| In the course of use and transportation,electronic equipment is inevitably affected by external dynamic environment such as vibration or shock.With the increasing miniaturization and high density of electronic equipment,various electronic components,such as functional chips,resistors and capacitors,are connected with printed circuit boards mainly through welding,so the vibration characteristics of PCB directly affect the reliability of the electrical connectors on the printed circuit board in the dynamic environment.In order to improve the reliability of the electrical connector on the printed circuit board,this paper studies the characteristics of the printed circuit board and the effect of the electronic components on the vibration characteristics of the printed circuit board.By means of experimental modal analysis and finite element modal analysis,experimental modal analysis is used to verify and modify the finite element model to improve the accuracy of the finite element model.Finally,the phenomenon of low voltage arcing in the charging process of the electric power connector of the mobile phone is reproduced.It is proved that the contact pair produces arc under the condition of the arc voltage below the metal,leading to the carbonization of the contact surface lubricant.The main research contents and achievements of this paper are as follows:(1)The material properties of printed circuit boards have both the properties of composite materials and the properties of viscoelastic materials.This paper takes the type FR-4 printed circuit board as the research object,first expounds the category and the composition of the printed circuit board,and then deduces the stress-strain relation of the printed circuit board using the linear elastic mechanics method according to the material of the printed circuit board,and then uses the viscoelastic theory to analyze the printed circuit.Finally,the stress-strain relationship of FR-4 copper clad laminate is deduced based on the theory of elasticity viscoelasticity consistency.(2)According to the testing method of viscoelastic material characteristics,the viscoelastic material properties of printed circuit boards are measured.Firstly,the principle and application range of two main methods of measuring viscoelastic materials,bending resonance method and forced non resonance method,are compared.Then,according to the characteristics of printed circuit board,the best measurement method is selected.In this paper,the flexural resonance method is used to measure the elastic modulus of printed circuit boards.The measurement method of loss factor is based on the free attenuation method.The specific process of measurement is to first determine the natural frequency of the sample,and then use the non-contact electromagnetic vibrator to stimulate the sample to achieve the stable state and then suddenly withdraw the excitation.Finally,the loss factor of the sample is obtained according to the free attenuation vibration wave.By changing the length of the specimen,the loss factor and modulus of elasticity at different frequencies can be measured.The measurement results show that the elastic modulus and loss factor of PCB are not constant,and are related to the frequency.(3)Taking the bare board of FR-4 printed circuit board as the research object,a modal experimental system for measuring the modal parameters of printed circuit boards,natural frequencies and mode shapes,is designed.Due to the light quality of printed circuit board,if a contact sensor,such as an accelerometer,is used,then the accelerometer will cause great error in the measurement results.Therefore,this paper does not use the traditional contact sensor to measure the response,but uses the non contact sensor eddy current sensor to measure the response.In addition,the excitation method is driven by the electric exciting platform,so the measurement accuracy is improved to the maximum.(4)In the modeling process,the material nonlinearity of PCB was first considered,and a more accurate finite element model was established.The modeling process is as follows:first,using the material parameters provided by the manufacturer,the model is established according to the isotropy,and then the finite element modeling method of the printed circuit board is determined by comparing the inherent frequency calculated by the finite element software and the natural frequency obtained by the numerical method,and then the material parameters obtained by the experiment are carried out.The printed circuit board is modeled according to the anisotropic material.Finally,the finite element model is further modified with the natural frequency and vibration mode obtained from the experimental mode,and the nonlinear characteristics of printed circuit board materials are taken into account.The research results show that the established finite element model of printed circuit board is more accurate than the traditional method.(5)The effects of components on the vibration characteristics of printed circuit boards are studied by using accelerometers to simulate components.As the electronic components are usually fixed on the printed circuit board by welding,to study the influence of the vibration characteristics of the printed circuit board when the electronic components are located at different positions,the actual operation is complicated if the specific components are used,and the difficulty can be overcome by the accelerometer simulation,and it can be very convenient.The natural frequencies of accelerometers on different positions of printed circuit boards can be measured.The finite element software is used to establish the model,and the natural frequencies and mode shapes of accelerometers on different positions of printed circuit boards are obtained.The error between experimental results and finite element software simulation results is very small.The results show that when the accelerometer is located at different locations,the natural frequency decreases and the vibration modes are different.(6)The contact between power source and printed circuit board under vibration and the low voltage arcing fault due to instantaneous interruption are studied.A vibration system is designed to make the contact pairs break periodically,and a circuit system for monitoring the instantaneous faults is designed to reproduce the fault phenomena.The mechanism and the conditions of fault generation are studied through experiments.To sum up,this paper studies the hot and difficult problems that have not been solved in the impact of the dynamic environment on the reliability of electrical connectors,and has obtained the pioneering research results in the following four aspects.The stress-strain relationship of printed circuit board is analyzed by viscoelastic mechanics for the first time.The viscoelastic material characteristics of printed circuit board are measured by experiment for the first time,and the experimental data are fitted with Matlab software.For the first time,the effect of electronic components on the vibration characteristics of the printed circuit board is studied by using the accelerometer to simulate the electronic components.The micro arc phenomenon of the electrical connector under the arc voltage is studied for the first time. |
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