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Desoldering SMT Chips?

Discussion in 'Electronic Repair' started by Madness, Jun 18, 2007.

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  1. Madness

    Madness Guest

    I need help here. I have an Apex AD-660 DVD that has extremely low
    analog sound. I've pulled the board that contains the A/V jacks on it.
    On the *bottom* of it, there are two 8-pin SMT chips that I believe are
    op-amps (yet there's a mess of through-board discrete components on
    top!). I was able to find equivalents for them, but how do I desolder
    the damn things??? I don't think my $15 RackShack soldering iron is
    going to cut it!
     
  2. Arfa Daily

    Arfa Daily Guest

    From experience, these are likely to be line out buffer amps, but I would
    have said that it is unlikely that both or even one of them is faulty,
    unless something nasty has been done on the jack side of things. I would
    have thought that the most likely suspects are that the analogue sound has
    been disabled or muted somewhere in software, or that a supply rail to the
    opamps has gone missing, often due to a short circuit decoupling
    electrolytic somewhere. I get faults like this on various manufacturer's
    items on a regular basis, and SM chips are, on the whole, pretty reliable.
    This exact symptom was very common a couple of years back, on a particular
    Toshiba DVD chassis, and was caused by a short circuit cap. It's also very
    easy to accidentally turn off the PCM analogue mixdown to the line out
    sockets via one of the menus, often several layers down in the "Audio Setup"
    menu suite.

    If it's definitely not a 'soft' issue, you should first 'scope the inputs to
    the chips, and analyze the DC conditions on their pins, but if you do need
    to ultimately get them out, there are several ways. With a little 8-pin, my
    preferred way is to use an 8-pin-shaped bit on my Antex temp controlled
    station. That unsolders all 8 pins at once. Flood the pins with new solder
    first, and before Smitty says so, add a little liquid flux to the equation
    !! ;~} The Antex bit with a long sloping edge does a pretty good job too,
    but only one side of 4 pins a a time. My next method is to flood the pins
    with new solder, then wick it all off with new, good quality soldamop. I
    then slide a piece of thin wire like rework wire or wirewrap wire under all
    the pins on one side, and anchor its end by touch soldering to any
    convenient joint. It is then just a case of pulling firmly away, and
    slightly up from, the pins with the free end, whilst touching each pin in
    turn with the tip of a fairly hot iron. Each one will pop up from the board,
    and the wire will pass under the pin at the same time, removing any remnants
    of solder. Be careful that the last pin to be left attached, doesn't twist,
    taking the pad with it.

    After that method, the final one is by using hot air. There are good people
    on here who swear by all sorts of ways of doing this using paint stripper
    guns and the like, but I've always found that this is tricky for the
    amateur, and is best done with a proper hot-air rework station.

    To solder in new ICs, make sure first that the pads are wicked absolutely
    flat, then carefully position the new chip ( right way round of course ).
    Whilst holding down the centre of the chip, touch one corner pin with a
    fairly hot iron. The remnants of solder on the pad, and the tinning on the
    leg, will normally be enough to tack it in place. Recheck the positioning of
    the other legs, then tack the opposite corner. Now add some liquid flux, and
    go ahead and solder normally. Don't worry if the solder 'blobs' between a
    couple of pins here and there at this point, but the liquid flux will go a
    long way to preventing this. When all pins have been soldered, check with a
    magnifying glass, and remove any solder shorts with a hot iron and thin
    gauge soldamop, applied to the vertical faces of any affected pins. Recheck
    that you've removed the blob, whilst still leaving a fillet of solder
    between the pad and the pin. Removal and refitting is not hard at this
    level - it just needs a bit of care. If you have some scrap boards around,
    practice first.

    Arfa
     
  3. Andy Cuffe

    Andy Cuffe Guest


    For small chips like that, you can remove them easily using a regular
    soldering iron. One method I use is to apply a blob of solder to all
    4 pins on one side. This will allow you to melt the solder on those 4
    pins at the same time so you can lift up that side. Remove the solder
    from the lifted side with solder wick and then repeat on the other
    side.

    An alternate method is to simply cut the pins with a sharp blade, then
    remove them from the board with solder wick. Be careful not to cut
    into the board traces.

    Note that the chip is probably glued to the board, so it will take
    some force to remove it. Practice on a junk board if you can.

    Having said that, I doubt this is your problem. Op-amps are very
    reliable.
    Andy Cuffe

     
  4. Steve Wolfe

    Steve Wolfe Guest

    I need help here. I have an Apex AD-660 DVD that has extremely low
    http://www.sparkfun.com/commerce/present.php?p=SMD-HowTo-3

    steve
     
  5. Cold_Cathode

    Cold_Cathode

    2
    0
    Jun 19, 2007
    Thinking 'outside of the box'


    Use a magic marker and stripe the solder black. Now for the fun part - on a nice sunny day - use a magnifying glass to melt the solder. Wear sunglasses - to see what you are doing. Use an airblast to blow off molten solder or have your assistant puff into an appropriately sized (and placed) drinking straw. Finalize by cutting with an exacto blade. I used a jumbo fresnel lens to remove a 100-pin cpu socket. It worked but it gets pretty smelly. In fact - this is prolly toxic and a bad idea - not recommended - inside or outside the box. :eek:

    Shop-style hot-air gun is good for component removal. (Wear leather gloves.) I like the thought of adding more solder - to encourage heat distribution. This may (will) also lower the freezing point giving you more 'thermal lattitude.' Normally, commercial boards use higher melting point solder alloys .. tough to desolder.

    Solder braid is good yet any old speaker cable (multi-strand) works just as well. (At one one-hundreth the cost.) Roger that! Over and out : )
     
  6. w9gb

    w9gb Guest

    No it will not. I wish you guys would quit buying that junk -- and even buy
    a good used unit -- for the same price -- you se them on eBay (and let
    quality go to trash!)
     
  7. msg

    msg Guest

    Madness wrote:
    If you don't mind a little homebrewing, consider building an SMT
    hot air pencil; here is my version:

    http://www.cybertheque.org/homebrew/smt-rework-tool

    Regards,

    Michael
     
  8. me

    me Guest

    or unsolder one leg at a time. Heat each leg with a soldering iron, and
    lift the leg with tweezers. For the last two or three just heat and lift
    the part...
     
  9. bz

    bz Guest

    Hard to do with surface mount technology. No legs to lift.

    And the pads are CLOSE together.

    BTW, thanks to MSG for the cool idea of converting a cheap soldering iron
    to into a hot air device, and the well written description of how to do it,
    step by step.





    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
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