During end-of-production-line testing of electronics
assemblies, multiple functional test failures (electrical short circuit /
low resistance) were identified.
Investigation found that tin whiskers growing from RF
shields/brackets became dislodged during the electronics assembly
process. The loose whiskers were scattered onto un-insulated pins of
a nearby connector producing undesirable short circuits
The RF shield was tin-plated steel with an intermediate
copper barrier layer between the tin and steel. The user of this RF shield
had specified that a nickel barrier be used instead of copper; however,
the product was not built to specification.
Within 5 to 6 months of plating these RF shields,
thousands of tin whiskers had formed with many of them extending as long
as 2.5 millimeters or more.
During the assembly process where these shields were being
installed over the electronics module, the assemblers were unaware of the
presence of tin whiskers on the shields. This is not surprising since tin
whiskers can be extremely difficult to see with the unaided eye due to
their extremely narrow cross sections (typically only a few microns or
less across).
Failure analyses of modules that failed
end-of-production-line testing identified detached tin whiskers had
bridged exposed conductors in the assembly producing the observed
electrical faults. In one example, a tin whisker bridged adjacent
connector pins whose pitch was 100 mils. As a result of the
failures, a careful inspection of the module was performed at which
point the extensive tin whisker growth on the RF shields as well as
loose tin whisker debris scattered within the module were identified as
the root cause of the problems observed.
Source: Anonymous
