PCB Technical - Three important measures to improve the thermal reliability of printed boards

Three important measures to improve the thermal reliability of printed boards

Provide fast, professional and legal technical development, product development and auxiliary development technical services for various electronic enterprises, research and development institutions. Zhilian Technology continues to develop and innovate. Based on the basic knowledge of thermal design, the selection of heat dissipation methods, thermal design and thermal analysis technical measures in PCB design are discussed. It is put forward that thermal analysis and thermal design are three important measures to improve the thermal reliability of printed boards.

1. The importance of thermal design

In addition to useful work, most of the electrical energy consumed by electronic equipment during operation is converted into heat and dissipated. The heat generated by the electronic equipment causes the internal temperature 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.

SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and the temperature rise of the equipment seriously affects the reliability. Therefore, the research on thermal design is very important.
2. 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 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 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.
2.1 Electrical power consumption
(1) Analyze the power consumption per unit area;
(2) Analyze the distribution of power consumption on the PCB.
2.2 The structure of the printed board
(1) The size of the printed board;
(2) The material of the printed board.
2.3 The installation method of the printed board
(1) Installation method (such as vertical installation, horizontal installation);
(2) The sealing condition and the distance from the case.
2.4 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;
2.5 Heat conduction
(1) Install the radiator;
(2) Conduction of other installation structural parts.
2.6 Thermal convection
(1) Natural convection;
(2) Forced cooling convection.

The analysis of the above factors from the PCB 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.
3. Thermal design principles
3.1 Material selection
(1) The temperature rise of the conductors of the printed board due to the passing current plus the specified ambient temperature should not exceed 125 degree Celsius (commonly used typical value. It may be different depending on the selected board). Since the components installed on the printed board also emit some heat, which affects the operating temperature, these factors should be considered when selecting materials and the design of the printed board. The hot spot temperature should not exceed 125 degree Celsius. Choose thicker copper clad as much as possible.
(2) In special cases, aluminum-based, ceramic-based, and other plates with low thermal resistance can be selected.
(3) Adopting multi-layer board structure helps PCB thermal design.
3.2 Ensure that the heat dissipation channel is unblocked
(1) Make full use of the components layout, copper skin, window opening and heat dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure that the heat is smoothly exported to the PCB.

(2) Setting of heat dissipation through holes
Designing some heat dissipation through holes and blind holes can effectively increase the heat dissipation area and reduce the thermal resistance, and improve the power density of the circuit board. For example, a via hole is set up on the pad of the LCCC device. Solder fills it in the circuit production process to increase the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper pad on the back through the through holes or blind holes to be dissipated. In some specific cases, a circuit board with a heat dissipation layer is specially designed and used. The heat dissipation material is generally copper/molybdenum and other materials, such as printed boards used on some module power supplies.

(3) Use of thermally conductive materials
In order to reduce the thermal resistance of the heat conduction process, a thermally conductive material is used on the contact surface between the high power consumption device and the substrate to improve the heat conduction efficiency.