Master Thesis - OUFTI-1

4.2.5 Recognition of components eects
When components are soldered to a PCB, they locally increase the mass and stiness of
this PCB. So, to be accurate, especially when a large number of components are present,
the FE model of the PCB must include these eects.
Theoretically, this problem can be solved by creating a detailed FE model of each
individual component present on the card, as illustrated in Figure 4.7. However, as already
mentioned in section 4.2.1, this solution, which brings a high level of detail in the model,
is not justied if only the PCB response is required. Indeed, in this type of model, each
lead of the component must be modeled using three rotational stiness elements, for which
the stiness must be determined by a trial/error approach. This requires too much time
and eorts for the expected results and so, this type of modeling is generally less favorable
than simplied methods.
Figure 4.7: Detailed model of a component [45]
Dierent levels of simplication are possible. They are summarized here:
• Simple method: The components eects are completely neglected. The reasoning
behind this method is the following one: ignoring the stiness increases the PCB
response and decreases the natural frequencies, whilst ignoring the mass decreases the
PCB response and increases the natural frequencies. So, each eect "compensates"
the other. This method is useful when no data about the components properties and
location are available.
• Global mass smearing method: The mass of each component is spread out over
the entire area of the PCB. The stiness contributions are completely neglected. This
method, which could be appeared not accurate, is based on the fact that neglecting
the stiness is a conservative approach (for the reasons exposed in the previous point).
To apply this method to a particular model, the card must be weighted and then,
its mass must be divided by the volume of its PCB to obtain the global density to
apply on this one.
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