Hi, Dave!
I was interested to read about the studies you and Dr. Tench have done
on tin oxide, and hope to be able to read more after I can get my
hands on the publications you cited, but don't keep me in suspense -
I'd still like to know whether my initial statement that tin oxide
grows a very thin protective coating that essentially then stops
growing is right or wrong. The thicknesses cited by Klein Wassink are
1.5 nm (nanometers) immediately, 2 nm after a week, 3 nm after a year,
and 6 nm after 20 years. Even in boiling water the thickness after 4
hours is only 4.5 nm. Using as a rule of thumb three atoms per nm,
that means that the oxide layer on any given tin surface that hasn't
been heated is 5 to 15 atomic layers. At such dimensions, it is a
little difficult to distinguish between a surface layer of SnO2 and of
SnO with chemisorbed oxygen.
If your studies have shown that the native oxide on tin or solder at
room temperature is much thicker than this, and that it continues to
grow, could you please provide some numbers? A simple citation of
thickness vs. time would be sufficient to allow comparison with what
I've quoted. Also, do you have data for the role of water vapor at
room temperature on the oxide growth rate?
Incidentally, let me mention that a great way to study oxidation of
tin and solder would be by ellipsometry - a technique very familiar to
people who measure oxides on silicon for device fabrication. Ellipso-
metry uses elliptically polarized light and measures changes in the
angle of polarization of the reflected light. It does require a
mirror-smooth surface (irregularities smaller than the wavelength of
the light being used), but it is able to detect sub-monolayer cover-
age, and to distinguish types of oxide.
I don't understand why putting something in a freezer to slow down a
reaction rate should sound strange. Actually, what seems strange to
me is to continue to put things in dry nitrogen without getting solid
evidence that that helps. Surely the cost of all that nitrogen and
the storage containers must be substantial. (It also seems strange to
me to do vacuum baking as if the vacuum could somehow suck water
molecules out of solids, but maybe that's getting too far afield.)
Your comment about using SERA to study what happens in a freezer has
me concerned that you may be confusing two different things. My com-
ments about the benefits of the freezer had to do not with the rate of
oxide growth (remember, I claimed that the oxide doesn't grow measur-
ably even at room temperature and hence that there is no benefit to
storage in dry nitrogen), but the rate of reaction between tin and
copper. I don't see how you could use SERA to study that. If you are
talking about the freezer retarding the conversion of SnO to SnO2,
then there is no doubt that it will, just because reducing temperature
reduces reaction rates in general.
There are two other issues that need to be considered here: 1) tin
whiskers and 2) tin pest.
1. Based on what I've read, I'd have to conclude that immersion tin
could develop whiskers, but need not. My thanks to Yisrael Leshman
for passing along information via TechNet on a new immersion tin that,
reportedly, does not form whiskers and does preserve solderability for
longer than people have usually experienced. There is another ques-
tion to be dealt with, though, and that is, what happens if tin whis-
kers do form on a printed board that then gets soldered? Certainly if
the entire assembly is heated above 232 C (tin's melting temperature),
the whiskers will all melt (to form tin balls?), and there will be no
residual stresses to drive further whisker formation, so how much
concern is justified? (Incidentally, in order for a wave- or hand-
soldered through-hole assembly to develop a complete fillet of the
type that people expect, the destination side of the board must rise
at least above 183 C (solder won't wet to a surface that is below its
melting temperature), and practically, it must get substantially
hotter than that, so if whiskers were even a theoretical risk, it
wouldn't take much of a process adjustment to ensure that it got above
232 C.)
2. My thanks to Joseph Haimovich for raising the issue of tin pest as
a potential problem of putting a tin-plated board in the freezer. Tin
pest is the conversion from a metallic crystal structure of tin
(white) to a nonmetallic, powdery form (gray). According again to
Klein Wassink (p.148, 2d edition), "the transformation may occur after
a long incubation period at temperatures below 13 C, the rate of
transformation being highest at approximately -30 C." I don't know
what the term "long incubation period" means, and I don't know if
anyone has data on tin pest forming with electroless tin on copper.
If someone were interested, it obviously wouldn't cost very much to
find out, although it would take a while, and of course, the most that
anyone could say would be, "It didn't happen during my investigation."
They wouldn't be able to say, "It never occurs" (and this applies as
well to studies of whisker formation). If it did occur, it seems
certain that that would spell the end of any protection of solderabil-
ity which the tin would provide.
Gordon Davy
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