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Date: | 13 Jan 1997 15:07:47 -0700 |
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What is an appropriate method to determine the amount of desiccant to
pack with a VME assembly which is populated with moisture sensitive
plastic encapsulated microcircuits?
The packaged assembly may sit in shipboard and landbased spare
inventory for 15 or more years. The packaging technique is as
specified in MIL-STD-2073-1C Method 51 (formerly IIc) with ESD
protection. The formula in MIL-STD-2073-1C for calculating the
quantity of desiccant to use does not have a storage time component
and I have not found a storage duration mentioned in the standard.
The similar industrial specification is EIA/JEP124. The method for
calculating desiccant quantity does have a storage time multiplier. I
plan on using an ESD, vapor barrier bag per MIL-B-81705C Type I, Class
1 which measures about 12"x16".
The MIL-STD calculation would require 4.2 units of desiccant. The
EIA/JEP calculation would require 3.5 units of desiccant for a 12
month storage duration. The slide rule calculator provided by the
desiccant supplier says to use 1/2 unit of desiccant. If I use the
EIA/JEP method for the 15 year duration then I would need to include
53.5 units of desiccant which would require more space than the VME
card. This larger amount seems extremely impractical. The 1/2 unit
quantity seems too little for the anticipated storage time.
I would appreciate comments and ideas regarding this issue.
Jim Baker
Principal Hardware Design Engineer
General Dynamics Defense Systems
100 Plastics Ave., Room 2154
Pittsfield, MA 01201
413-494-7152
email: [log in to unmask]
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