2.4.2 RANDOM VIBRATION PROFILE

In order to obtain an optimum RVSS, it is necessary to obtain an effective
random vibration profile. The principal parameters in random vibration
are the number of axes vibrated, the input stimulus to the equipment,
and the duration of the vibration. Due to the varying
parameters of electronic hardware and the complexity of optimizing the
vibration levels, there is no one profile for all equipment.The
required method of determining the maximum allowable PSD levels is to
calculate the input profile to the PWA on actual hardware using the
methodology contained in Section
5 or similar methodology. The values that can be determined using
Section
5 for the maximum allowable PSD input level are excellent starting
values. These values are calculated to establish a baseline for the
vibration levels. The maximum PSD input level determined is the largest
possible PSD level that the PWA can withstand in each axis due to the
limitations of the worst-case part. Therefore, the maximum allowable
PSD level in each axis determined by the method presented in Section 5.3 shall be reduced by 25 %
before beginning any vibration proof test. If failure occurs,
the proof test shall be terminated and the failure analyzed to determine
if the failure was a latent defect or if the PWB/parts were
overstressed. If the failure was due to latent
defects, the PSD level may be raised and the test continued using screen
failure analysis techniques until a plateau is reached where
surfacing of latent defects is optimized, but below PWA
overstressing. Once this plateau is achieved, other identical
PWAs shall be tested to insure production variations in
mechanical strengths do not result in overstressing a portion of
the lot. If the first proof test doesn’t produce any fallout,
the PWA is most likely being understressed. The
PSD levels shall be raised and proof testing continued by following the
screen-failure analysis technique described above. In no event shall
the PSD levels exceed the maximum values determined by Section 5.3 or .04
g^{2}/Hz.