A question has been raised over a couple of the comments I made in a 
     previous TechNet posting regarding the growth of tin oxide on solder 
     and of copper-tin intermetallic compound.  I stated that tin (or 
     solder) quickly forms a protective oxide layer a few hundred atomic 
     layers thick, and the growth then effectively stops.  While I didn't 
     say so, I was referring to oxidation under ordinary room conditions, 
     not, for example, in an oven or in steam.  My basis for making that 
     statement is R. J. Klein Wassink, _Soldering in Electronics_ (2d 
     edition, 1989, p.221, quoting original work by others):
     
          Fresh [SnO] layers have a thickness of 1,5 nm and grow 
          approximately _logarithmically_ [emphasis added - not 
          parabolically] with time: at room temperature 2 nm after a 
          week, 3 nm after a year and only 4,5 nm after 20 years (1 nm 
          = 10 angstroms).... The effect of lead (in solid solution in 
          the tin) on the oxidation rate of tin can be neglected.... 
          The tin in the alloy oxidises preferentially, so that for 
          solder alloys the main oxide is SnO, just as for pure tin...
     
     I know that Dr. Tench of Rockwell has been studying the oxidation of 
     tin and tin-lead in recent years, and maybe he or someone else has 
     data that conflicts with the above.  Perhaps very high relative 
     humidity at room temperature promotes faster or thicker growth.  On 
     the other hand, perhaps his findings relate to oxidation at elevated 
     temperature or in steam.  I'd be interested to see some data on this 
     question.  To the best of my knowledge, the oxide that exists on tin 
     or solder is SnO if it hasn't been exposed to deliberate heating or to 
     steam (in fact, it is this difference that limits the predictive power 
     of steam aging for solderability testing, since steam seems to promote 
     the formation of thick SnO2), but if my info is out of date, please 
     let me know. 
     
     I also commented about putting a printed board with immersion tin in 
     the freezer.  The reason for doing so was not to reduce the rate of 
     oxidation of tin, since that reaction is practically complete before 
     you could get it cold, but to reduce the rate of reaction between tin 
     and copper.  (If it is true that SnO slowly converts to SnO2, then 
     freezer storage would potentially be a benefit by reducing the 
     conversion rate.  Also, I don't mean to imply that putting boards in a 
     freezer would be economically justifiable, since I don't know whether 
     it is or not, but only to identify what is technically possible.)
     
     In general, reaction rates decrease exponentially with decreasing 
     temperature, and the temperature difference between room temperature 
     and a typical home freezer is about 40 deg. C, so that should be 
     enough to reduce the rate of reaction between tin and copper by better 
     than an order of magnitude. (I haven't looked up the activation energy 
     or done the calculation, but I'm sure that this is so.)  A commercial 
     freezer gets a lot colder than this, of course, and could be used to 
     reduce the reaction rate by several orders of magnitude. (One order of 
     magnitude would mean that a coating that would preserve solderability 
     for a day at room temperature would preserve it for ten days in cold 
     storage; two orders of magnitude would mean preserving it for a 
     hundred days.)
     
     Since my prediction was based not on my experience or anything that I 
     had read, but only on general prinicples, I was encouraged to see the 
     TechNet comment by Robert Welch (who has had a lot of experience - and 
     incidentally, his point about the importance of having the copper be 
     scrupulously clean was well taken, too):
     
        And if the project gets hung up for some reason don't fret, 
        just wrap them up like Cristmas leftovers and keepem in the 
        fridge.  
     
     Finally, I received expressions of concern about the possible harm to 
     a printed board by putting it in a freezer: "condensation, volatiles 
     loss, etc."  I can't figure out what the risks are here:
     
     1. Condensation. The condensation would occur when the board was 
     removed from the freezer - frost would form and then melt, but so 
     what? The board spent a lot of time during its fab in solutions a lot 
     more aggressive than a layer of frost or water.
     
     2. Volatiles loss. If a board needs to be stored for a minimum time at 
     room temperature to ensure that volatiles are lost before it is 
     soldered, that's news to me.  The only volatile compound that I can 
     think of in a printed board is water.  I know that a board can lose - 
     or gain - water during room-temperature storage (depending on how much 
     water it contains to start and the RH of its storage environment).  I 
     also know that a board can be baked before soldering to reduce the 
     risk of forming measles (or worse) during soldering, but that would be 
     true regardless of its storage time or temperature.  A board would not 
     gain a significant amount of water during freezer storage since the 
     permeation rate would be so low.  What am I missing so that I don't 
     perceive a risk?
     
     Gordon Davy

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