Master Thesis - OUFTI-1

4.2.6 Modeling of the chassis
A general rule of thumb when a FE model is created, is to always model the next level
up from the structure. In the case of PCBs modeling, this level is the chassis.
If the chassis is not included in the model, it may severely aect the accuracy of this one,
unless the chassis will be extremely rigid in comparison to the PCB. Indeed, in reality, the
PCB is xed on the chassis. So, to obtain accurate results, it is a current practice to attach
the PCB on this structure during experimental tests. Thus, the boundary conditions are
identical to the ones encountered during the lifetime of the PCB and the obtained dynamic
response is more accurate.
4.2.7 Denition of the boundary conditions
Once the FE models of the PCB and the chassis are created, the next step is to
combine them together. To achieve this, rigid and/or exible FE assembly methods are
used, depending on the studied case:
• In terms of translational displacement, most xing methods are very sti. So, they
can be modeled using rigid assembly methods.
• In terms of rotational displacement, all xing methods display some exibility. So,
the use of rotational local stiness assembly methods, is required.
In this last case, the main diculty is to determine the value of the rotational stiness.
Two situations are possible:
• No prototype of the PCB is available. In this case, the options are very limited: the
value of this stiness can be estimated basing on subjective experience, or a detailed
FE model of the joint must be created (which takes, as already mentioned, too much
time in several cases).
• A prototype of the PCB is available. In this case, tests can be performed and the
value of the stiness can be determined using a trial/error approach. However, it
is important to note that methods developed in some works [50], were originally
intended for use with card-lock style xing mechanisms, which provide clamping
force along the entire edge of a PCB. So, these methods should be used with caution
in the case of locally xed PCBs (e.g., bolted, ...).
4.2.8 Introduction of damping
Several methods exist to measure experimentally the damping of a given structure (see
references [51] to [53]):
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