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Stripping Sn/Pb solder from pins

Discussion in 'Electronic Design' started by Spehro Pefhany, Jul 22, 2013.

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  1. Anyone tried to do this? I'm interested in stripping solder from a
    nickel layer on small (say 1mm diameter) pins, without removing the
    nickel barrier, so they can be replated with gold. Got a couple
    hundred pins to do. The parts are very expensive and cannot be
    shipped.

    Thanks for sharing any experience or ideas

    Best regards,
    Spehro Pefhany
     
  2. tm

    tm Guest

    Acetic acid and hydrogen peroxide will react with the lead to form lead
    acetate. That should easily wipe off. Not sure what it will do to the Ni.

    tm
     
  3. I want to get down to the nickel so I can brush plate gold back onto
    the pins wot some cretin soldered to.
     
  4. axolotl

    axolotl Guest

  5. Absolutely no experience.
    I guess I'd try heating and some mechancial wiping, blowing first.
    Do you have to get it all off?
    How much money can I spend?
    Could you heat it in a vacuum to drive the last bits off?

    George H.
     

  6. The problem with Gold is not corrosion. The problem with Gold is
    intermetallic embrittlement.

    The 'cannot be shipped' refers to no external job. must be done in
    house. Justt a guess.
     
  7. Guest

    From yahoo answers, looks like solubility is best way:

    1) Mechanical removal is usually not practical because of static electricity considerations, and, you can't get to all sides of the device. It should be mentioned, however, the IC leads probably could withstand bending out flat (like gull-wings) so that you could get access to both sides, and use mechanical-abrasive methods to remove the lead (electronic grade baking soda abrasive blasting techniques for example, or residue-free CO2 ice blasting,mechanical sanding, etc.). Depends on the static sensitivity of your device and especially how you 'ground' the device in a custom-built enclosed fixture to eliminate static damage. Is labor-intensive, but doable to salvage an expensive device - suitable for low volumes - can be subcontracted to local shops for 'rework', but you'll have to spend time confirming that the ***very strict*** anti-static measures are implemented and adhered to on theshop floor. I don't think any post-rework electronic testing is really going to catch static damaged parts (some will only be 'wounded' and will failprematurely - short-term testing won't find this), so your only recourse is to avoid damage in the first place. Bead-blast residues are relatively small and inexpensive to capture and dispose of.

    2) Chemical methods would take advantage of the solubility of lead in certain liquids at room temperature (i.e, acids). FUNDAMENTAL PROBLEM: aqueous-based solvents WILL be absorbed into the plastic. Further, acidic ions will remain trapped in the crevices by capillary action and migrate over time bydiffusion directly into through plastic to the electronic device where corrosion could occur. So the requirements for dissolution by acid are: Acids that would eventually cause device failure are excluded (this eliminates HCL due to chloride migration to the wirebonds and subsequent corrosion), acids that are difficult to rinse are excluded (sulfuric acid is notoriously difficult to rinse). Further, it would only make sense to encapsulate the epoxy IC package in a protective covering to isolate the acid from the plastic packaging in the first place (thus tooling and extra labor expense is involved). Acetic acid is a weak organic acid that *might* be usable, in that its diffusion through plastic is lower due to the larger molecule size compared to HCl. You'd have to check acetic acid against solubility of lead vs.its compatibility with gold, silicon, aluminum, copper, epoxy used in IC manufacturing. THEN you'd need to 'tin' the leads with some other metal (another expense). After rinsing, the plastic packages would need to be 'baked'to drive off the moisture (and hopefully acetic acid residues if baking temp is above boiling point of acetic acid) - this must be done in a oven specially designed for removing humidity with a rechargable dessicant (an ordinary oven won't get the dew-point low enough; you need -80°F dew point orthereabouts), and you'd need to confirm by laboratory methods the acetic acid was gone. Acetic acid is so slow acting that I don't think you'd want to wait around for it to work (time is money), but whatever the candidate acid, it would be one that you could remove the traces of completely, and wouldn't damage the device. You'll need to work with your plating shop to select the ideal acid if you take this route. To confirm the effectiveness of your rework procedure, the devices really should be tested with '85/85'-typedevice testing to confirm you haven't introduced a problem with the acidicremoval of lead, and you'd need to confirm solderability with a soldering test to ensure the moisture introduced in cleaning was indeed removed from the epoxy and didn't affect solderability. Wet chemical methods seem like the most logical or attractive approach at first glance, but really introduce a host of problems that you'd need to test and confirm to ensure were avoided by your manufacturing procedure. Hard experience shows: you could **REALLY** introduce problems and eventual failures in an otherwise reliable product if you don't carefully consider the impact of wet chemical methods. And remember, if you're not the person doing the work, the reliability is delegated to the person doing the work (you know what that means), so objective testing methods and manufacturing process procedures are fundamental to making sure the parts aren't damaged in handling/treating. Also, the Pb in the dissolved acid is now a waste disposal problem and will need to be precipiated/concentrated before sending to a hazardous waste disposal unit (also at great expense).

