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Desoldering basics

Discussion in 'Electronic Repair' started by [email protected], Dec 18, 2006.

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  1. Guest

    Hi all,

    I have a small surface mount amplifer/filter chip on a packed PCB that
    I need to replace. In order to solder in the new one I obviously need
    to desolder the old one first being as careful as possible not to
    damage anything else on the board. I've never desoldered before and
    was wondering if someone could give me the basics or atleast point me
    to a tutorial for desoldering in this type of application. Thanks so
    much for any advice,

    Kev
     
  2. Meat Plow

    Meat Plow Guest

    google

    http://www.scienceprog.com/ease-way-to-desolder-smd/
     
  3. Guest

    What type of package?
     
  4. It's often worth crunching up the faulty chip with cutters to remove most
    of it before applying heat. Sounds brutal but the important thing is not
    to damage the PCB and prolonged desoldering is likely to. Once you've
    reduced it to just the soldered 'tails', remove each one individually
    using solder braid.
     
  5. Arfa Daily

    Arfa Daily Guest

    If you really have no experience of desoldering, then I would venture to
    suggest that you are dicing with death to attempt successful removal of a
    surface mount chip on a packed board, as a first project ...

    You really will need to practice first on a scrap board. I usually start by
    flooding the pins with new solder, then wicking off as much as possible.
    This will remove much more solder than just going for it with the existing
    solder, which will not melt and flow up into the solder wick very well. I
    then slide a length of single strand rework wire under all the pins down one
    side, and tack solder the end to any convenient solder pad. If you then
    gently pull on the free end of the wire, away from the chip body, whilst
    applying heat to each pin in turn, they will come up off the board cleanly,
    as the wire passes below them.

    When most of the pins are separated from the board, you may see the chip
    body start to move. Be careful then that the removal of the last couple of
    pins does not twist the IC and pull the last couple of pads off the board.
    If you do not see the IC moving, it is likely glued to the board. The glue
    bond is usually easily fractured by inserting a blunt scalpel blade under
    the edge of the IC body, and twisting. When the IC is off the board, re-wick
    the pads to make sure that they are very flat, and remove any traces of
    glue.

    Position the new IC very carefully, making sure that it is the right way
    round, then tack two pins at opposite corners. Check the pin alignment again
    with a strong magnifying glass. When you are satisfied that it is aligned
    correctly, apply some liquid flux to one row of pins, then just go ahead and
    solder with the smallest iron tip and finest gauge solder that you have.
    Don't worry about solder-flooding bridging the pins at this point. If you
    are using a good quality liquid flux, this should be minimal anyway, if you
    are not going mad at applying solder. Repeat for the other one or three rows
    of pins, depending on package type.

    When you have finished, examine your work with your magnifying glass again -
    a jeweller's loupe is ideal for this - and then go back to any pins that
    have bridged, and apply your solder wick with your iron tip to the vertical
    faces of the affected pins. This will remove the excess solder causing the
    bridge, without compromising the joint. Clean down with flux remover and an
    old toothbrush, and recheck the work with the magnifier. You should find
    that the job you have done is nearly as good as the original flow soldered
    product.

    Arfa
     
  6. N Cook

    N Cook Guest

    Hint for de-soldering surface mount ICs.
    Use a hot-air paint-stripper,1400W,500 degree centigrade,with 2 level heat
    control to prolong element life.
    Form a ring of silicone covered wire around the IC{to isolate the remaing
    components on the pcb.Push a thin piece of wire
    under one side of the IC and form a loop around the IC,repeat on the other
    side;this is to remove the IC when the solder melts, tug on these wires
    while heating up to ensure minimum heating contact time.
    Place a slab of PTFE with right size hole cut into and
    clip pcb and slab together with clothes pegs/Bulldog clips etc.
    If the IC is for re-use then cover body of IC with heat insulating material
    or blast IC with freezer spray.Allow the hot-air gun to get up to heat{say 1
    minute}before applying to IC.
    For more crowded boards make "conical" shrouds to surround the IC. I used
    some PTFE strip that i had but thin paxolin or similar but drilled and wired
    together would probably do. Cut 4 small trapezoids from the PTFE strip.
    Fixed together with all long edges one side and short edges adjascent on the
    other side. Fixed together with paper staples but for the smallest shroud
    for 8 pin SM had to wire together the final join. Forms a sort of truncated
    cone, frustrum, in shape. Tie to the PTFE cone (to stop the blower blowing
    it off) with
    copper wire or temporarily solder to distant points.
    When practised the heated contact time should be less than
    2 seconds - no board distortion or collateral damage surprisingly.
    If you can't get the tugging wires under the IC then pass
    under a few pins at each corner.
    Because this tugging frees the IC at the earliest moment, the solder
    on the board is not fully melted and leaves a profile for localising
    the new IC in place and then solder pin by pin.
    SAFETY NOTE:- ensure good ventilation, use safety goggles,and beware of very
    slight risk of combustion.

    other hints and tips on
     
  7. N Cook

    N Cook Guest

  8. Dave

    Dave Guest

    Hope you've got a single-sided, single layer PCB, put the flat side on an
    upside-down hot iron, remove SMD's (all of them 'cuz they all melt) from
    board. Doesn't go into replacement of said parts.
     
  9. N Cook

    N Cook Guest


    so a no-no , i take it
     
  10. Hi!
    Probably a site of questionable reputation...I see a sign of the video and
    then get redirected promptly to a "DMV lemon law" page or something.

    William
     
  11. FatTony

    FatTony Guest

    do it the RIGHT way and either

    (a). throw it out & buy a new one.

    (b). get a PACE or Weller hot gas bonder/debonder
    tool/station and do it the way the manufacturer
    (and repair depot) would.

    All these methods are half ass "poor man" approaches
    that might work, but it's highly dependent on your
    skill level (and if you have to ask how to remove an
    SMT chip, you don't have the skill level).

    the other question is, if you have no desoldering
    experience whatsoever - what kind of hands on electronics
    troubleshooting have you done to arrive at the conclusion
    that particular chip needs replacement ?

    Go look on E-bay and you'll find many hot gas
    debonding tools to rework SMT boards.
     
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