Master Thesis - OUFTI-1
Master Thesis - OUFTI-1 Master Thesis - OUFTI-1
4.2.1 Accuracy of the model First and foremost, it is important to note that the accuracy of the FE model which will be created, is strongly related to the decision-making. Indeed, if the results required are the component internal stresses, a detailed FE model is necessary. However, the long time required to build and solve this type of model can not be justied when only the PCB response is required. In this case, it is possible to greatly simplify the model by using several methods which will be discussed later. In addition, the accuracy of a FE model will be mainly dependent on various sources of error, including: • Manufacturing variability, which will cause deviations in the vibration response of supposedly identical PCBs. This variability includes not only material and assembly properties, but also dimensional tolerances applied during the manufacturing procedure. • Inaccuracies in the denition of the model input parameters. They can result from the modeling assumptions used or the impossibility to obtain reliable values of these parameters (no prototype available, ...). • Errors in the solution process (e.g., linear solutions in non-linear situations). The major diculties encountered during the creation of a PCB FE model are so: • To specify the input parameters, namely: stiness, density, boundary conditions and damping. The accuracy with which these parameters are specied, will determine the accuracy of the predicted response. So, experimental tests must be performed to determine reliable values of these parameters in order to obtain an accurate FE model. • Variation of these parameters due to manufacturing, assembly, wear, ... An example of manufacturing variation is shown in Figure 4.1, where the same test is performed on supposedly identical PCBs. It can be directly seen than the responses obtained are really dierent one from the other. 73
Figure 4.1: Example of manufacturing variation [44] • Limitations due to parameters and variation sources which can not be easily specied, such as non-linear eects, limitations of the FE mesh, physical eects that are too complicated to be easily included in the model (i.e., air or acoustic inuences), ... Most often, these diculties are overcome by including appropriate SF in the model. 4.2.2 Creating FE models of PCBs The procedure to follow for creating FE models of PCBs, can be divided into 5 parts: • Determination of the PCB properties • Recognition of components eects • Modeling of the chassis • Denition of the boundary conditions • Introduction of damping 4.2.3 Determination of the PCB properties The PCB properties include: Young's modulus, Poisson ratio, density, thickness, ... The specication of these properties represents the most dicult step in creating a PCB FE model. In general, their values may be provided by manufacturers. However, these values are not always exact (as shown in Figure 4.2). So, it is better to determine them by ourselves. 74
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4.2.1 Accuracy of the model<br />
First and foremost, it is important to note that the accuracy of the FE model which<br />
will be created, is strongly related to the decision-making. Indeed, if the results required<br />
are the component internal stresses, a detailed FE model is necessary. However, the long<br />
time required to build and solve this type of model can not be justied when only the<br />
PCB response is required. In this case, it is possible to greatly simplify the model by using<br />
several methods which will be discussed later.<br />
In addition, the accuracy of a FE model will be mainly dependent on various sources<br />
of error, including:<br />
• Manufacturing variability, which will cause deviations in the vibration response of<br />
supposedly identical PCBs. This variability includes not only material and assembly<br />
properties, but also dimensional tolerances applied during the manufacturing<br />
procedure.<br />
• Inaccuracies in the denition of the model input parameters. They can result from<br />
the modeling assumptions used or the impossibility to obtain reliable values of these<br />
parameters (no prototype available, ...).<br />
• Errors in the solution process (e.g., linear solutions in non-linear situations).<br />
The major diculties encountered during the creation of a PCB FE model are so:<br />
• To specify the input parameters, namely: stiness, density, boundary conditions and<br />
damping. The accuracy with which these parameters are specied, will determine<br />
the accuracy of the predicted response. So, experimental tests must be performed<br />
to determine reliable values of these parameters in order to obtain an accurate FE<br />
model.<br />
• Variation of these parameters due to manufacturing, assembly, wear, ...<br />
An example of manufacturing variation is shown in Figure 4.1, where the same test is<br />
performed on supposedly identical PCBs. It can be directly seen than the responses<br />
obtained are really dierent one from the other.<br />
73
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