PCB Tech - Master the heat dissipation technology of PCB circuit boards

The importance of thermal design.

The heat dissipation of the PCB circuit board is a very important link, so what is the heat dissipation technology of the PCB circuit board.

For electronic equipment, when the equipment is working, a certain amount of calories will be generated, causing the temperature in the equipment to rise rapidly. If the heat is not released in time, the equipment will continue to heat up. Due to overheating, the reliability of the equipment will decrease. Therefore, good circuit board heat dissipation is very important.

The factors of PCB temperature rise are analyzed.

The direct cause of the increase in the temperature of the printed board is that the existence of the circuit power consumption device changes with the change of the power consumption.

There are two phenomena of temperature rise on printed boards.

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

(2) Short temperature rise or long temperature rise. When analyzing the thermal time of PCB, it is usually analyzed from the following aspects.

The power consumption is 2.1.

(1) Analyze the power consumption per unit area.

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

The structure of the printed board is 2.2.

The size of the printed board (1).

Materials for printing plates.

Install 2.3 printed boards.

Installation mode (such as vertical installation, etc.).

Second, the distance between the seal and the housing.

2.4 Thermal radiation.

(1) The emissivity of the printed board surface.

(2) The temperature difference between the printed board and the adjacent surface and its absolute temperature.

2.5 Heat transfer.

Install the heat sink (1)(1).

(2) Transmission of other installation structures.

2.6 Thermal convection.

First, natural convection.

Second, forced cooling convection.

The analysis of the above factors of PCB is an effective way to solve the problem of temperature rise of the printed board. These factors are usually related to dependencies in products and systems. Most factors should be analyzed according to the actual situation, and only according to specific conditions can the temperature rise and power consumption be correctly calculated or estimated.

Three PCB thermal design methods.

1 Heat dissipation through the PCB board itself..

2 High heating equipment plus radiator heat conduction plate.

When a small number of PCB devices have a higher heating capacity (less than 3), when the temperature does not decrease, a heat sink with a fan can be used. Improve the heat dissipation effect. When the amount of heating equipment is greater than 3), a large heat dissipation cover (board) can be used. It is a special kind of heat sink, according to the position and level of the heating equipment on the PCB board, or choose different components on a large flat heat sink. Fasten the heat dissipation cover on the surface of the components as a whole, and contact each component to dissipate heat. However, due to the high degree of consistency of the components, the heat dissipation effect is not good. Usually, a soft thermal phase change thermal pad is added to the surface of the component to improve the heat dissipation effect.

For free convection air cooling equipment, it is best to arrange integrated circuits (or other equipment) or horizontal length.

4 Use reasonable wiring design to realize heat dissipation.

Due to the poor thermal conductivity of the resin in the plate, the copper foil wires and holes are thermal conductors, which increases the residual rate of the copper foil and increases the thermal conductivity holes.

Four conclusions.

(1) The temperature rise of the wires of the printed board due to the current should not exceed 125°C (commonly used typical value). The board selected may be different. Since the components are mounted on the printed board, some heat will also affect the operating temperature. When selecting materials and printed boards, it should be considered that the hot spot temperature should not exceed 125°C. Try to choose thicker copper foil.

(2) In special cases, thermal resistance plates such as aluminum-based ceramic substrates can be used.

3.Multi-layer board structure helps PCB thermal design.

3.2 Ensure that the heat dissipation channel is unblocked.

First, make full use of technologies such as copper skin, window openings, and heat dissipation holes to establish a reasonable and effective low-heat barrier channel to ensure that the heat is smoothly discharged from the PCB.

The design of the heat dissipation hole can effectively improve the heat dissipation area, reduce the thermal resistance, and increase the power density of the circuit board. For example, a via hole is provided on the pad of the LCC device. In the circuit production process, the solder can quickly propagate through the holes or blind holes to the metal heat dissipation layer or the copper pad on the back to improve the thermal conductivity. In some cases, special design and use of heat dissipation layer circuit board heat dissipation materials, such as printed boards used in some module power supplies.

In order to reduce the thermal resistance of the heat conduction process, a thermally conductive material is used to conduct heat conduction on the contact surface between the high power consumption device and the substrate.

In order to improve the heat dissipation conditions, a small amount of copper can be added to the solder paste to improve the heat dissipation conditions. After soldering, the solder joints under the equipment have a certain height. The gap between the device and the printed board increases convective heat dissipation.

3.3 Layout requirements of components.

(1) The software thermal analysis PCB design controls the internal maximum temperature rise.

(2) You can consider mounting high-radiation components on a printed board.

(3) The uniform distribution of the heat capacity of the board should be careful not to concentrate the high power consumption equipment upstream of the airflow, if it is unavoidable. And to ensure that enough cooling air flows through the concentrated area of heat consumption.

4) Make the heat transfer path as short as possible.

(5) Make the heat transfer cross section as large as possible.

(6) The layout of components should take into account the influence of heat radiation on surrounding components. Heat sensitive parts (including semiconductor equipment) should be kept away from heat sources or isolated.

It is better to keep the capacitor (7) away from the heat source.

Eighth, pay attention to forced ventilation and natural ventilation.

(9) The air duct of the additional daughter board equipment is in the same direction as the ventilation.

(10) Try to keep a sufficient distance between intake and exhaust.

(11) The heating equipment should be placed above the product as much as possible, and enter the air flow channel when allowed.

(12) Parts with high heat or high current should not be placed in the corners and surroundings of the printed board, as long as they can be installed on the radiator and far away from other equipment. And ensure that the heat dissipation channel is unblocked.

(13) The peripheral equipment of the small signal amplifier (the smallest possible temperature drift equipment).

(14) Try to use metal chassis or chassis for heat dissipation.

3.4 is required for wiring.

(1) Steel plate selection (reasonable design of printed board structure).

(2) PCB wiring rules.

(3) Plan the minimum channel width according to the current density of the equipment; pay special attention to the channel wiring at the seam.

(4) The large current line should be as surface as possible; if the requirements are not met, the use of bus bars can be considered.

Fifth, minimize the thermal resistance of the contact surface. For this reason, the heat conduction area should be increased, the contact surface should be smooth, and thermally conductive silicone grease can be coated if necessary.

(6) Consider the stress balance and increase the thickness of the thermal stress point.

(7) The heat dissipation copper skin adopts the heat dissipation stress window method, and the heat dissipation solder mask method is used to open the window correctly.

(8) Large-area copper foil on the surface can be used as appropriate.

(9) The ground mounting holes on the circuit board adopt larger pads, and make full use of the mounting bolts and the copper foil on the surface of the printed board to dissipate heat.