    3) Solubility: One possible method worth investigating: how was the Pb applied in the first place? Was the Pb electro-plated on the IC leadframe? (in which case acid could not reach inside the encapsulated package to dissolveanyway), or, was it applied through a wave-solder or reflow method? Lead, like most metals, are soluble in other liquid metals (similar to mercury amalgams). Depending on your devices, it might be possible to run them again through a wave-solder bath with the objective of dissolving the Pb in a wave-solder bath that only consists of, say, tin-silver. This has the advantage of not introducing moisture & acids into the plastic which you eventuallywill need to remove later at great expense, and, the packages are 'dried' by the high temperatures, and, any oxide layer that existed on the Pb-tinned leads are now refreshed with new tin-silver tinning. Might be good idea to package in nitrogen bags for storage if you don't anticipate using them up right away to keep the tinned leads 'fresh' and oxide free. Only significant drawback: under the requirements of the law, you'd need to confirm thatwith your 'rework' method that the lead has indeed been dissolved to levels permitted by law; this however is basically a non-recurring expense with process monitoring (it also gets you off the hook if you show due diligence). Further, if anyone tests the lead frames, they would only find tin-silver on the surface of the lead frames (you'd need to dig into the fine print of the law, but traces of Pb that is buried deep in the product might be exempt under certain instances). The only way to find the Pb would be to grind up the IC into a fine powder and look for a Pb signature on a spectrograph, and I don't think that is the intent of this initial RoHS initiative.

    It could be some combination of the above would work for you also.
    Source(s):
    http://www.rohs.gov.uk/content.aspx?id=5
    http://www.infoplease.com/ce6/sci/A08291…

    http://answers.yahoo.com/question/index?qid=20060628124719AAiDWml
     
  8. Not much of a constraint. Actually, I lied, it can be shipped.
    Miscommunication. Apparently "welded" means something different to
    other people.
    Probably not safe, cracking of the insulation ($$$$$$$$) and lead
    fumes.


    Best regards,
    Spehro Pefhany
     
  9. Not BS. I was originally told it was welded into a big chunk of a
    multi-million dollar "thing", but it turns out not. Still better not
    shipped.
    Turns out it doesn't have to be done in-situ, but still I'd hand carry
    the d*mn things.


    Best regards,
    Spehro Pefhany
     
  10. Would it be an optionto machine off the sn/pb and Nickel, then
    replate?

    Cheer
     
  11. Robert Baer

    Robert Baer Guest

    Replace tin/lead with gold? Are you nuts? Gold is SO less reliable
    especially in light of the corrosion problems.

    What is this "cannot be shipped" BS?
     
  12. DoN. Nichols

    DoN. Nichols Guest

    Hmm ... you are asking for *total* removal, which probably needs
    a chemical attack. And the solder wets the nickel very well, to make it
    more complex.

    For a first pass (perhaps to make the chemical pass whatever it
    is more efficient) there are various mechanical and tricky ways to do it.

    Are these used pins with solder and wire fragments in them?
    Onews with the wire fragments removed but lots of solder, or ones which
    were solder-tinned from the factory. I'll assume below that it is the
    middle ground above, and what I am offering will leave you somewhat
    close to the factory-tinned level.

    For just a few, without a solder cup for wire ends, I would grip
    them in a solder-free area with needle nose pliers or the like, dip in
    rosin flux and then in a solder pot to get it up to the melting point,
    and then strike the hinge part of the pliers against a wood block, thus
    flicking off *most* of the solder (but not all).

