Since early 60s it was decided that 5,000 ppmv moisture was maximum allowed 
for hermetic packages. Corresponds to a number of monolayers water on the 
chip or substrates. Converted to about 1um of leakage hole and molecular 
Helium flow it means 5x10exp-8 atm cc per sec. If you seal with maximum 10 
ppmv O2 this will give 10 years without getting dewpoint over the dangerous 
level. However, with 1 volt chips and line space as little as 0.1 um, and 
life expectancy as much as 20 years or more the old limits are questionable 
by many. Some recommend a level of exp-11. Then we leave molecular flow and 
goes to diffusion . Have anyone of you implemented that ?
Inge 

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