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The latest on tin whiskers in lead-free soldering?

Discussion in 'Electronic Design' started by Peter, Jul 27, 2013.

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  1. Peter

    Peter Guest

    A lot was said about this c. 2005 but the whole discussion seems to
    have died down.

    Here in the UK, most people have now moved to lead free solder. A lot
    used the Control & Monitoring Equipment ROHS exemption, which is valid
    till 2017 and this protects you fine if you sell direct to many small
    customers, but if you have big customers you can't use it because most
    big firm customers are bullying their supplier base with surveys
    demanding a confirmation of total compliance on ROHS & REACH...

    At work we tested about 30 hand solders and all but two were absolute
    crap. I cannot see who could use the others - except maybe with a
    *very* hot iron e.g. 400C+ and zero quality control. The two which
    worked both contained silver (SAC solder). One was GBP 45/0.5kg (5x
    more than normal solder) and the other, which is actually pretty good,
    is GBP 65 (from Almit in Japan). Per product, the cost of the hand
    solder is negligible however. Neither of the two flows well; they do
    good joints but basically the solders stays where you put it. It
    doesn't like to flow into a gap e.g. if soldering a TO220-style
    package onto a PCB by the tab on it.

    But it is in SMT reflow soldering that the whisker troubles happened.
    I read the Swatch story; obviously they found a solution eventually.

    I wonder if perhaps several factors helped:

    1) The silver stabilises the solder and stops whisker forming. I found
    most reflow soldering is done with SAC solder, despite its hugely
    bigger cost.

    2) The industry stopped the quest towards ever finer TSOP package pin
    spacing. We use 0.65mm pitch which is probably OK. The really dense
    stuff went to BGA which is very well spaced out.

    3) Much electronics is consumer stuff and nobody gives a **** if it
    packs up after a few years...

    The military retain their exemption for ever, presumably for a good
    reason.

    The problem I have is that out products routinely run for 20 years, in
    cabinets, at an elevated temp, perhaps +50C.

    Any views?
     
  2. Robert Baer

    Robert Baer Guest

    * ---------- our -------------^
    SAC has a higher MP than Sn96.3Ag3.7, and i understand that it has
    soldering problems, some of which you allude to.
    Those higher processing temps degrade everything..

    The "plain" tin/silver alloy mentioned above seems to flow as easily
    as original tin/lead (aka 60/40 or thereabouts) and i have made zero
    adjustments to iron temp.
    Reflow also works like a charm.
     
  3. Robert Baer

    Robert Baer Guest

    What diff? The arts have so little solder on them that machs nicht.
     
  4. rickman

    rickman Guest

    I thought the Toyota incident was relatively recent. But looking at the
    NASA paper I see the car was a 2005 model. It just took five years to
    get the info to NASA... or maybe it took nearly five years for the
    whisker to form?
     
  5. Peter

    Peter Guest

    This is scary and it seems that nothing has actually changed, and all
    that the EU has achieved is to trash long term product life.

    I do find this hard to believe...
     
  6. Peter

    Peter Guest

    The 2.6V is actually the voltage needed to break down the oxide layer
    which forms on the whiskers and which stops them shorting things out.

    The whiskers form without any power applied to the component(s).
     
  7. Syd Rumpo

    Syd Rumpo Guest

    It's a bit like the first gulf war when we saw on the TV news a laser
    guided bomb blow up a bunker by flying down the ventilation shaft. The
    same one, over and over and over again.

    Every time this comes up, these same pictures come out. If whiskers
    really are such a problem why are there not many hundreds of pictures?

    FWIW, I've been using lead-free for many years for other than ROHS
    reasons, and once you're used to the difference, it seems easy and reliable.

    cheers
     
  8. Peter

    Peter Guest

    Googling suggests that the issue of whiskers shorting out component
    *leads* may have been solved by using some gold palladium etc plating
    on them. Maxim suggests that this may be standard on their chips so I
    guess others have done something similar.

    What is a worry is that, immediately post the ROHS shafting of
    electronic manufacturing back c. 2005, a lot of chips went to tin-only
    plating on the leads, and this WILL grow whiskers. They grow up to 1mm
    per year. They grow rapidly in vacuum which is why this has been a
    huge issue in space and high altitude (unpressurised) applications.

    But I see nothing having obviously changed on whiskers growing out of
    lead-free solder *joints*. There is a suggestion that SAC (silver
    loaded) solder suffers less from whiskers, but it's only a degree,
    nowhere near an elimination.

    There is one conformal coating, Arathane 5750, which stops whiskers
    growing. It is very expensive (though probably insignificant on a
    per-PCB basis), very hard to get here in the UK, but most importantly
    conformal coatings are very expensive to apply. I have used them for
    many years, and they are a pain in the ****. You have to let the PCBs
    dry off, etc. We use CC on PCBs which have little toroidal
    transformers as it helps secure the windings in place. But it cannot
    be used on PCBs that have connectors, or mounting holes through which
    you are expecting earthing continuity, etc, unless you do fancy
    masking.

    My guess is that anybody using pure-tin-plated chips ought to stick
    some Arathane 5750 onto the pins, at least. A quick spray through a
    template would do it.

