Fully agree, Ed, mostly everything around us belongs to the
'greater-than-zero' category. For instance, being a waiter at Nobel's
solemnity, the possibility of dropping wine on the queens gala dressing is
not zero, a fact that may cause your hand to shake a little...and a
catastrophe is there. However, despite numerous such dinners and a wide
spectra of waiters, I believe that has never happened. The risk is greater
than zero, but all behave like it is zero.

(If the waiters read this, they would become uncertain, because the longer
the time, the greater the risk, because P>0....and finally Year 2560, a
waiter is so nervous that HE fulfils the equation.)

Inge


On 12 June 2013 23:59, Ed Popielarski <[log in to unmask]> wrote:

> Inge,
>
> Although only one, we did have an opportunity to capture a tin whisker
> causing a short on a lead free assembly. I did get a terrible photo of it,
> but it was there nonetheless. Based on this occurrence, the risk is greater
> than zero.
>
> Ed Popielarski
> Engineering Manager
>
>
>                                970 NE 21st Ct.
>                               Oak Harbor, Wa. 98277
>
>                               Ph: 360-675-1322
>                               Fx: 206-624-0965
>                               Cl: 949-581-6601
>
>
> https://maps.google.com/maps/myplaces?hl=en&ll=48.315753,-122.643578&spn=0.011188,0.033023&ctz=420&t=m&z=16&iwloc=A
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge Hernefjord
> Sent: Wednesday, June 12, 2013 2:43 PM
> To: [log in to unmask]
> Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
> whiskers
>
> NASA have the most spectacular registry of whisker growths, some of which
> are compatible to sort of metallic psitacosis. E.g. whole cable 'ladder'
> supports covered with Tin 'wool'. If we limit the tour through the NASA
> Whiskerland to PWBs only, then how many of you have seen real schrecklichen
> cases? I have been working for a company that has produced 10ths of mllions
> of boards, and I have just found a handful serious examples. We DID have
> some really pudgy issues (loved to perform analysis), but they were beside
> the PWB world. The vigilance on the whisker theme at Ericsson has been
> active for years, despite that very few boards were reported from the
> myriade of customers. Therefore I have the question: what are the risks of
> tin whiskers today?
>
>
> On 12 June 2013 21:28, Stadem, Richard D. <[log in to unmask]>
> wrote:
>
> > Only a lurdane engineer would disagree with you on that clarification,
> > Wayne.
> >
> > -----Original Message-----
> > From: Wayne Thayer [mailto:[log in to unmask]]
> > Sent: Wednesday, June 12, 2013 1:54 PM
> > To: Stadem, Richard D.; TechNet E-Mail Forum
> > Subject: RE: [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> >
> > Perspicacious?  Damn, Richard, that's a big word!  Had to look that
> > one up.  No disagreement with the basic sentiments you've expressed:
> > Using the data as a guideline for how to prevent problems with tin
> > whiskers would be wrong, and peeling conformal coating is rarely a good
> thing!
> >
> > -----Original Message-----
> > From: Stadem, Richard D. [mailto:[log in to unmask]]
> > Sent: Wednesday, June 12, 2013 2:24 PM
> > To: TechNet E-Mail Forum; Wayne Thayer
> > Subject: RE: [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> >
> > I am not sure I follow your first statement, Wayne.
> > It would be foolish to treat this as just a strange anomaly from just
> > a conformal coat bonding perspective.
> > It would be perspicacious to treat it as a reliability issue from the
> > perspective of a catastrophic failure due to electrical shorting
> > issues, whether through or under the coating.
> > NASA purposely selected a rather benign environment to show that
> > whisker growth can take place both through and under coating in an
> > office environment (to illustrate that even in a best-case scenario
> > they will
> > grow) and they were using tin-plated brass simply because that is more
> > prone to provide the stress interface that tends to produce tin whiskers.
> > However, in real life, and depending on the product, you would
> > typically have a much worse thermal cycling scenario, and plenty of
> > stress interfaces to grow plenty of whiskers. Just because they did
> > not penetrate the 2-mil thick coating does not mean they won't short out
> under the coating.
