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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