Hi! I guess I would disagree with the statement that immersion silver may
not be suited for harsh environments. I have seen all of the industry pwb
surface finishes fail a Battelle mixed flowing gas test. One portion of
the equation on the survivability of any pwb surface finish is the type
and robustness of the electronics enclosure. Rockwell Collins uses
immersion silver surface finishes in a number of "harsh" environments very
successfully and has done so for over 10 years. General statements on the
environmental robustness of pwb surface finishes tend to blind folks on
possible applications of a specific pwb surface finish that they may have
been able to utilize. There are many inputs to the robustness assessment
- just need to look at all of the issues with due diligence.
Dave Hillman
Rockwell Collins
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Charming Chan <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
10/29/2009 11:17 AM
Please respond to
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Subject
Re: [TN] Immersion Silver for Class 3 Boards
Hello George,
Thanks for your input.
If so, most of telecom products need to work in harsh even severe outside
environments as well, I'm not sure if immersion silver is robust enough to
survive. Some papers show that immersion silver without conformal coating
get failed in Mixed Flowing Gas Test of NEBS.
Immersion Silver works in the well controlled and protected environments
while it may not work in harsh and severe outside environmentals.
Best Regards
Charming Chan
-----Original Message-----
From: Wenger, George M. <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, Oct 28, 2009 11:12 pm
Subject: Re: [TN] Immersion Silver for Class 3 Boards
Charming,
We try to avoid conformal coating whenever possible because of many issues
ssociated with manufacturing and repair. We did have one product that
goes in
n outdoor cabinet and during initial deployment had an issue with water
ngress, which has subsequently been corrected, that we put conformal
coating on
ne board. We'd like not to use the conformal coating but every now and
then
omething happens were a customers contractor forgets to close the cabinet
door
o we've left the conformal coating on that one PCBA. That PCBA is a small
ower input interface at the back of the cabinet which is the first
location
ater would touch if the equipment was improperly installed. The other
PCBA's
in the system are not conformal coated.
Regards,
eorge
eorge M. Wenger
ndrew Solutions
enior Principal FMA/Reliability Engineer
0 Technology Drive, Warren, NJ 07059
908) 546-4531 (Office) (732) 309-8964 (cell)
[log in to unmask]<mailto:[log in to unmask]>
_______________________________
rom: [log in to unmask] [mailto:[log in to unmask]]
ent: Wednesday, October 28, 2009 10:29 AM
o: [log in to unmask]; Wenger, George M.
ubject: Re: [TN] Immersion Silver for Class 3 Boards
Hello George,
I'm curious, have you performed conformal coating on immersion silver
surface
inish to improve reliability? Thanks a lot.
Best Regards
harming Chan
----Original Message-----
rom: Wenger, George M. <[log in to unmask]>
o: [log in to unmask]
ent: Thu, Oct 1, 2009 1:20 am
ubject: Re: [TN] Immersion Silver for Class 3 Boards
Most everyone who knows me realizes that I am a strong proponent of
immersion
silver surface finish for several reasons. Many of you may have heard me
state
the main reason but not believed what I had to say. So I'll repeat it
again; we
use immersion silver surface finish as a solderability indicator that our
PCB
fabricator didn't have a problem with incomplete tin strip or leave a
solder
mask residue on the PCB features and they should be solderable. For a
several
year period when fine pitch components were beginning to appear on
products I
was assigned to work for an extremely large PCB fabricator and we were
being
pushed by our assembly factory customers to provide a flatter surface PCB
surface finish for fine pitch assembly. Our logical choice was to use
Imidazole
OSP because we had up and running in the factory and our customers were
using it
for old technology wave solder product. When we switched to Imidazole for
fine
pitch surface mount boards everything looked okay for a while be every now
and
then one of the assembly customers would complain that there were features
on
their boards that weren't solderable. After a lot of evaluations it was
realized that in any large fabrication operation processes don't run with
100%
perfect yield. Checking back on boards used for previous non-fine-pitch
products we realized that the HASL boards being supplied to customers were
solderable because every feature on the boards had solder on them. The
reason
we knew that was because we did 100% inspection at the end of the HASL
line and
if we saw any features on an 18"x24" panel with exposed copper (it's
amazing how
easily a non-solder-coated copper feature on a panel with thousands of
features
can be detected with the naked eye) the panel was put back through the
HASL
machine again hoping that the cleaning or micro etch or hot air leveling
fusing
fluid or the hot molten solder would clean off or burn off any incomplete
tin
strip or solder mask residue and the feature would accept solder. If it
didn't
the second time we'd run it a third time and a fourth or fifth time in
necessary
before we knew better and limited reruns to only three. When we switched
from
HASL to Imidazole we no longer had the visual confirmation that all the
surface
mount features were free of incomplete tin strip or solder mask residue
because
those contaminates were so thin the features appeared to be copper and
after
Imidazole they still looked like copper. During that time period we also
participated on the five year NCMS PCB Surface Finishes and Pb-Free
efforts
looking at new surface finishes for fin-pitch and Pb-Free assembly. Yes
there
was lots of doubt that a surface finish better than HASL could be found.
