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Soldering Help

Discussion in 'Troubleshooting and Repair' started by drumsticksplinter, Mar 28, 2010.

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  1. drumsticksplinter


    Mar 28, 2010

    I'm looking for a little advice in repairing a PCI card for a PC. The card in question is an industrial data acquisition card with 64 isolated digital outputs. Unfortunately due to my own fault i think i have blown one of the darlington transistor arrays as i was only using 1 output at the time and it has taken out 8 (which is also the amount of channels of the transistor array). My question is about actually replacing the IC. My soldering skill is usually very good, but I've never de-soldered / soldered a multi layer PCB before and I'm afraid i might destroy the card all together......:eek: Could anybody please give me some pointers to minimize my chances of destroying a very expensive card completely.

    Many Thanks in advance,

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you doubt your skills, find someone who has better skills.

    Having said that, the general method is as follows:

    0) Make sure you note which way around the IC goes!

    1) Remove solder

    2) IC falls out

    3) Replace IC

    4) Solder IC.

    The only hard part is step 1. There are various methods. A lot of the difficulty factor depends on whether the leads are soldered on both sides.

    You can

    a) melt the solder and suck it up with a solder sucker,

    b) use solder wick to soak up the solder,

    c) use a special soldering iron tip that melts the solder on each leg of the IC at once

    d) use a rework tool with an appropriate nozzle to melt the solder over the entire area.

    e) clip the IC legs and remove them one by one.

    I would recommend (b). Note that in some cases it can be beneficial to add more solder to the joints. Place the wick over the joint and heat the joint through the wick. You'll see the solder get absorbed in the wick, and in the best case you'll be left with a lead sitting in the middle of a hole with no solder connecting it to the board. If there is a very small bridge of solder left, you can often break it by pressing the lead away from it with a jeweller's screwdriver. The idea is that all leads are loose and the IC just falls out.

    The benefit of (b) is that the IC remains in a condition for re-use (which may be a good thing if it's not faulty!) but only if you're fairly quick and skilled.

    Option (e) is pretty easy, but destructive. Use very small side cutters to clip each lead of the IC and remove the body of the IC. Then, one by one, grasp the legs with pliers and use your soldering iron to melt the solder, allowing you to remove the lead. Use solder wick to clean up the holes. This method requires three hands.

    This option is perhaps safest for the board since you don't need a lot of skill and (if you are less skilled) results in the lowest chance of overheating things or pulling off tracks.
  3. drumsticksplinter


    Mar 28, 2010
    Thanks for the reply Steve!

    I think i will definitely go with the method you recommended. De-soldering is something I've had fairly little experience at. Just another quick question, what wattage iron would you recommend? Unfortunately I've not got the benefit of a temperature controlled unit (yet), but i have got 15w, 30w and 50w irons.

    Many Thanks for your help!

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Oh, if it's a double (or more) sided board, and you go the lead clipping route, remember not to pull the pin out until the solder has melted on both sides of the board!

    The soldering iron you use depends on the technique you're using, and on how hot they get.

    With solder wick, it may be beneficial to use the higher wattage iron if it has a tip that allows heat to be applied to a couple of pins at once. This is beneficial if it allows you to suck up the solder faster. Solder wick is a pretty good conductor of heat and low wattage irons may not be able to quickly heat both it and the joint up, leading to more heat being applied to the board.

    If you have to hold the soldering iron in the pins for more than 2 or 3 seconds, it's taking too long.
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