Analysis of the structure of surface mount circuit modules
When only considering that the structure is subjected to static load, static finite element analysis can be used. However, in actual work, the structure is often subjected to dynamic loads that vary with time. If the dynamic load is large, or although not large, but the frequency of the force is close to the natural frequency of a certain order of the structure, the structure will resonate, which will cause high dynamic stress and cause structural strength damage or non-uniformity. Allowable deformation. Therefore, it is necessary to make a dynamic analysis of the structure, that is, to analyze the natural frequency of the structure, the main vibration mode (mode), and the dynamic deflection and dynamic stress under the action of dynamic load.
Dynamic analysis refers to the analysis and study of the dynamic characteristics of the system based on the system dynamics model based on the external excitation and actual working conditions of the system. It specifically includes the analysis of the inherent characteristics of the system and the analysis of the dynamic response. The inherent characteristics of the system include parameters such as the natural frequencies, modal shapes, and modal damping ratios of the system. The purpose of inherent characteristic analysis: On the one hand, it is to avoid resonance or harmful vibration mode when the system is working; on the other hand, it is to lay the foundation for further response analysis of the system. Therefore, inherent characteristic analysis is a problem that needs to be solved first in system dynamic analysis. Response analysis is to calculate the various responses of the system under the action of external excitation force, including displacement response, velocity response and acceleration response. The system’s response to external excitation leads to principal stress and dynamic displacement inside the system, which affects the service life and working performance of the product, or produces greater noise. The purpose of response analysis is to calculate the dynamic response of the system to various possible exciting forces and control it within a certain range.
When finite element analysis of structural dynamics is carried out, modal analysis is first carried out. Through modal analysis, the natural frequency and modal shape of the structure can be determined to avoid resonance at the same frequency as the excitation frequency, and then harmonic response analysis can be performed. Through harmonic response analysis, the steady-state response of a linear structure under a load that changes with time according to the harmonic law can be determined, and the response value (usually displacement) of the structure at several frequencies versus frequency can be obtained.
(1) Modal analysis
The mode is the natural vibration characteristic of the structure. Each mode has a specific natural frequency, damping ratio and mode shape. Natural frequency and mode shape are important parameters in the design of dynamic load-bearing structures. These modal parameters can be obtained by calculation or experimental analysis. Based on the principle of linear superposition, a complex vibration system can be decomposed into the superposition of many modes. Such a decomposition process is called modal analysis.
Modal analysis is used to determine the vibration characteristics (natural frequencies and mode shapes) of the structure and its components in the design. It can also be used as a starting point for other more detailed dynamics analysis, such as harmonic response analysis, spectrum analysis, and transient dynamics analyze. The basic process of modal analysis consists of 4 steps: ①modeling; ②loading and solving; ③expanding modal; ④checking the results. The modeling of modal analysis is similar to that of other types of analysis, but only linear behavior is effective in modal analysis. If a non-linear element is specified, it will be treated as a linear element. Material properties can be linear or non-linear, isotropic or anisotropic.
The vibration mode is the inherent and integral characteristic of the elastic structure. By determining the characteristics of the main modes of the structure in a certain frequency range through the modal analysis method, it is possible to predict the actual vibration response of the structure under the action of various external or internal vibration sources in this frequency band.
Since the quality of the actual structure is continuously distributed, any actual structure can be said to have infinite degrees of freedom. But if all structures are calculated according to infinite degrees of freedom, it is not only very difficult, but also unnecessary. In actual calculations, infinite degrees of freedom are replaced with multi-degree-of-freedom systems, thus simplifying the calculation of natural frequency.