Diagnosis of Malfunctions in Complex Electronic Assemblies. Final Report. Page: 63 of 66

of Mylar tape and the liquid crystal mixture is brushed onto the surface of
the tape. The liquid crystals should be applied evenly. As soon as the cry-
stals turn red, indicating that the lower range of the mixture is at ambient
temperature, testing may begin. Body or hand heat imparted to an insulator
such as a Formica table top will affect the color of the liquid crystals. The
unit under test was isolated from the table by a folded tissue, and careful
handling prevented any serious trouble from extraneous heat sources. Care
must be taken not to contaminate the R or L components to prevent a change
in the mixing ratio required to obtain color at a given temperature. Separate
eyedroppers were used for each mixture and individual eyedroppers were
used for each bottle.
Routing and _'low-Time Effects. Another type of failure that sometimes
could not be isolated occurred when an electrical rejection could iiul Le
repeated or was lost in evaluation. The isolation between operator error.
test equipment, and the production unit cannot always be made from data
from one-time reject information. If the fault can be found by troubleshooting
at the time of failure, there is a better chance for isolation; but if the unit is
removed from the tester by inspection and sent to troubleshooting, an in-
spection error or equipment failure may be lost. In such cases, a technical
plan which severely tests those circuits associated with the purported failure
is normally outlined and, if no failure is found during this investigation, the
unit is submitted to the complete inspection testing again in accordance with
a defined anonymous-failure procedure in the electrical specifications.
One definition of a successful troubleshooting attempt might require that the
requested rework correct the discrepant parameter and bring it within speci-
tications and normal distribution limits, but a more rigid definition requires
that the rejected part be found defective when removed from the assembly
and tested separately. In the more-complex higher-level. assemhlies, a
small percentage of rejected units failed to meet the last requirement, but
apparently met the requirements of the first definition. Such incompatibilities
can be caused by one or more of the following.
" Operators can make errors in verifying the purported failure.
" In cases where critical circuits limit the degree of margin in design, the
worst-case design condition cannot be applied, and when many componentE
are involved, unique tolerance buildups may cause marginal conditions
between two particular subasserblies or between testers at different
levels of testing.
" An intermittent condition can exist where the. failure is very critical to
environmental parameters such as temperature, humidity, or electrical
or physical stresses.

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Losure, J. A. Diagnosis of Malfunctions in Complex Electronic Assemblies. Final Report., report, January 1, 1971; Kansas City, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1031893/m1/63/ocr/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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