Hi Gordon -
Do you really put things in the freezer? I have used a refrigerator as
a method for extending solder paste use life but you have to take
precautions for condensation, volatiles loss,etc. I would like to see
the data that shows a tin/lead oxide growth is significantly different
at 23 C than at 0 C - I have doubts that all of that effort/logistics
of using a freezer would be beneficial. I agree with your comments
except for one section:
"A tin-bearing finish quickly develops a protective oxide coating of a
few dozen nanometers (a few hundred atomic layers).There is no further
oxidation unless the finish is porous, no matter how long the storage.
The thickness will be the same if the part is stored in your desk
drawer, your garage at home, or in dry nitrogen."
If we were talking about nickel or aluminum I'd agree but tin finishes
form an initial SnO structure and then over time convert to SnO2 oxide
structure. There have been several studies using wet chemistry, Auger,
and SERA methods demonstrating that the oxide changes and is not
"stable". Even in nitrogen storage the oxide grows (especially if your
nitrogen gas purity is poor) - it grows slowly put it does grow. The
studies have also shown that solderability of your board or component
is dependent your flux's ability to breakdown the type of surface
oxide present - i.e. SnO2 gives many flux chemistries fits whereas SnO
most fluxes handle easily. The problem many of us face in industry is
that the flux-oxide type interaction gets complicated by the thickness
of the solder coating - just as you pointed out.
Potatoes in solder, finishes in freezers, solder paste in
refrigerators, Joy in waterwashes - we should have all taken home
economics in school!
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator _________________________________
Subject: RE: GEN: Shelf life for components...
Author: [log in to unmask] at ccmgw1
Date: 1/16/97 11:04 PM
G K Bhat asks about a "standard shelf life" for components, presumably
referring to solderability. Since most components have a tin-bearing
finish, that is what is discussed here.
While it is often asserted that solderability deteriorates with
storage, (and sometimes it really does), it is important to understand
that solderability _need_ not deteriorate, and parts that are twenty
years old will solder just fine, even without an elaborate (expensive)
storage environment. In some cases where old parts are found not to
solder well, they weren't very solderable when they went into storage,
and no one noticed.
To the best of my understanding, there are two conditions that cause
loss of solderability during storage:
1. A finish that is very thin. (See my recent TechNet posting on
immersion tin coatings.) This is typically seen on such low-cost
items as terminals, for which the price of the finish is a significant
fraction of the cost of the part.
2. A finish that is porous. Plating salts remain at the base of the
pores and in a humid environment promote corrosion. While in princi-
ple it would be possible to prevent this mechanism by storing in a dry
environment, a better prevention is not to accept parts that have a
porous coating. Porosity is a characteristic of plating that is thin
or not properly controlled. Reflow of a plated coating, or dipping in
solder, removes porosity. Note that it is entirely possible for a
plated finish to not be porous, and in the case of a rectangular lead
(see below), there is a benefit to a plated finish.
About ten years ago, steam aging was added to the solderability test
methods (IPC and military). Manufacturers of military integrated
circuits found that to pass the test with steam aging, they had to dip
the leads in solder after burn-in. They were reluctant to do that,
because it was inconvenient to perform a manufacturing operation after
electrical test, but they did it anyway, and these parts went, in just
a couple of years, from the ones with the worst solderability to the
ones with the best. The solder provides a pore-free finish that
remains solderable indefinitely without special storage provisions. (I
believe that the steam aging reveals a porous finish by accelerating
the corrosion to such an extent that the solderability is lost.)
Incidentally, the issue of "oxidation" needs to be addressed, since
the term is so often used in discussing loss of solderability. A
tin-bearing finish quickly develops a protective oxide coating of a
few dozen nanometers (a few hundred atomic layers). There is no
further oxidation unless the finish is porous, no matter how long the
storage. The thickness will be the same if the part is stored in your
desk drawer, your garage at home, or in dry nitrogen.
The only case I know of where a part which has passed the solderabil-
ity test after steam aging fails it later is on rectangular solder-
dipped leads. Because of the geometry, the amount of solder protec-
tion along the four sharp edges that bound the major and minor flats
is minimal. Also, it is hard enough to judge solder coverage along
the minor flats (typically only about 5 mils (125 micrometers) wide,
let alone the edges themselves, so the people performing dip-and-look
solderability testing for the component manufacturers may well not
notice lack of solder coverage in these regions. The wetting balance
is also insensitive to such cases because the area affected is only a
small part of the total area tested. (A plated (unreflowed) finish
covers the lead much more uniformly than does a reflowed or dipped
finish, and for a small-diameter round lead, may be thicker than is
achievable by dipping.)
There are three ways I have been able to think of for dealing with
this possible local loss of solderability:
1. Store the parts in a freezer.
2. Test again just before soldering and redip if necessary.
3. Alter the acceptance requirements for the solder connection so that
wetting to the sharp edges is unneeded. Since, as just stated, the
area affected by the loss of solderability is small, the loss of
strength due to lack of wetting to this area is also small.
Gordon Davy
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