From my past dealings with OSPs (its been a number of years), I found
the same as stated below. However, I also found it was really tough to
tell if the OSP was present or not after multiple exposures. I'd be
interested to know exactly how this is tested. Either way, I don't think
it really mattered much whether the OSP was present. People felt better
thinking it was. Exposed copper was more of a cosmetic or perception
condition than a reliability concern. The hardest thing is convincing
folks that exposed copper is OK. Remember that this is old information
and certain situations or conditions may prove this wrong for certain
applications.
Regards,
Paul Terranova
Analytical and Environmental Test Services Lab
Digital Equipment Corporation
200 Forest Street
Mail Stop: MRO3-1/D2
Marlboro, MA 01752-3085
' Phone: (508)467-3109
* Fax: (508)467-6796
* Email: [log in to unmask]
WebSite: http://www.digital.com/lab-services
-----Original Message-----
From: Bill Fabry [mailto:[log in to unmask]]
Sent: Tuesday, May 26, 1998 12:18 PM
To: [log in to unmask]
Subject: [TN] Re[2]: [TN] HASL
alternative.Exposed copper
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
[log in to unmask]
______________________________ 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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