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TIP: A simple and safe way to desolder SMD SOICS etc

Discussion in 'Electronic Repair' started by N_Cook, Jul 24, 2013.

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  1. N_Cook

    N_Cook Guest

    I discovered this via needing to isolate Vcc of a SOIC where the trace
    is under the IC and a multilayer board.
    I have a reel of 0.08mm diameter tungsten wire , tungsten probably not
    critical and diameter only needs to be small enough to go under SOIC pins.
    I've since tried, pin by pin, of an 8p SOIC on a scrap board with a
    success rate of 8 out of 8 isolating each pin , first time. Leaded
    solder, PbF situation not tried yet

    Feed length of the wire under a pin , long enough to grab hold of both
    ends. Pull outwards and slightly upward while quick soldering iron melt
    of the pin solder. The wire pulls through, but surprisingly , 8 out out
    8 times so far, the solder has not rejoined across the gap. Then a
    sliver of mica under the pin , in my Vcc situation, to fully
    isolate for testing purposes.
    But of course where hot air is out of the question , the IC is required
    for salvaged re-use etc, this method pin by pin could be used even in
    restricted space,
    to salvage a whole SMD IC. I wonder if a single loop per side of a DIL
    or quad footprint IC , and progressive single pin melting , would work
    with 8 or more pins in one operation - worth trying some time
     
  2. Surface-mounted stuffs are not supposed to be repaired as traditional
    chips, right? :)

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  3. N_Cook

    N_Cook Guest

    Not accessible in tnis case, its buried inside the board somewhere unknown.
    While at it for traces that are accessible. Sometimes in those sets of
    Cintride/"diamond" abrasive addons for Dremmels, a diabolo shaped one.
    Like 2 cones back-to-back, the 3mm or so diameter outermost cone base is
    excellent for cutting neat + tight slots in fine pcb copper traces.

    I will try kevlar fibre from an optical data link instead of tungsten
    wire, to see if that works also
     
  4. N_Cook

    N_Cook Guest

    A twist of kevlar fibres works if you can get the cord under and around
    the pin
     
  5. N_Cook

    N_Cook Guest

    Etch revision wire must be compatible with solder, I would have thought
    the technique would require an incompatible material
     
  6. N_Cook

    N_Cook Guest

    Must have been on a thread I was not following. I think I would have
    tried it myself if I'd seen it.
    Just tried removing a 50 pin quad format SMD IC. Convenient tiny plated
    hollow vias all over the board to pass the tungsten wire through. First
    run I decided to run flux over the pins to assist desoldering but that
    made the solder recoalesce on half the pins. As tungsten wire is awkward
    to handle, used an artery forceps to clamp the wire under the pcb via
    and another to hold the free end of the wire, and tension angles just
    slightly off line to avoid bending the pins.
    Tried another 50 pinner and 4 runs with satisfying twang on release each
    time, with only 2 pins having rejoined solder under , which of course is
    no difficulty to deal with individually when the other 48 are released.
    Only a couple of minutes to desolder a 50 pin SMD
     
  7. N_Cook

    N_Cook Guest

    I've not come across the term or actuality of revision wire, only pcb
    trace repair copper foil of various thicknesses/widths. Tungsten is
    surprisingly strong for only 1/10mm diameter, which is probably an
    advantage for this use and quite rigid for the insertion under the pins
    process. Ill probably ditch the masking off and hot air desoldering of
    ICs and move over to this method. I used tungsten wire there but only to
    apply lift-off force to the IC while applying hot air.
     
  8. N_Cook

    N_Cook Guest

    I find solder perfidious stuff. Cut a trace for diagnosis purposes and
    the solder will not bridge the gap when it comes to repairing the cut.
    Resolder one pin of a SMD and there is a good chance of soldering to
    the neighbouring pin and for good measure some of the solder migrating
    under the IC. I would never of believed that running a wire between pcb
    pad and pin of a SMD swathed in molten solder , would split the solder
    into 2, the vast majority of times, so would never have considered
    trying it.
     
  9. N_Cook

    N_Cook Guest

    "here" being usenet but on uk.d-i-y, end apr 2013, a place I rarely
    visit except to post a buildings type query
     
  10. N_Cook

    N_Cook Guest

    I just noticed the production date on my roll of tungsten wire - 1954, I
    think it came from auction sell off of stuff at CEGB Marchwood,Hampshire
    labs
     
  11. notme

    notme Guest

    Surface-mounted stuffs are not supposed to be repaired as traditional

    I believe he was being humorous. (note smiley...)
     
  12. N_Cook

    N_Cook Guest

    or even just plain old wire-wrap wire, it has to be strong enough to be
    pulled tight over those square corners of the pins without fracturing.
    If I can remember where it is I'll dig out some of it and see how
    solderable it is and whether it works for this SOIC removal purpose
     
  13. N_Cook

    N_Cook Guest


    Tried some ancient Bicc-Vero wire-wrap wire (Kynar sleeving stated on
    the drum, order code faded with age), surprisingly as made just down the
    road from me, stated as 30AWG. I don't know how to strip off long
    lengths of the sleeving but for 6 inch length, grab stripped wire end in
    pliers and compress by pushing with fingernail+finger, the Kynar/ptfe?
    along and off the wire. Incidently , by heating and stretching this , in
    effect sleeving, will form even finer spaghetti tube. The wire does take
    solder and I can confirm can be used for SMD SOIC removal
     
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