Sn whisker growth was investigated from submicron Sn layers on Cu substrates in thermal
vacuum conditions to compare the growth differences in ambient and space conditions.
Cu substrates were covered with 500 nm thick Sn by PVD. The samples were kept at 50
°C and 8.3x10-6 mbar for 1000 h in order to simulate the space conditions. Numerous
Sn whiskers were found on the samples after some days of sample preparation. More
but shorter whiskers with different structures were developed in a vacuum than in
ambient conditions before. The vacuum-grown whiskers had segmented block-like bodies
with a plain surface, while ambient-grown whiskers had a twisted body with grooves.
TEM investigations found a weak correlation between the crystal structure and the
shape of the different whiskers. However, it proved that the high mechanical stress
of the Cu6Sn5 intermetallic layer growth initiated the interface flow mechanism, which
transported Cu6Sn5 into the whiskers. The vacuum-grown whiskers contained approximately
half the amount of Cu6Sn5 inclusions than the ambient-grown ones, which could be related
to the more uniform stress relaxation of the oxide-free surface in the vacuum. The
higher amount Cu6Sn5 inclusions could cause the twisting of the whisker bodies in
ambient conditions.
