Avionics equipment, vibration and shock caused by the failure will significantly reduce its reliability, resulting in extremely serious consequences. In the vibration test of avionics conducted, PCB also often occur. PCB assembly through dynamic analysis, design can effectively reduce the probability of failure in the test environment, improve the reliability and quality avionics products.
Kinetic analysis is based on dynamic analysis. PCB components through dynamic analysis can establish its dynamic model. Only established dynamics accurately model can only be effective from the kinetic analysis. Therefore, this article attempts to finite element analysis (FEA) and experimental modal analysis (EMA) a combination of pre-test analysis techniques for certain avionics PCB assembly (Figure 1) analysis of the dynamic characteristics and the establishment of the Finite element analysis model kinetic PCB assembly.
A finite element modal analysis
As a sophisticated numerical analysis techniques, finite element analysis techniques (FEA) has been widely used in electronic equipment PCB assembly dynamic analysis. And, FEA can help engineers design more reliable PCB assembly through the beginning of the design predict potential failures and fatigue. In this paper, an avionics PCB assembly (Figure 1) as the research object, its dimensions (length × width × thickness) of 133.5mm × 79mm × 1.8mm, screw the housing electronic equipment in the four corners by PCB on. The PCB component dimensions and fixation are similar to the standard test PCB regulations, only a few large thickness. Components and connectors using surface mount technology (SMT) and PCB assembly, in which the main components of the package as BGA, QFP and SOP.
Figure 1 target PCB assembly
1.1 Finite element analysis model
Physical properties of the material parameters of the various parts of the composition of the object PCB assembly shown in Table 1. According to the PCB assembly geometry information and related material information, established in ANSYS finite element analysis model (Figure 2). Due to the overall PCB assembly is obtained as shown by the dynamic performance data itself rather than the details of the data part, and therefore the modeling of the components and connectors simplified. Specifically, rectangular and square blocks to simulate components, connectors used to simulate the shape substantially. Finite element analysis model are used in various parts of the three-dimensional solid elements (SOLID187) to mesh (mesh using solid elements, although increasing the amount of computation to a certain extent, but from CAD to CAE model workload greatly reduce, conducive to the promotion of engineering applications), and connected components and the PCB, between the connector and the PCB are using multi-point constraint (MPC) to simulate. Also, due to the rigidity of the chassis is much greater than the stiffness of the electronic PCB assembly to simulate the PCB components and equipment chassis screw connection in the finite element model is applied to a fixed support constraints in the four corners of the screw holes.
Finite element model of the object in Figure 2 PCB assembly
1.2 Finite element modal analysis results
Establish a finite element model of the object PCB assembly, and use the block Lanczos method (Block Lanczos Method) modal analysis. In the form of modal analysis is obtained by solving the system characteristic equation, characteristic equation is generally a multi-DOF system can be represented by the formula (1), and to get the eigenvalues and eigenvectors of the system, which is inherent in the system of vibration frequencies and mode shapes.
Where, [M] - mass matrix systems, finite element analysis, modal mass matrix formed by the cell assembly; [K] - stiffness matrix systems, finite element modal analysis assembled by the element stiffness matrix is made; {X} - displacement vector system; ω-system characteristic values.
By modal analysis, has been the object of using four screws PCB assembly first three natural frequencies and mode shapes, detailed in Table 2. The PCB assembly first-order vibration mode of a bend, the second-order vibration mode is reversed, and the third-order vibration mode is sinusoidal curve. These modes and get the four screws under the JEDEC standard board similar.
Figure 3 PCB assembly first vibration mode (FEA)
Figure 4 PCB assembly second-order vibration mode (FEA)
Figure 5 PCB assembly 3rd-order modes (FEA)
2. Experimental Modal Analysis
Experimental Modal Analysis is a comprehensive number of engineering disciplines, it is through the establishment of test "device" with an estimated frequency response function, system identification, recognition results validate four steps to get the system modal parameters: natural frequencies, mode shapes, modal damping . Results of experimental modal analysis is often used to test the validity and accuracy of the finite element analysis model. The effectiveness of the finite element analysis model established in this paper to examine the object PCB assembly and modal analysis results correctness of the PCB components experimental modal analysis.
2.1 Experimental Modal Analysis System
Modal test system used in this paper by the exciter, force sensors, clamps, test objects, a laser vibrometer (IVS200), Dynamic Signal Analyzer (DP730), data acquisition recording software (SignalCalc730) / modal analysis software (ME 'Scope V4) and PC configuration, shown in Figure 6.
Figure 6 constitute experimental modal analysis system
In order to make a consistent boundary conditions PCB assembly modal analysis and finite element boundary conditions in the experimental modal analysis object, the object PCB assembly through four 15mm high-clinching standoffs with screws in the fixture plate. Specifically as shown in Figure 7. Experiment using sine sweep excitation test objects by laser vibrometer to adopt in response to shock PCB assembly, dynamic signal analyzer and data processing software to calculate the frequency of each point on the PCB assembly response function (FRF), the final use of the mold modal analysis software from modal parameter identification system.
Figure 7 is installed on the target PCB assembly jig
2.2 experimental modal analysis results
Select the number of points equal to the distance from the object PCB assembly, obtain the frequency response function of each point (FRF) by way of by-point scanning, and then identify the modal parameters of PCB components.
Figure 8 PCB assembly first vibration mode (EMA)
Figure 9 PCB assembly second-order vibration mode (EMA)
Figure 10 PCBS 3 order vibration mode (EMA)
3. Comparison of results and discussion
In order to test the results of the finite element modal analysis and experimental modal analysis of the consistency of the results, and thus determine the validity and relevance of the experimental model of the object created by the finite element model of the PCB assembly needs to be finite element results with experimental modal analysis modal analysis results were compared, specifically in Table 4. Finite element modal analysis results of the comparison process and experimental modal analysis results need to go through two steps: first, the natural frequency value comparison between the two, and then for both modal degree of correlation (MAC) for comparison. Comparison with the natural frequency of the most common steps, but if the results of the finite element results and experimental modal analysis modal analysis than strictly in accordance with the order of the corresponding words, only the natural frequency of the comparative risk of error will exist appear. This is due to the same frequency as the system may correspond to two values of two different modes. Therefore, only a combination of these two steps in order to accurately determine the relevance of the finite element analysis model and the experimental model.
4. Summary
Text in PCB assembly avionics equipment for the study, the first to establish a finite element analysis model, and calculate its first three natural frequencies and mode shapes using the block Lanczos method. Then, in order to test the correctness of the calculation and finite element model to determine the degree of correlation with the actual PCB assembly, using experimental modal analysis system for PCB component object experimental modal analysis. Finally, the calculation results and experimental results comparison shows: modal finite element analysis and experimental modal analysis results in good agreement with the dynamic characteristics of the data obtained in the text object PCB assembly is correct, reliable, and the finite element model The dynamics can be used to respond to subsequent analysis.
