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As electronic devices continue to be miniaturized, thermal PCB board design becomes more and more important. The small size and compact layout lead to a higher temperature rise of components, thus greatly reducing the reliability of the system. For this reason, starting from the principle of heat transfer, this paper uses ANSYS finite element software to analyze the temperature field distribution of key components on the printed circuit board (PCB) during operation and determines the high-temperature area and low-temperature area of the PCB. The temperature field of PCBs with different layouts is calculated by an example, and a more reasonable layout method is obtained by comparison. Optimize the layout, reduce the temperature of the PCB board, and improve the reliability of the system.
1 Introduction
The continuous miniaturization of electronic equipment makes the layout of the PCB board more and more compact. However, the unreasonable PCB board layout seriously affects the heat transfer path of the electronic components on the board, which leads to the failure of the reliability of the electronic components due to the temperature increase. , that is, the system reliability is greatly reduced. This also makes the temperature rise problem of the PCB board rise to a certain height. According to reports, 55% of the failure factors of electronic equipment are caused by the temperature exceeding the specified value. Therefore, for electronic equipment, even a decrease of 1 °C will reduce the failure rate of its equipment by a considerable amount. For example, statistics show that the failure rate of electronic equipment in civil aviation will decrease by 4% for every 1 °C reduction. It can be seen that the control of temperature rise (thermal design) is a very important issue. The heat on the PCB is mainly due to power dissipation components, such as transformers, high-power transistors, and high-power resistors. Their power consumption is mainly dissipated into the surrounding medium in the form of heat conduction, convection, and radiation, and only a small part is dissipated in the form of electromagnetic waves. Therefore, in order to improve the stability and reliability of electronic components on the PCB board, it is necessary to clearly understand the power consumption of key components on the PCB board and the temperature field distribution on the board, so as to achieve a reasonable layout. When conducting thermal simulations, finite element or finite difference methods are usually used to solve heat transport and fluid flow equations. This paper adopts finite element analysis. The finite element is more accurate for solving complex geometries, allowing to refine the mesh in some areas, such as parts of a plate or system that are more interesting than others, the mesh can be refined in these areas, while other areas mesh. A little sparser. But mesh refinement can't jump directly from one density to another, only gradually.
2. Basic heat transfer principle and ANSYS finite element thermal simulation process
ANSYS finite element thermal simulation process
In this paper, the geometric model is created by ANSYS software, and the solid model is created by bottom-up and top-down methods. In the process of creating a solid model, due to the complex structure of electronic components, the solid model can be simplified for the convenience of mesh division and the accuracy of the results, and SOLID87 10-node elements suitable for the division of irregular-shaped elements are selected.
3. Finite element solution of temperature field
3.1 Example analysis of two-dimensional temperature field
Layout 1: Chip1, Chip2 side by side, Chip3 next to Chip1 side. The temperature was 101.5°C and the temperature was 92.7°C.
Layout 2: Chip1, Chip2 side by side on one side, Chip3 on the other side of the PCB. The temperature was 90°C and the temperature was 70.7°C.
3.2 Comparative analysis
1) Comparing the analysis results of the two final simulated temperature fields, it can be clearly found that the temperature and temperature of layout 2 have been greatly reduced (about 10∽20℃), which is very impressive for the thermal reliability of electrons. For example, statistics show that the failure rate of electronic equipment in civil aviation will decrease by 4% for every 1 °C reduction. It can be seen that the control of temperature rise (thermal design) is a very important issue. Thereby improving the reliability of the equipment.
2) The two temperature field distribution diagrams both reflect the same problem: when the components are densely distributed, the temperature field distribution is irregular, and the high-temperature and low-temperature regions cannot be determined. Therefore, when laying out the PCB board, full attention should be paid to the dense area of power dissipation components, where no or less heat-sensitive components should be placed as much as possible.
3) The convective heat transfer coefficient in the finite element analysis is different for different component values, and if only the point measurement results are used for calculation, the h value will be small, so some corrections must be made. The h value with large power consumption is slightly larger. , and then compare the calculated and measured results, and continuously adjust the h value until it is basically consistent.
4) In different temperature field distributions, although the displayed colors are the same, the temperature values represented by the same color are different. They are used to indicate the trend from high-temperature areas to low-temperature areas.
5) Boundary conditions are also very important, and the boundary conditions given during modeling must be correct.
3.3 Example analysis of 3D temperature field
There are three chips on the PCB, the layout and all parameters are the same as 2.
4. Conclusion and Analysis
1) On the surface, the three-dimensional temperature field simulation result is not as good as the two-dimensional ideal, but in fact, it is not the case. The temperature indicated in the 3D simulation is the component die location, where the temperature is actually higher than the component surface temperature. Therefore, the simulation results for layout 2 are reasonable.
2) The 3D model is more complex. For the accuracy of the simulation results, the chip material can be regarded as being composed of three layers of different materials to simplify the model.
3) The establishment of the 3D model and the processing of the results consume a lot of energy and time, and the material and structure requirements are more detailed and specific than the 2D model. Although 3D simulation can get more information, 2D can also quickly get an approximate temperature field distribution. Therefore, in practical applications, these two methods can be selected according to the PCB board specific actual situation.
