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February 1997

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[log in to unmask] (DAVY.J.G-)
Date:
Mon, 24 Feb 1997 15:47:18 -0500
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     cbyxbee reported having occasional "problems with moisture in the 
     soldering operations" of boards and wondered whether the problems 
     might be due to packaging them in paper instead of something that 
     would protect them from moisture.
     
     I thought that I'd see a response by now; since I didn't, I'll try, 
     but before sticking my neck out too far, I'd like to have a better 
     idea of what constitutes "problems with moisture in the soldering 
     operations".  In a previous posting I described how blow-holes are 
     caused by moisture that is present in improperly drilled holes as part 
     of the crystal structure of occluded copper salts; this moisture 
     cannot be driven off by any baking you'd be willing to consider, but 
     the blow-holes are rarely so severe as to threaten the reliability of 
     the connection.
     
     The condition called "measling" is also caused by moisture: previously 
     dissolved water molecules in the resin of the board are evolved as a 
     gas during the sudden heating of soldering.  This condition has also 
     not been shown to threaten reliability, but its occurrence can be 
     diminished by baking the board before soldering.  I'm guessing that 
     the problems you refer to are one of these.
     
     Given that a board must be exposed to air of 40% to 60% relative 
     humidity while parts are being installed, it will pick up some 
     moisture during this time if it was dried before.  How hard is it 
     worth trying to keep the board dry during assembly, to avoid having to 
     bake it before soldering?
     
     Incidentally, let me mention again that the use of vacuum to promote 
     drying, however much it may appeal to one's intuition, is worthless.  
     Vacuum cannot reach into the plastic to pull water molecules to the 
     surface, and that's the step in the drying process that takes the 
     time: waiting for water molecules to diffuse to the surface on their 
     own, driven by a concentration gradient, at a speed that depends only 
     on the temperature.  In comparison, removing water molecules from the 
     surface takes no time at all in an ordinary oven.
     
     To make it seem a little more reasonable that vacuum doesn't help, one 
     can calculate that if the relative humidity at room temperature is 
     50%, that corresponds to a partial pressure of water vapor of about 
     0.015 atmosphere.  The partial pressure in the oven will be the same 
     (as long as there is any kind of venting), but since the temperature 
     is much hotter, the RH will be much lower: 1.5% at 100 deg. C, for 
     example.  So the effect of the vacuum, no matter how good, is to 
     reduce the RH only slightly.  Since the vacuum may make it more 
     difficult for the item to reach temperature, it might even slow things 
     down.  The only case I have seen where a vacuum bake was worth doing 
     was where the item to be dried out had only a small orifice in the 
     wall through which all the water molecules had to pass.  But in the 
     case of a board, there is no restriction to water molecules leaving 
     the vicinity of the surface.
     
     Also, if a board is dried, putting it in a plastic bag won't help keep 
     it dry, because water molecules from the air will go right through the 
     bag, probably much faster than they will enter the board.  You could 
     add desiccant to the bag, or store the board in a dry environment, but 
     is it worth it?  If there is some special situation that I have not 
     covered, post again and I'll try again.
     
     Gordon Davy

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