Hi Arnold! Ugg - being subjected to a mixed flowing gas test (aka. the
"Battelle gas test ") is a tough requirement to pass! The one question I
always ask when someone suggests that printed wiring assemblies be
subjected to a mixed flowing gas test is "in the actual use environment,
will the individual pwas be exposed directly to the use environment or will
they be in an enclosure?". Its funny how some testing requirements have
absolutely no relationship to their use environment. The majority of the
time, the pwas will be housed in an enclosure - so the mixed flowing gas
test should be conducted with the pwas in their enclosure! However, I have
seen instances (like those your described) where direct contact to a mixed
flowing gas type environment is realistic and in those cases I have seen
designs favor gold and tin/lead surface finishes to combat the corrosive
impact of the mixed flowing gas. Downhole drilling electronics have
utilized a pwb finish of 96Sn4Ag for many years because of the nature of
their use environment. There was an Rockwell Automation paper presented at
the 2005 SMTAI Conference documenting how a leadfree HASL finish performed
well in a mixed flowing gas test. And a Doug Pauls suggestion would be to
consider the use of a conformal coating as a additional material choice to
combat an aggressive use environment. Just relying on the pwb surface
finish to fight the battle alone is not always the best choice. If you use
an immersion silver finish in a mixed flowing gas use environment, look at
your other design options (i.e. conformal coating, the enclosure
qualities) to help achieve good performance. Good luck!
Dave
Arnold Offner
<aoffner@phoenixc
on.com> To
"(Leadfree Electronics Assembly
09/07/2006 04:30 Forum)" <[log in to unmask]>,
PM [log in to unmask]
cc
[log in to unmask]
Subject
Re: [LF] Immersion Silver and
tarnish
Hello David,
I've always been impressed with the level-headed answers to the Lead-Free
(read RoHS-related) topics you have covered.
And wondered if you or any other LF members have any experience with
PCB-assemblies that have to pass the ISA 71.G3 or ISA.71 GX mixed gas
tests.
In the Industrial Control, and especially harsh chemical, environments
encountered in petrochemical, paper and pulp, and some power utility
applications, we are faced with issues that would be related to the use of
immersion silver and SAC solder on PCB's faced with Chlorine and H2S gases.
What mitigating methods (PCB finishes, Solders and Fluxes) should be
considered to prevent silver whiskers from occurring ?
Kindest regards,
Arnold Offner
Phoenix Contact Inc. - USA
Industry Standards Manager
Program Lead - RoHS & WEEE Directive -USA
Box 4100
Harrisburg, PA 17111-0100
United States of America
Telephone: USA - 717 - 948 3469
Cellphone: USA - 717 - 215 0155
Facsimile: USA - 717 - 944 1625
E-Mail: <[log in to unmask]>
"David D.
Hillman"
<ddhillma@ROCKWEL To
LCOLLINS.COM> [log in to unmask]
Sent by: Leadfree cc
<[log in to unmask]
> Subject
Re: [LF] Immersion Silver and
tarnish
09/07/2006 05:16
PM
Please respond to
"(Leadfree
Electronics
Assembly Forum)"
<[log in to unmask]
>; Please respond
to
ddhillma@ROCKWELL
COLLINS.COM
Hi Genny - I recommend avoiding any type of cardboard whenever possible to
avoid the sulfur contamination issue altogether. However, if you put an
immersion silver pwb wrapped in silver saver paper in an unsealed poly bag
which then sits in a cardboard box, you should not have any sulfur
contamination issues in most typical cases. Immersion silver can have some
level of tarnish which will not impact solderability - Don Cullen of
MacDermid presented a paper at an IPC Fall meeting documenting an
approximate level of acceptable tarnish. I will jump up on my soapbox and
recommend that you never use a "pearl pink eraser" to "clean" a solderable
surface. The use of a pearl pink eraser is an old metallurgist tool of
determining whether a corrosion/tarnish attack is a surface phenomena or a
subsurface phenomena (one of my mentor's taught me the trick many moons ago
when dealing with sheet metal corrosion) - unfortunately some misguided
soul long ago considered this process to be a "cleaning" process and now we
stuck with an incorrect idea. You would be amazed at the residue that can
potentially be left on a "treated" pad after a pearl pink eraser treatment.
Dave Hillman
Rockwell Collins
[log in to unmask]
Genny Gibbard
<Genny.Gibbard@VC
OM.COM> To
Sent by: Leadfree [log in to unmask]
<[log in to unmask] cc
>
Subject
[LF] Immersion Silver and tarnish
09/07/2006 02:58
PM
Please respond to
"(Leadfree
Electronics
Assembly Forum)"
<[log in to unmask]
>; Please respond
to
Genny Gibbard
<Genny.Gibbard@VC
OM.COM>
Are cardboard ESD boxes a source of sulfur that could tarnish Immersion
Silver PCBs?
If you have tarnish on a PCB, would you ever try to remove it with an
eraser?
Just wondering...
Genny
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