> > Not to sound like a metal farmer, but tin whiskers do not grow every
> > so far apart like planted corn; in certain situations they will sprout
> > like grass under a rail fence in an Iowa pasture.
> >
> > The separation of the coating is a secondary reliability concern
> > compared to the electrical shorting possibility under the tent. But
> > both are a concern.
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
> > Sent: Wednesday, June 12, 2013 12:44 PM
> > To: [log in to unmask]
> > Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> >
> > Awesome presentation, thanks for posting the link.  But it would be
> > foolish to treat the results as anything but a strange anomaly which
> > may or may not have applicability in "the real world".
> >
> > For those who didn't check it out, this is a NASA study where they've
> > been watching tin whiskers grow on a piece of tin plated brass sitting
> > in an office environment for 11+ years.  The polyurethane conformal
> > coat is on half of it.  No whiskers penetrate the 2 mil conformal coat
> > because the combination of this urethane's high internal cohesive
> > strength, it's flexibility, and it's relatively poor adhesion to
> > plated tin cause any whiskers to push the coating off of the
> > surrounding flat (unwhiskered) tin plating, creating a "circus tent"
> > structure around the whisker, with the whisker as the center pole.
> > The material continues to delaminate until the load on the tin whisker
> is high enough to cause it to fail by buckling.
> >
> > Whether the polyurethane adheres to the pcb assembly being coated in a
> > comparable way is open to speculation.
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
> > Sent: Wednesday, June 12, 2013 10:28 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> >
> > I did find the paper that started the discussion about the magic
> > thickness being .002" but I still agree with the general rule stated
> below.
> >
> >
> > http://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-I
> > PC-Tin-Whisker.pdf
> >
> > I will patiently wait for Dave to come back.
> >
> > Phil
> >
> > From: [log in to unmask] [mailto:[log in to unmask]]
> > Sent: Wednesday, June 12, 2013 6:10 AM
> > To: TechNet E-Mail Forum; Bavaro, Phillip @ MWG - TW
> > Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> >
> > Phil,
> > While this is an answer I "should" know, I don't.  Dave Hillman
> > regularly attends and presents at the CALCE yearly conference on
> > whiskers and so he keeps up on all of that.  At present, my esteemed
> > colleague is bumping his head on rocks, kayaking upside down, on some
> white water in North Carolina.
> >  He should be back in the office on Monday and will no doubt answer then.
> >
> > From our discussions, the general rule is still "no conformal coating
> > prevents whiskers".  A thicker coating may cause the whisker to expend
> > more energy punching through and yet more energy to punch through an
> > adjacent coating on a lead (usually resulting in buckling), but I have
> > yet to hear about some magic thickness of any kind of coating that
> > completely mitigates whiskers.  But I could be wrong.
> >
> > Dave?
> >
> > Doug Pauls
> >
> >
> >
> > From:        Phil Bavaro <[log in to unmask]<mailto:
> > [log in to unmask]>>
> > To:        <[log in to unmask]<mailto:[log in to unmask]>>
> > Date:        06/11/2013 02:26 PM
> > Subject:        [TN] minimum thickness of Type UR Conformal coat and tin
> > whiskers
> > Sent by:        TechNet <[log in to unmask]<mailto:[log in to unmask]>>
> > ________________________________
> >
> >
> >
> > Doug et al,
> >
> > Is there a disagreement in the industry as to what minimum thickness
> > of urethane is required in order to mitigate tin whisker concerns?
> >
> > I am hearing that the .003+/-.002" does not provide enough of a minimum
> > thickness and that the number is as high as .004".   I can understand
> > wanting the minimum being raised to .002" but higher than that would
> > seem to make the process much more difficult to control.
> >
> > I have a potential customer asking if we measure the thickness on the
> > individual component leads which is another can of worms it seems.  We
> > always used flat samples to document our thicknesses.
> >
> > I did not get to attend this years APEX so I might have missed the
> > latest data.
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