Dave-
I wasn't trying to indicate the iNEMI statements are incorrect so much
as incomplete, and verging on misleading. As everyone who has responded
so far has pointed out, the data suggests that ImAg deserves a better
rating in their table of Hi Rel PCB finishes. The report suggests that
ImAg on PCBs is at risk for silver migration in corrosive environments,
but the IPC/UL study suggests that it is no more at risk for migration
in these environments than most other finishes. Also, they did not
choose to list any risk for "black pad" for ENIG, nor a risk of joint
embrittlement for electroplated Au.
Any reasonable person who is wondering about PCB finishes and picks up
that iNEMI paper will conclude the ImAg is a risky finish to use. That
appears to be an assumption by the authors of the paper without any
convincing data.
I know if one of my customers happens to pick this paper up, they'll
limit my finish options to exclude ImAg. After all, everyone wants "hi
rel" if they can get it without much additional cost.
Wayne Thayer
>>> [log in to unmask] 12/19/2006 5:14:57 pm >>>
Hi Wayne! There is a big difference in the migration behavior of
electroplated silver and immersion silver. Some of the differences are
due
to the specific chemical baths, the nature of the deposition mechanics,
the
deposit thickness, and the deposit structure. The IPC committee work
that
George W. described included electroplated silver samples in their
testing.
And to quote Doug Pauls, the propensity of silver to migrate "depends"!
Don
Cullen has presented some data on the creep corrosion of immersion
silver
in a very harsh, high humidity environment. The INEMI statement is
not
incorrect but there are a number of other use environment dependences
(such
as a sulfur contaminated atmosphere, etc.) which need to be
considered.
Many high performance product companies have determined that
immersion
silver is an applicable finish for their use environments. There will
be
some information on immersion silver durability in the next revision
of
Clyde Coomb's Handbook.
Dave Hillman
Rockwell Collins
[log in to unmask]
Wayne Thayer
<WTHAYER@EVITECHN
OLOGY.COM>
To
Sent by: TechNet [log in to unmask]
<[log in to unmask]>
cc
Subject
12/19/2006 12:29 Re: [TN] Fwd: RE: [TN] iNEMI Hi
rel
PM Pb-free finish report
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
Wayne Thayer
<WTHAYER@EVITECHN
OLOGY.COM>
Steve-
Thanks for the link.
That paper actually increases my questions about the iNEMI
recommendations:
First, it IS about whiskers grown from silver, not electrochemical
migration. The author claims to have seen such whiskers "6-8 cm long
and 1mm in diameter"!
Second, on page 4, it specifically mentions whiskers also being grown
on AgNi alloy, while the iNEMI recommendations list Ag over Ni as one
of
their preferred finishes for separable contacts. Maybe the plated
coatings won't make enough AgNi to matter, but it would probably have
been a useful tidbit to add to the recommendations paper.
The author, Bella Chudnovsky (Square D circuit breakers), of this
paper
actually concludes that Tin might be better in some situations for
addressing this problem, since it isn't as reactive to the sulfurous
environment found within a pulp plant and the whiskers are relatively
tiny. She does temper this by recommending the use of Tin alloys
which
have higher whisker resistance.
She reports that thinner platings of silver actually makes the
situation worse, but the thinnest she could find data reported on was
still 2 microns--much thicker than ImAg. The reasoning is that the
base
metal, copper, contributes to the reaction (copper was found in the
whiskers in single digit percentages).
She claims to have solved her problem with a combination of air
filtration and circulation to minimize H2S and drop the temperature,
contact lube (to form a film to further block sulfur), and regular
maintenance solved her issue (circuit breakers failing in a pulp
plant).
Sounds like it would be an interesting challenge to attempt to grow
these structures on PCBs with ImAg. Growth accelerated greatly above
140C, and you want about 1ppm H2S to get the precursors started.
Wayne
>>> [log in to unmask] 12/19/2006 12:19 pm >>>
Wayne writes:
"...The UL paper you and I both found suggests that ImAg is no more
susceptible to electrochemical migration (I hear you Bev!) than most
other finishes. The subject paper contains language which gives me the
impression some hydrogen sulfide tests were done with ImAg and the
results were of concern, but the only obviously relevant paper in the
reference area was 38 Chudnovsky, Bella, "Degradation of power
Contacts
in Industrial Atmosphere: Silver Corrosion and Whiskers". Proceedings
of
the 48th IEEE Holm Conference on Electrical Contacts, 2002 which I
don't
have ready access to, but I suspect it doesn't concern ImAg."
Hi Wayne!
I found it for you:
http://tinyurl.com/oo23f
-Steve Gregory-
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