    With solder cups, what I would do is grip it in some kind of pin
    vise, and then heat with a soldering iron and either use a vacuum solder
    removal iron or use small braid soaked in rosin flux to wick up as much
    solder as possible.

    For quantities greater than your couple of hundred, the
    mechanical method could be a vibratory feeder to an automated pin vise
    Perhaps load a half dozen or more in arms fixed to a common hub, hit
    with a hot air flow and while hot, spin the hub so the solder at the
    outer ends is flipped off and collects as a film on the inside of the
    splash shield, and then hit with a blast of cold air before dropping
    them into a hopper.

    And probably OSHA will consider it a hazmat zone by the time you
    have all that oh so deadly lead mixed with tin scattered all over. :)

    But -- this still leaves you with needing a chemical method.
    For the quantities you are talking about, it may still be less expensive
    to simply buy new ones with the desired gold plating.

    The cleaning methods I listed above would not be commercially
    cost-effective. they are what a hobby user would do to clean terminals
    stripped from old equipment for re-use.

    Good Luck,
    DoN.
     
  13. tm

    tm Guest

    Hey, how about tumbling in a material softer than Ni but harder than Pb?

    Find someone who reloads ammo and try it with corn cob media.

    tm
     
  14. dependsin why you need to but, how about putting the parts in machined
    gold plated sockets and solder from top inside only?

    -Lasse
     
  15. Tim Williams

    Tim Williams Guest

    Although, HCl and H2SO4 are astonishingly slow when it comes to nickel.
    The stuff is pretty much noble (hence its popularity!).

    Acidity alone doesn't determine corrosivity: if a complex is formed, metal
    will dissolve much faster. Copper dissolves faster in HCl + H2O2 than
    H2SO4 + H2O2, because it forms a green chloride complex (there is also a
    reduced form with a deep brown color, which is probably familiar to anyone
    who's used this brew to etch PCBs), while sulfuric basically does nothing
    special with copper.

    Hydrofluoric acid isn't actually very strong, but because it forms a
    complex with silicon (hexafluorosilicate), it's one of the few chemicals
    which dissolves glass.

    Oxidation potential is, of course, a big force. Electrolysis can beat the
    pants off any chemical, for obvious reasons. (There's literally nothing
    more "acidic" on Earth than the LHC -- one definition of acidity is
    "proton donator", and a naked proton beam at ~light speed can't really be
    stopped from "donating" to anything!) Among chemicals, this means zinc
    dissolves faster than iron faster than nickel, while copper pretty much
    doesn't at all (in acidic water alone). If you add an oxidizer (nitric
    acid, H2O2, hypochlorite, etc.), less energy is spent generating hydrogen
    and more doing the reaction. (Bubbling decreases or stops when an
    oxidizer is used, unless another gas is produced -- nitric usually gives
    off NO and NO2 fumes, nasty things.)

    There is, of course, no chemical which is a stronger oxidizer than
    fluorine, which will literally burn through anything on the periodic table
    besides pure oxygen and the noble gasses (which, except for helium and
    neon, are all known to form compounds with fluorine anyway, they just take
    some persuading).

    Tim
     
  16. Syd Rumpo

    Syd Rumpo Guest

    'Sugar of Lead' (lead acetate) gives a sweetness to wine which had
    oxidised. Just hang some lead in your wine and banish that vinegary taste!

    Cheers
     
  17. Syd Rumpo

    Syd Rumpo Guest


    I would try polishing the solder off, first having removed excess with
    solder wick and flux. Maybe make a tool for a Dremel comprising that
    thick polishing felt but with <1mm axial hole to fit over the pins, and
    use with a little metal polish paste.

    And keep a swear box nearby, you could make a fortune.

    Cheers
     
  18. Syd Rumpo

    Syd Rumpo Guest

    Connect the pin to ground, I suppose.

    If you can't, make sure your polish is a bit conductive, maybe add some
    water if necessary and ground the Dremel if it's floating.

    I actually grew up near a polish club. It was years before I realised
    that it was in fact a Polish club.

    Cheers
     
  19. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    I think the applied voltage is more important than the other electrode
    material.

    Bye
     
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