate the heat dissipation capacity of the PCB, it is necessary to calculate the equivalent thermal conductivity of composite materials composed of various materials with different thermal conductivity.

For equipment that uses free convection air cooling, it is best to arrange integrated circuits (or other devices) in a vertically long way or a horizontally long way.

They should be placed on the same PCB according to their heat and heat dissipation. Devices with low heat or poor heat resistance (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed. The highest flow of the cooling airflow (at the entrance), and the equipment that generates a lot of heat or heat (such as power transistors, large-scale integrated circuits, etc.) is located at the most downstream air flow of the cooling.

In the horizontal direction, high-power components should be placed as close to the edge of the PCB as possible to shorten the heat transfer path. In the vertical direction, high-power components should be placed as close to the top of the PCB as possible to reduce the temperature of other components during operation.

Temperature-sensitive components should be placed in the lowest temperature area (such as the bottom of the device). Do not place it directly above the heating device. The multiple devices are preferably staggered on a horizontal plane.

The heat dissipation of the PCB in the device mainly depends on the airflow, so the airflow path should be studied during the design process, and the device or PCB should be properly configured. When air flows, it tends to flow in places with low resistance. Therefore, when configuring equipment on a printed circuit board, avoid leaving a large air space in a certain area. The same problem should be paid attention to in the configuration of multiple printed circuit boards in the entire machine.

Avoid concentrating hot spots on the PCB, distribute the power to the PCB as evenly as possible, and keep the temperature performance of the PCB surface uniform. In the design process, it is usually difficult to achieve strict uniform distribution, but it is necessary to avoid areas with too high power density, lest hot spots affect the normal operation of the entire circuit. If necessary, 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 the circuit design.

Place the components with the highest power consumption and maximum heat generation near the best place for heat dissipation. Do not heat the heat sink unless it is placed on the corners and peripheral edges of the printed circuit board. When designing a power resistor, please choose a larger device as much as possible, and leave enough space for heat dissipation when adjusting the layout of the printed circuit board.

When the high heat dissipation device is connected to the substrate, the thermal resistance between them should be minimized. In order to better meet the thermal characteristics requirements, some thermally 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 to disperse the device.

Connection of component and substrate

(1) Minimize the length of component leads;

(2) When selecting high-power components, the thermal conductivity of the lead material should be considered, and the lead with the largest cross-section should be selected as far as possible;

(3) Choose components with a large number of pins.

Equipment packaging selection

(1) When considering the heat dissipation design, please pay attention to the packaging instructions and thermal conductivity of the components;

(2) Consideration should be given to providing a good thermal path between the substrate and the device package;

(3) Air separation should be avoided in the heat conduction path. In this case, a thermally conductive material can be used for filling.