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 *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************