3.4.7 HERMETICALLY (FINE/GROSS SEAL)
Hermeticity or leak testing is a screen performed on hermetically sealed
packages (those with cavities) to detect package sealing defects such
as cracks and holes, improperly formed seals, and improperly sealed
lids. A package is hermetic if the leak rate is small enough to
maintain the desired internal atmosphere for the desired life of the
part. The detection of leaks in hermetic packages and removal of
them from the production lot will keep potential corrosion related
defects from becoming field reliability problems. Exposure of a package
to helium (fine) or fluorocarbon (gross) gases at elevated pressures provides
a screening technique for the detection of leaks in a limited leak
range for hermetically sealed packages.
Hermeticity tests must be performed as two separate tests. This is
necessary because devices having different magnitudes of leak rates
cannot be detected by one single leak test method. Fine leak test
methods will not detect devices having large holes in them and gross
leak test methods do not have the sensitivity to detect fine leaks.
Fine leak tests must be performed prior to gross leak tests, if the
fluids used in the gross leak test methods could contain particles
large enough to plug fine leak holes.If a one-micron filtration system
is used the order of hermeticity testing may be reversed.Other
hermeticity testing methods can be found in MIL-STD-883 Method 1014.
Other techniques include radioisotope fine leak, penetrant dye gross
leak, weight gain gross leak, and Nid gross leak.
Fine and gross leak tests are designed to determine the effectiveness of
the seal, detecting leaks ranging from 5x10-8 to
1x10-5 atm cc/set. They screen out the defective seals that
will produce latent failures when exposed to moisture or corrosive gaseous
contaminants. These tests should be one of the last screens after all
part handling has occurred.