[tinwhiskers] NASA comments ...
- From: "Bob Landman" <rlandman@xxxxxxxxxxxxxxxxx>
- To: "tinwhiskers@xxxxxxxxxxxxx" <tinwhiskers@xxxxxxxxxxxxx>
- Date: Fri, 14 Mar 2008 09:18:43 -0400
Dr. Henning Leidecker at NASA GSFC just replied to me - please be sure to read
all the way to the bottom of this post as he's replying to several posts on
this forum.
Bob Landman
==================
Dear Bob,
As before, my remarks are interpolated, and you may repost.
Best,
Henning
=============================
-----Original Message-----
> On Behalf Of Bob Landman
> Sent: Thursday, March 13, 2008 9:14 PM
> Subject: Re: Conformal Coating? When Reliability Goes Astray
>
> John,
>
> I can't speak for NASA but I do read and hear what they say
> and they say AVOID pure tin at all costs.
Goddard Space Flight Center / NASA prohibits the use of tin coatings on or near
electrical components. We prohibit the use of cadmium anywhere, but it often
sneaks in as a finish on connectors. And we are increasingly prohibiting the
use of zinc coatings.
We assay our components as they arrive, to check that our contracts are
honored: this is expensive --- and in line with your remark "at all costs".
> While the solder alloys are not pure tin, the soldering process does
> not wet the entire surface of the parts that have become pure tin
> plated.
Wetting the entire surface is rare.
> Furthermore, I'm told that no matter what the solder alloy is, if
> there is pure tin under it, the pure tin still
> has a propensity to whisker and push up through the alloy.
If the solder has lead in it, and is applied as a hot liquid, then the lead
alloys nicely into the tin and no long whiskers grow. But evaporating or
electroplating some lead onto the pure tin is not so effective at getting the
lead into the pure tin.
If the tin surface has already grown some whiskers, and then a leaded alloy is
flowed on, the whiskers can sometimes remain: they melt at 232C (and the tough
oxide on their surface can maintain them as needles encased in a shell, at up
to 260C and sometimes a bit more), while eutectic lead-tin solder melts at 183C
and might be applied at less than 250C.
> From http://nepp.nasa.gov/WHISKER/background/index.htm
==> Jay Brusse lists what we believe can help.
And we work to keep this description up to date <==
-----Original Message-----
> On Behalf Of Wayt, John
> Sent: Thursday, March 13, 2008 9:05 PM
> Subject: Conformal Coating? When Reliability Goes Astray
>
> Hello everyone,
>
> I was going to ask the question to the group at some point anyway, but
> in light of the NASA comment I thought I'd ask now.
>
> Since they see it necessary to re-plate their parts with tin/lead,
> does this mean that there has not been adequate research into the use
> of trinary or quaternary lead-free solder alloys for whisker
> mitigation or are they not comfortable with the research
> that has been published?
>
> John Wayt
We have been reviewing papers published since 1950-ish on solders, and there
are only a few that track the specimens over a decade or more. (There are
hundreds that track for up to a year or so.) The long duration experiments
show variable results, except that lead quenches the growth of tin whiskers in
all experiments. (Reduction in density, reduction in length, causes whiskers
to curve instead of grow long and straight, is effective in concentrations as
low as
0.5%)
It takes a couple of years to build our spacecraft (well, sometimes less than a
year, but this is unusual). Then, the planned missions are 5 years and up to
15 years; and, we would like them to last even longer. So studies that last
(say) three years are of less than no interest to us.
We have seen dozens of groups announce "We have solved the tin whiskering
problem with our (patented and proprietary) formulation!", only to abandon
advocating their mix after a year or two, when their specimens start sprouting.
We continue to watch, and hope for something useful to us..
>-----Original Message-----
> On Behalf Of Bob Landman
> Sent: Thursday, March 13, 2008 8:19 PM
> Subject: Re: Conformal Coating? When Reliability Goes Astray
>
> Joe,
>
> Someone two weeks ago (maybe three?) commented on the tin whiskers
> teleconference that a new AMD part was found to have whiskers on all
> the leads (I think it was a TQFP package).
> I don't know if it was conformally coated or not but what struck me as
> a significant data point was the fact that the part was "new" since we
> tend to think of whiskers on parts that have some age on them. The
> person has not further commented.
> I hope he's on this forum now (he didn't have the details with him at
> the time he commented) or that someone else here heard the comment,
> got the name of the person (I'm new to
> the telecom so didn't catch it).
>
> As for a whisker having to penetrate another pin of the package, I
> don't believe that's necessary. Whiskers at different potentials,
> according to Jay Brusse at NASA Goddard, will
> attract each other.
Yep. They attract. But they might not touch, depending on their stiffness and
the field strength.
And, if touching, the potential might not be large enough to breakdown both
oxide films. This breakdown could be as large as 90V, for two films, although
it is more likely to be in the range
0.3 V to 10V.
> If conformal coatings were sufficient then why is NASA spending so
> much money to have parts dipped in hot tin/lead solder at Corfin
> Industries?
Many of our circuits cannot work when conformally coated --- among other
things, the dielectric loading can be damaging.
And a coating does not work when it is not there --- sometimes, there are gaps
in coatings, or deliberately uncoated parts (like connector pins and sockets).
So we FORBID PURE TIN. We conformally coat everything we can, in order to
guard against other conductive debris, which happens all too often. When we
find that we have gotten pure tin parts anyway, and we can't take the stuff out
without huge expenses, and we find that we can bury all the dangerous zones
with conformal coating, then we investigate what that is likely to do to the
reliability.
Got to run now.
Best,
Henning
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