Bob: We have used OSP finishes for over 15 years. At first we also had great
concern about possible field failures due to exposed copper. Our products
are used in all parts of the world in some fairly tough environmental and
corrosive conditions. Water treatment plants, refineries, coastal and ship
board, high humidity, HVAC/refrigeration, etc. Most of our equipment also
has a lifetime warranty.
We did a variety of testing the most severe being MFG (mixed flowing gas).
At that time we worked with IBM Endicott on conducting these tests.
Basically it's a controlled atmosphere chamber that has corrosive gases in
known concentrations. This is then correlated to a test coupon in the
chamber that can predict years of service life. We used the most severe, I
believe it was MFG-1. ASTM has similar tests at different corrosive levels.
Bottom line there was no problems with circuit corrosion, unit failure or
signal alteration. In fact I still have a box of the original meters from
that test in my office. I open them up, inspect and turn them on every few
years. Of course I realize my office is not the harshest of environments but
in still works as a sanity check.
However I would make the following points:
1. OSP/Copper finish in not good for switches or contacts. We typically use
electroplated hard gold or PTF(polymer thick film).
2. If the case or enclosure is not well sealed, this does not mean air or
water tight, there can by entry points for corrosive fluids that could cause
damage. However these would probably also be an issue even with a HASL
board.
3. As far as the new immersion Ag's and Sn's I wouldn't expect them to be
any worse than the OSP. In fact they are probably better. However I would
have the same concerns as OSP's regarding switches and contacts in long term
exposure to corrosive atmospheres.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Robert L. Lazzara
Sent: Tuesday, July 26, 2005 1:23 PM
To: [log in to unmask]
Subject: [TN] PCB Finishes: Final or otherwise
Virtually all lead-free bare PCB finishes are intended as interim, pre-
assembly finishes. Yet I also see a growing occurence of circuit features
that are sent into the post-assembly world without protection of assembly
solder, solder mask or conformal coating. Which begs the question:
Are the lead-free PCB surface finishes safe in the field as final finishes?
There's a design before me at this writing: It has nearly 100 test points.
All immersion silver. They'll go through assembly as immersion silver, then
they'll go out into the field as immersion silver. The assembler is very
interested in knowing how long the silver can sit on his shelf and still
promote soldering, but hasn't any interest in how long immersion silver
will sit in the field and still protect copper.
Immersion silver isn't being singled-out: Trade it with OSP or imersion
tin, and I don't feel any better or worse.
In-fact I have yet to find any producer of immersion silver, immersion tin
or OSP that rates their products as final, in-service finishes.
But it's happening...
I'm not too concerned about lead-free HASL or even ENIG, but the immersion
and OSP coatings seem like long term risks in circuit reliability.
Anybody else thought about this?
Is there a paper that can be recommended?
Bob Lazzara
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