Smack!



On 13 June 2013 06:03, Stadem, Richard D. <[log in to unmask]> wrote:

> Regarding #3 in Joyce's post below, tin whiskers in space are several
> thousand times more likely to lead to catastrophic electrical failure than
> tin whiskers in electronics here on earth.
> In space, in a (nearly) perfect vacuum, there is little or no oxygen. A
> tin whisker short can handle much more current than it can here on earth.
> Most tin whisker shorts on earth actually go undetected, because the power
> going through them will burn them away before the current surge can cause
> any real harm in most cases. In space a tin whisker will pass many
> milliamps of current for a very long time,  enough to cause the device to
> fail.
> So as Inge pointed out, although tin whiskers are rare, once is too many
> times for a satellite costing millions, or for any avionics, traffic
> controls, etc, where people's lives are at stake. Or for braking systems in
> automobiles, or life sustainment systems in the medical field, or......
>
> And I totally agree with Joyce's point number 4, why a total ban if as
> little as 3% lead in the solder will greatly reduce any chance of
> formation? RoHS regulations do not allow more than .1% lead in solder. Why?
> But then again, why any ban in the first place?
>
> Because, in the European Union, they did not perform due diligence before
> passing the lead ban into law. The politicians fed on the public's
> ignorance. A total ban was the only thing acceptable, even though there was
> no proof that the presence of ANY AMOUNT of lead in solder presented any
> health threat to us. In fact, there was plenty of evidence at the time to
> the contrary, but who cared about that? The witch hunt for lead was on.
>
> As each and every day passes, the stupidity of the RoHS ban of lead in
> solder continues to be made more manifest than it was yesterday.
>
> Has there been any re-consideration by RoHS legislators regarding the
> total ban of lead in solder?
>
> If not, it makes the stupidity even more convincing.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
> Sent: Wednesday, June 12, 2013 10:01 PM
> To: [log in to unmask]
> Subject: Re: [TN] minimum thickness of Type UR Conformal coat and tin
> whiskers
>
> (1) Saw it in early 90s on connectors. Pure tin plated. It is hard to do
> FA on field return, you lost the evidence when you un plug the board.
> (2) Bad case when you have tin or zinc plated Tiles for cleanroom or on
> the roof.  Fan or air condition can blow them around - random failure in
> equipment.
> (3) NASA case is more persevered may be due to lack of disturbance in
> space.
> (4) Still don't understand why ban Pb, if add few % can eliminate it.
> (5) Good design - material and process can prevent it for sure if it NOT
> follow the politically correct way. It is a man made problem
> (unfortunately, those men are dress better, more powerful than you and me).
> --------------------------
> Sent using BlackBerry
>
>
> ----- Original Message -----
> From: Inge Hernefjord [mailto:[log in to unmask]]
> Sent: Wednesday, June 12, 2013 05:43 PM
> To: [log in to unmask] <[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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