Thermal reliability analysis and judgment of PCB

Date：2022-05-26 16:00:00Views：779

The Flotherm software proposed in this paper plays a great role in analyzing heat transfer, especially suitable for the analysis and judgment of thermal reliability of electronic circuit boards. The key to its application in the thermal reliability analysis of electronic circuit board is how to build a reasonable and effective Flotherm model, in order to ensure the accuracy and meet the capacity requirements of computer memory. Taking PCB circuit board as an example, the thermal reliability analysis is carried out by using Flotherm software. By obtaining the thermal analysis results, the operating temperature of each component in electronic equipment can be accurately calculated, and then compared with the allowable operating temperature, so as to judge whether it is in the operation state of ultra-high temperature. However, the layout of wires and components in electronic circuit board is very complex. If you want to obtain accurate modeling, you need to simplify each component to ensure the construction of correct calculation model.

1. Modeling of thermal reliability analysis of electronic circuit board

In the electronic circuit board analyzed in this paper, the main heater is composed of 9 MOS tubes and 6 integrated circuit blocks. When the components are in the working state, most of the lost power will be transformed into heat, so the simplification of components must be fully considered in the modeling process; For the copper foil laid as a conductor, when it is arranged in the PCB substrate, it can not only play the role of conductivity, but also conduct part of heat, and the thermal conductivity and heat transfer area are relatively large.

1.1 modeling method of PCB substrate

PCB substrate is an indispensable part of electronic circuit. It not only provides electrical connection between components, but also is the support board of components. PCB board mainly includes copper foil and epoxy resin substrate coated as wires. The thickness of copper foil is about 0.1mm and the thickness of epoxy resin substrate is about 4mm In order to prove that the use of copper foil will affect the heat transfer of PCB board, the simulation experiment is carried out by using Flotherm software.

Two copper blocks at constant temperature are placed in the epoxy resin substrate of PCB. The temperature of the left copper block is about 80 ℃, and the temperature of the right copper block is about 20 ℃. At the same time, a constant temperature of 20 ℃ is designed at the bottom of PCB as the boundary of conduction. Comparing the case with and without copper foil, it is found that although the added copper foil is very thin and thin, it has a strong guiding effect on heat, so this element can not be ignored in the modeling process.

1.2 modeling method of MOS transistor devices

MOS tube devices mainly include silicon wafer, bottom copper alloy heat sink, epoxy resin shell, etc. their thermal conductivity is 117.5w / (m ·℃), 301.5w / (m ·℃) and 0.82w / (m ·℃) respectively. Inside the MOS tube, the silicon wafer is a very important heating part, and its power is about 1W; When modeling MOS tube, the following problems should be considered:

(1) Simulation modeling of primary accuracy. Assuming that the MOS tube is a simple solid body, the physical parameters are set according to its actual situation.

(2) Combined with the internal structure of MOS transistor, the Flotherm software model is constructed; Under the natural convection conditions of room temperature (20 ℃) and high temperature (60 ℃), the heat distribution state of MOS tube needs to be calculated separately. The experimental results show that the results calculated by the model are basically the same whether in room temperature (20 ℃) or high temperature (60 ℃); Although the internal structure of MOS tube is very complex, the temperature difference between the internal and external surfaces is very small; Combined with the analysis of heat conduction effect, because the volume of MOS tube is relatively small and its main materials have good heat transfer performance, the thermal resistance from the interior to the surface of components is low and the temperature difference is relatively small. Then, in the actual process of building the model, the MOS tube can be directly assumed as a simple solid body. As long as it has the same heat generation rate, radiation and convective heat dissipation area as the actual object, the same simulation results as the detailed model can be obtained.

In order to verify the rationality of this conjecture process, an experiment can also be carried out: two MOS tube models are placed in the same PCB. On one hand, the complex hollow model is constructed according to the actual structure, and on the other hand, the solid structure is simplified according to the same shape; The heating power of 0.4W is also applied to two MOS blocks, and the effects of two different modeling methods are compared. The results show that the effects of the two models are basically the same.

1.3 integrated circuit block modeling method

Firstly, according to the actual structure of the integrated circuit block, the modeling is carried out, and the relatively small heating silicon body is surrounded in the shell of alumina ceramic. The thermal conductivity of the ceramic is 21 The whole component is connected with the base plate through the pin, and the thickness of the pin is about 0.1mm

In order to simplify the model, follow the principle of "same thermal conductivity" and merge the originally separate pins. Experiments show that although the structure of integrated circuit blocks is accurately modeled, the temperature difference between the interior and surface of components is relatively small. Therefore, using the same analysis method as the previous MOS transistor, it can be proved that the modeling of integrated circuit blocks can also be replaced by simple blocks.

The above analysis proves that in PCB circuit board, MOS tube and integrated circuit block can be modeled in a simplified way without affecting the final result.

2. Experiment and verification

After completing the simplified modeling of components related to thermal analysis in electronic circuit board, it is necessary to combine the simplified models of various parts to build a complete PCB Flotherm model, and calculate and analyze the results: first, simulate at room temperature. Take the power loss of each electronic component as the heat generation rate load of thermal analysis, apply 20 ℃ air natural convection on the surface of PCB, and the ambient temperature as the radiation boundary is room temperature (20 ℃), and calculate the heat distribution.

Combined with the practical application, the infrared thermal imager can be used to measure the working temperature of the electronic circuit board at room temperature, and the calculated value of the test point position can be compared with the actual measured value. Due to a series of simplification in the model, the error of the calculated value can be controlled within a certain range. From table 1, the temperature distribution calculated by Flotherm software is basically consistent with the measured value. It can be seen that the simplified modeling method is convenient and feasible.

The previous experiments were carried out at room temperature, but the environment of PCB circuit board may be different in practical work, so the above experimental methods can not be used to measure the distribution of temperature field. Then, according to the idea of simplified modeling, software can also be used to simulate the temperature field distribution of electronic circuit board in vacuum environment. For example, the heat sink is installed on the top of the PCB circuit board. At this time, the PCB substrate is connected with the bottom plate of the heat sink, and a 60 ℃ cooling air conditioner is connected at the same time. In the model calculation, the heat transfer boundary conditions are set at the bottom of the substrate and the heat sink respectively. According to the final results, under the condition that the cooling air conditioner works normally, the maximum temperature of the device is only slightly higher than that of the air conditioner, which meets the safe working range; On the contrary, if the failure state of cooling air conditioner is simulated and only radiation is used to dissipate heat, the maximum temperature of MOS tube will exceed the safe working range.

In conclusion, when analyzing the thermal reliability of PCB electronic circuit board, the modeling can be greatly simplified, which is more conducive to the modeling operation. For the heating components that need to be simplified, because the thermal resistance from the inside to the outside surface is relatively small, the electronic components with complex structure are treated as simple modules, and the thermal analysis of PCB electronic circuit board with Flotherm software can reduce the requirements of computer for memory and improve the calculation efficiency and accuracy. The experimental results show that this method has good scientificity, rationality and operability, and is easy to use. It can well complete the thermal reliability analysis of complex electronic circuit boards, and has a wide application space and application value.