Hi Yuheda!
Well, I found something else to check out, go to:
http://www.alphametals.com/products/techarticle/1999140018.pdf
This link talks about a study done on a "green color" beneath rosin flux
residues (typical copper corrosion indicator). In the introduction, it does
differentiate between a copper surface that is exposed to an environment that
is surrounded with halide ions and/or hygroscopic materials, and benign,
natural, copper oxidation process that is self limiting.
If an assembly is going to be exposed to the harsher environment, then again,
it should be coated whether it is plated or not...a tin lead plating won't
cause it to last any longer without a protective coating than bare copper
will...
From everything I've learned in doing this search, the biggest reason that
copper is plated with anything, or coated with an OSP, is to keep it
solderable. One other thing I ran across is SMOBC, some soldermasks are not
as "air-tight" as what we think they are, some are permeable to moisture...so
what now? Do we worry about the traces turning to powder?
Plating has nothing to do with circuit reliabilty. If the board is cleaned
well, and in a environment that is not full of salt and moisture, it will
perform as well as a plated circuit. But if in such an environment, it
doesn't matter if it is HASL'd, flash gold, or bare copper, both assemblies
will need some sort of coating to protect it from the elements.
http://www.pcfab.com/archive/2000/0007/risk.html
This link talks about various surface finishes; HASL, Benzimidazol, Imidazol,
Immersion gold on electroless nickel, Immersion silver on copper,
Electroplated palladium on copper, and Immersion gold on electroplated
palladium. There is nothing in this study that suggests a problem with
exposed copper...which as anyone who has ever worked with OSP's know that
solder doesn't flow out to pad edges as with HASL. The only problem that this
study points out is that there may be a problem with handling and multiple
heat cycles...I believe this is because this study may have been conducted
before there has been improved formulations of OSP's. There are now formulas
that are more robust to handling and multiple heat excursions...
I don't know if this will be any help, but I humbly think that there are too
many people stuck in the 60's about exposed copper, ya' know, the "China
Lake" syndrome..hehe.
To respond to post sent to me about the Statue of Liberty being all green;
hey, that's called a "patina"...something that is very valuable, if any of
you have any old copper sculptures or art, DON'T CLEAN IT!! You'll wipe the
value out...
Even with the "Green color" on the Statue of Liberty, I wouldn't ever think
of entertaining the thought of standing at the top of the torch, grounding
myself, and tell somebody to pour a bunch of volts in down at the bottom, and
not worry about getting fried from the copper failing because it was green on
the outside...
-Steve Gregory-
<< Hi Steve,
You see - that's exactly what I mean. Copper is very corrosion resistant,
however we are coating it with so many corrosion resistant finishings. I am
quite certain that someone examined this issue at least once.
I was asked by Susan - What exactly am I talking about, so here goes:
I am talking about the bad things that might happen to an isolated trace or
the PTH in several holes that are only copper plated. The environment can
vary from room to harsh.
The trigger to this "weird" question is a discussion that we've held
regarding tented via holes for BGA. We need to tent the via holes near the
BGA to avoid the entrapment of soldeballs inside the via holes. Everywhere
it is recommended that if the via holes are tented - they should be tented
on both sides (we are working with dry film soldermask, that's why we are
tenting and not plugging).
Then came our QA people and said "Oh no.....you have now a via hole inside
that is copper plated and protected only by a thin (2 mil) plastic film on
both sides. What will happen if this film is damaged and some flux or other
contaminant gets inside the hole?"
I sort of agreed with them but on the other side - the copper oxide is
acting as a kind of a barrier that protects the copper after the initial
surface attack. That is why we have so many copper objects that live almost
forever (telegraph wires for example).
Again, I am quite certain that we didn't invent anything new here and these
discussions were held (in one form or the other) in other corners of the
globe too. So, I want to find some of these discussions and if any work was
done to check who's right - these reports I am interested in.
I'll be very gratefull for any inputs.
Best regards,
Yehuda >>
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