5.5 Assembles to Be Considered For Vibration Screening Deletion
5.5 ASSEMBLIES TO BE CONSIDERED FOR VIBRATION SCREENING
There are several types of hardware that will not benefit from a vibration
screen and may even be degraded from an improperly applied vibration screen.
Some of the items that should be considered for deletion of vibration
screening are listed below. In many cases, the vibration screen may be deleted
on these assemblies if an effective thermal screen can be
5.5.1 WIRE WRAPPED BACKPLANES
Wire wrapped backplanes (WWB) often
receive a vibration screen at the NAVMAT levels. These screens generally prove
ineffective at finding workmanship defects but are helpful in “cleaning up”
debris that is left from the manufacturing of the backplanes. If the WWB’s
need to be cleaned, a .01 g2/Hz random vibration can be performed
that will remove the debris. This vibration should not be considered as part
of the screening process, but as a manufacturing process. WWBs and backplanes
should only be screened if they contain parts or workmanship that may benefit
from a properly developed screening process.
5.5.2 MECHANICAL HARDWARE
Purely mechanical hardware should not receive screening. Purely mechanical
hardware is defined as hardware that contains only mechanical parts. Other
systems that can be included in this category are mechanical systems that
contain very limited electrical hardware, e.g. antennas, fuse boxes, and
switch boxes. These units will not benefit from screening and should not be
screened. Mechanical devices will generally have a reduced life if they are
subjected to screening. The reduction in life is small but the screen will not
surface any defects that will justify screening.
Electronic displays, e.g. Liquid Crystal Displays (LCD) and vacuum
displays, should not be screened by either random vibration or thermal
cycling. Displays that are subjected to screen will experience degradation
and/or hard failures. Thermal cycling will create stress fractures in the
glass materials due to the differences in thermal coefficients of expansions
and vibration will crack the displays.