James Baker asks how to calculate the amount of desiccant to add to a 
     plastic bag containing an electronic assembly with "moisture- 
     sensitive" plastic encapsulated microcircuits.  The bagged assembly 
     may sit "in shipboard and landbased spare inventory for 15 or more 
     years."
     
     The way to calculate how much desiccant you need is to determine:
     
     1. The initial amount of water vapor in the bag (we'll assume zero)
     
     2. The amount of water vapor you can tolerate at the end of 15 years
     
     3. The rate at which water vapor gets through the bag, assuming some 
     average ambient temperature and humidity
     
     The answer to question 3 can be found if you have a figure for the 
     permeation rate, but the answer to question 2 is more problematic, 
     since many users find that plastic encapsulated microcircuits work 
     just fine at ordinary, or even moist, humidity levels.  They would say 
     that you don't need to bag your circuit at all.
     
     Before you go too far, you need to find out who says that the parts 
     you are using are "moisture sensitive", what they mean by that, and 
     why they say it.  In the cases I've heard of where a PEM was moisture- 
     sensitive, it was because of a deficiency in the way the manufacturer 
     packaged it, so the issue becomes one of familiarity with the manufac- 
     turer and his/her (its?) process controls.
     
     If you arbitrarily assume that the answer to question 2 is, say, a 
     partial pressure of 0.05 atmospheres (just below the vapor pressure 
     for ice at 0 C), then the question reduces to "how much water will get 
     through this bag in 15 years and how much desiccant do I need to 
     adsorb it, so that the residual partial pressure of water vapor is 
     0.05 atm?"
     
     Molecular sieve is the best desiccant, by the way, not only because 
     its gettering is effectively permanent, but because the amount of 
     water it will adsorb, as a percentage of desiccant weight, is high.  
     (You could also consider calcium carbide, if you can think of some- 
     thing to do with the acetylene that is produced by its reaction with 
     the water.)
     
     Two other possibilities that might be overlooked:
     
     1. If you were to store your spare microcircuits in the electronics 
     bays in which they will eventually be used, the ambient relative 
     humidity will be low, because the ambient temperature is high (at 
     least while the equipment is operating), whereas the absolute humidity 
     level is unchanged from outside.  It's the relative humidity that is 
     important in driving, or protecting from, moisture-induced failures.  
     The only function then of the bag is to protect it from ESD and 
     handling damage.
     
     2. If you were to store your spare microcircuits in a freezer, the 
     rate of moisture-induced failure mechanisms would be effectively zero, 
     because these mechanisms require liquid water, not ice, water vapor, 
     or water dissolved in plastic.  (I'm not claiming that this is 
     practical, mind you, just possible.)
     
     Gordon Davy

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