The formula used on wikipedia is for calculating the water pressure when you have water vapour in equilibrium with liquid (bulk) water in a confined volume, and that is hardly the case when a PCB is passing a soldering process.



On the other hand, using the formula PV=nRT may also give the wrong answer if n (the number of gaseous water molecules in the confined volume) is assumed be unaffected when the PCB is heated. At room temperature, most of the water molecules in the PCB will be bonded (adsorbed) to surfaces inside the laminate. This bonded water will have other properties than bulk water. Compare with an adsorbed water film on a hydrophilic surface which will form at relative humidities far below 100%. When a PCB is heated, the absorbed water will evaporate causing an increase of the internal pressure due both to an increase of the temperature and an increase of n in the formula PV=nRT.



The question is at what temperature will the absorbed water molecules be evaporated. Due to the strong bonding of adsorbed water molecules (compared to bulk water) the temperature probably needs to be considerably above 100C before all water molecules have been evaporated. If we assume that all of them have been evaporated at the soldering temperatures used for SnPb soldering, then n would be unaffected by a further increase of the soldering temperature and the formula PV=nRT could be used to calculate the increase in pressure. If this assumption is wrong and there still are some water molecules that have not been evaporated, an increase of soldering temperature would result in a larger increase in pressure, but far from an increase with 100%. Also, this means that the pressure will be far below 300 psi even for SnPb soldering.



A similar question has also surfaced when discussing the effect of absorbed humidity on blistering of conformal coatings. So far, I have not seen any measurements or relevant calculations of what pressures could be expected under these conditions. Do anyone else have knowledge of such measurements or calculations?



Per-Erik Tegehall





-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Jack Olson
Skickat: den 24 oktober 2011 16:27
Till: [log in to unmask]
Ämne: Re: [TN] How do we determine the Maximum Acceptable Moisture Content (MAMC) for a board?



you can check it on wikipedia...

http://en.wikipedia.org/wiki/Vapour_pressure_of_water



look at the chart,

compare the difference between 493Kelvin and 533Kelvin

(nearly double)



Jack



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