Wolf,
I would caution you not to downplay the tin whisker issue.
I am a consulting engineer, and a very busy one. Right now a lot of my
time away from GD is working tin-whisker issues. Although most of those
I see are on lead-free product, they are showing up on leaded products
as well, typically at the portion of the termination where the leaded
solder stops and the tin-plated area begins.
It is nearly impossible to completely coat 100% of the tin-finished
RoHS-compliant leads with tin-lead solder. Often it is simply not
feasible without automating the process, and as a result there are many
instances where the tin-finished portion of the lead is exposed near the
component body, and that is where I am continually seeing the tin
whisker growth on RoHS-compliant parts.
Behind the few articles published are many more cases of tin whiskers
that never go public.
The issue is such that many military, automotive, avionics, and
high-reliability companies are investing in fully automatic robotic
tinning cells such as those manufactured by V-Tek or Corfin to
automatically dip all of their component leads in Sn63Pb37 solder prior
to assembly and soldering. Other mitigation methods have not proven to
work very well, including conformal coating.
Many of the assemblies with whiskers are on ENIG-finished PWBs. I don't
see how the plating finish works as a mitigation scheme, except possibly
with HASL finishes where the HASL is tin/lead. The few microinches of
gold in the solder volume are not enough to present any effective
mitigation up around the top of the fillet.
The last thing I would recommend to anyone is to switch to ENIG. That is
simply switching from one set of problems to another.
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of SCHMIDT,
WOLF-DIETER - PFHO
Sent: Wednesday, April 30, 2008 1:27 AM
To: [log in to unmask]
Subject: [LF] tin whiskers / reports of nasa
Hello Steve,
in between I've read most of the available reports related to the your
link.
Nearly all reports point out that the tin cover on mechanical parts have
been the base for tin whisker growth:
<< DirecTV 3 ...... Boeing 601-model satellite ......
being caused by the growth of metal filaments in a relay .....
<<
>> Galaxy VII Lost ..... failures in Boeing 601
satellites........
tin whiskers grow in the vacuum of space on internal
tin-plated relay latching switches..... <<
(a lot of reports on the 601 - bad luck for Boeing products)
>> report by Gordon Davy1, Northrop Grumman Electronic Systems,
Baltimore, MD: "Relay Failure Caused by Tin Whiskers" <<
... and a lot more reports decribing the same.
Interesting are the two Westinghouse reports:
# diode lead as base of a whisker
# mechanical part of a potentiometer
The link
>> Patriot Missile: Anoplate WWW Site:
Suspected tin whisker related problems (Fall 2000) <<
gives explanations under which conditions tin whiskers will grow and
this is due to my knowledge as well. Tin covered mechanical parts will
be able to produce tin whiskers if mechanical stress ist introduced to
them. In all those cases the tin cover was made by galvanic processes
but not by melting tin on a copper or brass metal sheet. It is also well
known, that the risk of tin whiskers out of tin on copper (or alloys
containing a high percentage of copper) is much higher than whiskers out
of tin on a nickel barrier.
It is reported that even out of Sn60Pb40 you may have some tin whiskers
if the conditions are given.
The only report which not basicly refferes to a mechanical part is one
of the two mentioned Westinghouse reports. There the anode lead of a
diode was diagnosed to be the root of failure. In this very case it
would be interesting to know whether the whiskers have been grown out of
the lead at it's bend or not.
So in my oppinion the risk of tin whiskers on an printed board assembly
with electronic components (not regarding electromechanical parts !) is
quite low, especially when produced using reflow soldering. Then most of
the stress on components leads and the tin coverage - if there has been
some stress - will be removed. And the copper lines on the board
themselves normally cannot be mechanically stressed. An additional
method to be more aware of tin whiskers is to use gold over Nickel as
metal plating on the board - we have experience with this for about 15
years and thousands of boards with very good results.
I would like to know what's your thinking on the points mentioned
before.
Regards
Wolf-Dieter Schmidt
Industrial Engineering
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> -----Original Message-----
> From: Steve Gregory [mailto:[log in to unmask]]
> Sent: Monday, April 21, 2008 5:22 PM
> To: (Leadfree Electronics Assembly Forum); SCHMIDT, WOLF-DIETER - PFHO
> Subject: RE: [LF] The Cost of EU RoHS: $32.7B
>
>
> Hi Wolf!
>
> Here's a link you might want to look at:
>
> http://nepp.nasa.gov/whisker/failures/index.htm
>
> Steve
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