Nicholas:
If today's OSP coatings are properly applied, the board can be exposed
to multiple thermal cycles without degrading the OSP to expose copper.
The other factor is to take care that the assembly NOT be washed with
an alcohol-based solvent to remove all of the coating after assembly.
A certain amount of the OSP coating is removed with each thermal
exposure (e.g. reflow, wave or hot-air rework process). However,
modern OSP coatings can withstand 4-6 thermal cycles before TRULY
exposing copper to the environment, even if the copper APPEARS to be
exposed.
With reference to vias, most assemblers do NOT fill them with solder
at SMT assembly. As long as they are NOT used for any through-hole
wire or component attachment, any oxidation developed in the hole
should not create a reliability problem for the assembly. If you are
concerned with via failure, print solder paste in them and reflow it
with the SMT components.
I agree with Steve in that the rate of copper oxidation slows down as
the thickness of oxidation increases. After initial assembly,
oxidation of unused component pads or vias should not pose a problem
for PCBA reliability. IMO, the concern about oxidation should be
focused around the addition of components after initial assembly
(ECOs, upgrades, etc.), after the oxide has had a chance to grow and
create unsolderable surfaces.
Bill Fabry
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______________________________ Reply Separator _________________________________
Subject: Re: [TN] HASL alternative.Exposed copper
Author: "TechNet E-Mail Forum." <[log in to unmask]> at INTERNET
Date: 5/26/98 11:42 AM
Hi Nicolas!
As far as the exposed copper that results from an OSP process, there
isn't an issue as far as I know. Yes the copper will oxidize, but from the
little bit I know about metallurgy, it's my understanding is that the copper
will be coated with a layer of oxidation, and then it slows down dramatically.
For a illustration of that just think about some of the copper water lines in
old houses, the copper telephone lines exposed to the elements for years and
years...that shows that it takes quite a while for copper to deteriorate.
Somebody correct me if I'm wrong about this, but the exposed copper
problems that are defects usually have something to do with the exposed copper
being an indicator of some other problem, not the fact that the copper itself
is exposed. Oxidation of the copper is normally only a problem when trying to
solder to it...
C-ya,
-Steve Gregory-
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