Thermal Reliability Analysis Of Key Components On PCB

1. The heat on the PCB is mainly due to power consumption components such as transformers, high-power transistors, and high-power resistors. Their power consumption is mainly radiated into the surrounding medium in the form of heat conduction, convection and radiation, and only a small part is emitted as electromagnetic waves. Therefore, in order to improve the stability and reliability of PCB electronic components, it is necessary to clearly understand the power consumption of key components on the PCB and the temperature field distribution on the board to achieve a reasonable layout.

 

In the case of thermal simulation, the finite element or finite difference method is usually used to deheat the heat transfer and fluid flow equations. This paper uses finite element analysis. Finite elements are more accurate for solving complex geometries, allowing you to encrypt meshes in some areas. For example, if a part of the board or system is more interesting than other parts, you can encrypt the mesh in these areas, while other area networks Sparse. However, mesh encryption cannot jump directly from one density to another, and only gradually encrypts it.

 

2. Basic heat transfer principle and ANSYS finite element thermal simulation process

2.1 heat conduction

Fourier's law (also known as the basic law of heat conduction): (1)

Where: Q is the amount of heat transfer in time t, K is the thermal conductivity, T is the temperature, A is the plane area, and T is the distance between the two planes.

 

2.2 Surface heat convection

When the surface area is A, when the heat is transferred Q, when the temperature difference between the surface and the environment is Tw-Tf

    Q=hA(Tw-Tf) (2)

h is the surface convective heat transfer coefficient. The convective heat transfer coefficient can be calculated by this formula. In this paper, the natural convection heat transfer coefficient is mainly calculated by this formula. Here the heat radiation of the PCB board can be ignored, so it is ignored.

 

At the same time, it is worth mentioning the heat generation rate of the power consumption components on the PCB. The power dissipated by the power chip is expressed in ANSYS by the heat generation rate HGEN. The calculation formula is as follows:

     Where: P is power consumption and V is the volume of the component

2.3 ANSYS finite element thermal simulation process

This paper creates geometric models with ANSYS software and creates solid models with bottom-up and top-down methods. In the process of creating a solid model, due to the complex structure of electronic components, in order to facilitate the meshing and the accuracy of the results, the solid model can be simplified, and the SOLID87 10-node unit suitable for irregular shape unit division is selected.

 

3, finite element solution temperature field

3.1 Two-dimensional temperature field example analysis

 

Layout 1: Chip1, Chip2 side by side, Chip3 next to Chip1 side.

The maximum temperature is 101.5 ° C and the lowest temperature is 92.7 ° C.

 

Layout 2: Chip1, Chip2 side by side, Chip3 on the other side of the PCB. The maximum temperature is 90 ° C and the maximum temperature is 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 maximum temperature and the minimum temperature of the layout 2 are greatly reduced (about 10 ∽ 20 ° C), and this value is very impressive for the thermal reliability of the electrons. of. For example, statistics show that for every 1 °C reduction in civil aviation electronic equipment, its failure rate will drop by 4%. It can be seen that the control of temperature rise (thermal design) is a very important issue. Thereby improving the reliability of the device.

 

2. The two temperature field distribution maps all reflect the same problem: when the components are densely distributed, the temperature field distribution is irregular, and the high temperature zone and the low temperature zone cannot be determined. Therefore, in the layout of the PCB board, you should pay full attention to the power-constrained component area, where you should not release the heat-sensitive components 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 result is used to calculate the h value is small, some corrections should be made. The large power consumption h value is called slightly larger. , then compare the calculation with the measurement results, and constantly adjust the h value until it is basically consistent.

 

4. In the different temperature field distributions, although the colors displayed are the same, the temperature values indicated by the same color are different, and they are used to indicate the trend from the high temperature zone to the low temperature zone.

 

5. Boundary conditions are also important, and the boundary conditions given during modeling should be correct.

3.3 Three-dimensional temperature field example analysis

   There are three chips on the PCB, and the layout and all parameters are the same as 2.

4. Conclusion and analysis

1. From the surface, the simulation results of the three-dimensional temperature field are not as good as the two-dimensional ideal. Actually, this is not the case. The highest temperature indicated in the three-dimensional simulation is the component chip position where the temperature is actually higher than the component surface temperature. Therefore, the simulation results of layout 2 are reasonable.

 

2. The 3D model is more complicated. To simulate the accuracy of the results, the chip material can be viewed as being composed of three different layers of material to simplify the model.

 

3. The establishment of the 3D model and the processing of the results require a lot of energy and time, and the material and structure requirements are more detailed and specific than 2D. Although 3D simulation can get more information, 2D can also get the approximate temperature field distribution quickly. Therefore, in practical applications, these two methods can be selected according to the actual situation.