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Surface mount soldering/desoldering

Discussion in 'Electronic Basics' started by rgb-man, Feb 15, 2004.

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  1. rgb-man

    rgb-man Guest

    I have a board that has a faulty surface mount chip - the package is
    just over 1 inch square and has 176 pins.

    I have another faulty board (with missing parts) but which has the
    same part that I could, in theory, remove and transplant to the board
    with the faulty IC.

    Trouble is that I've never desoldered or soldered a surface mounted
    device in my life, least of all one with 176 pins! I have though
    soldered and desoldered plenty of chips with the usual plated through
    holes.

    Should I give up before I even start?

    If I decide to give it a go, will I need to buy much specialised
    equipment?

    Are there maybe any web sites which people here would recommend that
    are particularly useful?


    Thanks
     
  2. What kind of pins? Flat and J-leads are (relatively)
    easy; first suck up most of the solder with pre-fluxed
    copper braid (buy it as "desoldering braid") then GENTLY pop
    the pins loose from the pads with a BRAND-NEW Exacto knife.
    On the bad chip, you can just cut the pins loose from the
    package (with good quality fine-tip cutters) and desolder
    them individually to minimize the amount of heat you apply
    so as to avoid lifting the pads from the board. That may
    risk lifting the pads anyway though because cutting them
    produces a mechanical shock that has nowhere else to go.

    Then, straighten the leads on the good chip, pre-tin the
    target board pads with solder paste applied with a toothpick
    or very fine wire solder, tack the chip down at opposite
    corners (make sure you get it aligned just right or you'll
    have to start over), then solder all the pins on each side
    en masse by gently wiping the iron slowly along them (if you
    tinned the pads right you won't get bridges). With flat pins
    you can do them individually if you're feeling froggy; press
    the pin down with the Exacto while heating with the iron.
    I've never had solder stick to an Exacto blade.

    You may need a lighted magnifiying lamp to see what
    you're doing. Finally, check for solder bridges between pads
    (use the braid again to clear them). You'll want a very
    fine-pointed iron (preferably one that's temperature
    controlled), though I've done it successfully with a clumsy
    Weller gun in my younger days.

    So, you're looking at acquiring (in approximate
    descending order of cost); a magnifying lamp, a good
    sm-capable iron, maybe a pair of fine-tip cutters, an Exacto
    knife set, some fine wire or paste solder, and desoldering
    braid. Much of it you'll find indispensable for lots of jobs
    once you have it.

    A set of "third hands" or small bench vise will help hold
    the board steady, too.

    Get some practice with the other sm chips (if any) on the
    donor board first to build your confidence. Oh, and lay off
    the caffeine that day (ask me how I know).

    Mark L. Fergerson
     
  3. Put it in a pre-heated oven at gas mark 6, 2 hours should do it.
     
  4. Garrett Mace

    Garrett Mace Guest


    I have done this successfully with a $20 heat gun. It is best if you can
    come up with some kind of mount to hold it over the chip so you have two
    hands free to remove it.

    Heat guns, on the low setting, put out about 650 degrees F. That temperature
    will damage the chip internals, and if you have a moisture sensitive chip
    you might generate steam bubbles inside and damage the die with internal
    stresses. So the plan is to heat up all the pins as quickly as possible, tap
    with a dental pick until you see the entire chip wiggle, and then quickly
    flip it over with the dental pick and turn off the heat gun. This gets the
    chip off the board before the inside of the chip has the chance to heat up
    to dangerous levels. The ideal distance seems to be about 1.5 inches above
    the device. This is really no different from the way a professional hot air
    surface mount rework station operates, except they have metal airflow
    concentrators that direct most of the hot air directly to the outer edges.
    You could find a piece of ceramic tile that fits the top of the device, and
    use that to help keep air flow off the top of the chip. Perhaps even take
    some clay (real, not plastic) and form little pyramids of different sizes
    for different chips. My heat gun happens to blow most of the air around the
    edges of the barrel, so it works fine for me the way it is. I have used this
    technique on devices up to PQFP-208.

    For soldering, you want to keep in mind from the very beginning to use as
    little solder as you can get away with. Go ahead and buy the thinnest solder
    you can find, or a syringe of solder paste if you can get it. Use the barest
    amount of solder and a fine-tipped iron to tack down the chip in alignment
    on the board, then go around with a small screwdriveror razor blade and
    ensure every pin is in contact with the pad below it. Run a flux pen along
    the length of the pads. Then take a length of fine solder (.016" or smaller)
    and lay it across the pins, and use a LARGER soldering iron, about 40W, with
    a bigger tip to smoothly drag along the pins and solder. If you do it right,
    you should have nearly every pin soldered. If you used too much solder, it
    will wick up in between the pins and lodge itself right up against the chip,
    where it is difficult to remove even with solder wick. That's why you need
    the thin solder, because if you at any point have too much in one area it
    will wick up. You need the large soldering iron because it stores more
    energy and won't cool down before you get to the end of the row. ONLY use it
    for dragging though, and don't keep it in contact with any single pin for
    more than a second. Use the fine tipped iron for touchups.

    Alternately you could just run a thin bead of solder paste along the pads,
    position the chip, and heat up again with the heat gun. Solder paste is
    expensive so I don't do this with large chips that are easily done with the
    above method.
     
  5. I have had good luck slavaging big gull wing SMDs by slipping a piece of
    wire wrap wire behind the pins and temprarily tacking it down somewhere
    off to the side. Then after soaking up as much solder as possible with
    braid I touch the end pin with the iron and give the wire a light tug to
    lift the pin off the pad and proceed down the row.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  6. Jamie

    Jamie Guest

    you need a hot pencil with vacuums and a cross heating tip would help.
     
  7. Jamie

    Jamie Guest

    hey, thats a hell of an idea!
    i think i will try that next time!
    i do have some very small bare wire here that would work perfectly
    for that.
     
  8. rgb-man

    rgb-man Guest

    On Sun, 15 Feb 2004 10:16:54 -0800, Jamie

    Thanks very much for all the replies - now I've got to summon up the
    courage to have a go. :)
     
  9. Yet another use for wire wrap wire! I agree with Jamie;
    I'll have to give this a try.

    <humility rant>

    I was so sure I knew all the worthwhile tricks (assuming
    no access to "pro" tools) that I didn't bother looking for
    the kind of relevant website rgb-man asked for. Most of the
    non-pro stuff I know I figured out on my own (after ruining
    a sufficient amount of stuff to know what not to do) and
    learned from other techs, but obviously none of us can know
    everything. Might be a good idea to put all this up
    somewhere so all us hotshots can fill the gaps in our
    knowledge, or at least know what's appropriate to a given
    task without having to guess, or risk ruining something
    irreplaceable. I have no objection to leapfrogging the
    learning curve as long as I know why what I skipped isn't
    worth slogging through.

    Sometimes "hard-won experience" isn't worth the pain
    involved in winning it.

    Mark L. Fergerson
     
  10. David

    David Guest

    You might fold a piece of Aluminum and tape/glue it to the top of the chip.
    Aluminum reflects of IR radiation quite well and would therefore reduce or
    eliminate heating of the chips internals while leaving the pins exposed.
    Just FYI.
     
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