    Researching this leads to a really sh*t realisation that the EU has
    shafted all of us, and the big corporations (with the US ones behaving
    in the most disgusting manner when it comes to screwing suppliers)
    having a big play in it...
     
  9. Phil Allison

    Phil Allison Guest

    "Jeff Liebermann = lying radio ham cunthead "

    ** Yes it FUCKING is - you brainless, bullshitting TWAT !!

    1. Whenever the evidence SHOULD be there and is NOT.

    2. When there is simply no proper supporting evidence at all.

    FYI:

    The NUMBER ONE proof that something is 100% BULLSHIT

    - is that nothing *credible* makes it fact.

    Idiot.



    .... Phil
     
  10. Syd Rumpo

    Syd Rumpo Guest

    On 28/07/2013 09:59, Jeff Liebermann wrote:

    Higher softening and melting point for high temperature work. ROHS
    actually made things better in that regard as the Sn/Pb solder on
    component pins would often need to be removed before use. It doesn't
    take much Pb contamination to lower the melting point and weaken the
    solder joint, particularly with the small amount of solder used with
    surface mount.

    Sometimes Sn/Pb solder is used to deliberately contaminate a joint to
    aid component removal. This solder is kept in a locked cabinet.

    I've never seen a tin whisker in real life, and have spent many an hour
    looking at solder joints through a microscope.

    Cheers
     
  11. Peter

    Peter Guest

    I have just done hours of reading on this.

    One way to guarantee getting loads of whiskers is to plate *bright*
    tin onto some substrate and then compress the substrate.

    In that "1960s transistor" case above that is probably what happened.
    They started with a tin plated sheet and pressed it into the
    transistor casing, which compressed the tin coating inside it, causing
    massive whisker growth.

    So that may be an extreme case.

    I don't think bright tin has been plated onto IC leads. The tin I see
    is normally dull grey. That is just as well since the leads get bent
    into shape *after* plating.

    Also tin plated IC leads go back decades before ROHS, with no reported
    problems AFAIK. So maybe the problem was known in the industry for a
    long time, which is what my reading does confirm.

    The ROHS-related problems would be to do with the solder used, rather
    than component leads.

    I don't think component leads were ever plated with tin-lead, were
    they? Can you even do that? You would have to solder bath immerse them
    I think, which isn't going to be done with an IC lead frame. That will
    always be plated, and I don't think you can plate tin-lead as such.
     
  12. Fred Abse

    Fred Abse Guest

    Maybe that's what they wanted...
     
  13. Fred Abse

    Fred Abse Guest

    Traceability?
     
  14. Fred Abse

    Fred Abse Guest

    There are links from the above reference that explain in detail the
    difficulty of seeing, let alone photographing, whiskers. Many are missed
    simply because of incorrect illumination.

    Explain "seems easy and reliable". What products? voltages? environmental
    conditions? lifetime? soldering process?
     
  15. Syd Rumpo

    Syd Rumpo Guest

    On 28/07/2013 14:44, Fred Abse wrote:

    All that work for free? No.

    Cheers
     
  16. Fred Abse

    Fred Abse Guest

    IOW, you can't...
     
  17. Syd Rumpo

    Syd Rumpo Guest

    Well, I'm not fundamentally dishonest, and I don't have a vested
    interest. I have never once seen a tin whisker under my 20x stereo
    inspection microscope in many years of using lead free solders. I think
    the problem is greatly exaggerated and probably insignificant compared
    to other soldering issues. I suspect that, because lead free is touted
    as 'green' or 'European' it provokes a knee jerk negative response in many.

    But I'm not going to write a book. Have you seen any real tin whiskers
    yourself?

    Cheers
     
  18. Peter

    Peter Guest

    Having done a lot of reading, I think the industry has got away with
    it, due to a number of factors:

    1) The whiskers need an elevated temp to grow well, say +60C

    2) The whiskers need a reduced pressure to grow well (aircraft and
    satellite apps)

    3) The whiskers won't grow on component leads unless *bright* (smooth)
    tin plated, which most aren't (but some definitely ARE)

    4) Ref 3) what really helps is if the surface is under compression
    (concave curve e.g. the *inside* of a transistor case)

    5) SAC solder doesn't seem to have a big problem with whiskers because
    the surface finish is very dull

    6) IC lead pitch miniaturisation stopped c. 0.65mm pitch (went to BGA
    for high lead count packages)

    7) Very few products are going to fail within say a 2 year warranty

    8) IF SOMETHING FAILS IT GETS CHUCKED AWAY AND NOBODY LOOKS FOR
    WHISKERS!

    9) The vast majority of electronic mfg is consumer stuff which gets
    chucked away after a few years at most

    So there you are...

    I *would* be concerned about elevated temps and TSOP packages, in
    industrial products which are expected to work for years. Conformal
    coatings can be used there, or just use leaded solder and nobody will
    notice ;)

    The conformal coating I mentioned earlier is difficult to store (6
    month shelf life and a crazy price) and extremely hazardous if warmed
    up.

    Does anybody make SMT solder paste with say 2-3% lead? I guess it must
    exist because the military demand 2-3% lead.
     
  19. tm

    tm Guest

    You are totally full of shit. Totally!.
     
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