ENIG
looked like a better choice at that time (Black Pad issues weren't
publically
disclosed at that time) than immersion tin because evaluations showed
immersion
tin had shelve life issues and exposure to temperature and humidity
degraded its
solderability. We decided to run an evaluation using ENIG boards because
we know
it provided a flat surface and after ENIG plating we believed we could
tell the
difference between a gold colored ENIG pad and a copper pad that hadn't
accepted
ENIG plating. At the time when the evaluation was being planed AlphaLevel
immersion silver was being introduced so it was decided to include it
along with
ENIG. The evaluation didn't reveal any obvious problems with either
surface
finish but on some of the environmentally stressed boards the solder
didn't flow
out to the full extend on some on the ENIG features. All of our
experience up
to this point (i.e., HASL and OSP) was with solder joints made to copper
not
nickel. After extensive internal evaluations was decided to use immersion
silver surface finish. The reasons were simple:
1. It provided a visual indication before boards were assembled that there
were
no non-immersion-silver plated copper features indicating that incomplete
tin
strip or solder mask residue wasn't there and the features should be
solderable.
2. Solder joints were being made to copper not nickel.
3. No reliability issues were found in our extensive testing.
4. It worked.
Participation on the IPC-3-11g committee confirmed everything we had
determined
in our internal testing. In my mind the decision we made to use immersion
silver surface finish was the correct decision because in 12 years we have
no
documented reliability issues associated with the use of immersion silver
surface finish. Granted our products are only Class 2 telecommunications
products but they are deployed throughout the world in all kinds of
environments
and now in the 3G & 4G wireless world were service providers have having
to
deploy more active electronics at the tops of towers rather than in base
stations they are viewing the reliability of the products as Class 3.
As I'm sitting here at my desk typing the long email I look up every now
and
then and I see a picture hanging next to my monitor that I took on July
26,
2005. It is a photograph of my grandson sitting in front of a TV in my
son's
house in Friendswood, TX. He is pointing to a person on the TV who is the
lead
space shuttle robotics engineer standing up in the NASA JSC Mission
Control who
is talking to an astronaut during a space walk. That person is his father
(my
son) and my grandson is saying "that's my dad and I want to be just like
him".
My son has an ambition to some day be an astronaut and so does my
grandson. You
might ask what does this have to do with immersion silver. The answer is
simple
to me. With my FMA and reliability experience associated with immersion
silver
I would feel more comfortable if my son or grandson every because
astronaut
knowing that they were in space vehicles with electronics that used
immersion
silver surface finish that other options like ENIG or immersion tin.
My apology again for such a long response.
Regards,
George
George M. Wenger
Andrew Solutions
Senior Principal FMA/Reliability Engineer
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 (Office) (732) 309-8964 (cell)
[log in to unmask]<mailto:[log in to unmask]>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf
Of
lidden, Kevin
Sent: Wednesday, September 30, 2009 11:32 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] Immersion Silver for Class 3 Boards
Good point about the environmental issues. There are also recommendations
I
have seen that state ImAg is primarily a solderable surface protectant,
and not
the best selection as a final surface finish that may be exposed in the
application. In that case, the usual recommendations (that I have seen)
are
ENIG or HASL.
Kevin Glidden
Manufacturing Engineer
Luminescent Systems Inc.
-----Original Message-----
From: Thayer, Wayne - IIW [mailto:[log in to unmask]<
mailto:[log in to unmask]>]
Sent: Wednesday, September 30, 2009 11:22 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] Immersion Silver for Class 3 Boards
People panic about silver migration, which is not really an issue.
However, there apparently is a very serious problem that can occur on
uncoated
boards in high sulfur environments, where somehow the silver acts to
catalytically cause the copper to redeposit in random directions. Doesn't
even
need a voltage to cause this phenomenon!
A paper on this was brought up to this forum several months back. If you
search
the archives on ImAg you should be able to find it. The growths are nice
black
dendrites.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]<mailto:[log in to unmask]>] On Behalf
Of
arner, Chester
Sent: Wednesday, September 30, 2009 11:03 AM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: [TN] Immersion Silver for Class 3 Boards
The revised Immersion Silver IPC spec (4553A) indicates that Immersion
Silver is not recommended for Class 3 boards. There is no stated reason.
I have been looking into Immersion Silver for all the same reasons as
everyone else, to replace HASL for flatness etc. It looked very good
until the Class 3 problem.
Does anyone know why IAg would not be recommended for Class 3 boards?
Chet Harner Sr. Engineer Crane Hydro-AirePO Box 7722 Burbank Ca -
91510 818-526-2600 x3557
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