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the formation of thick SnO2), but if my info is out of date, please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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