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Desoldering: Thin layer of solder left on pads

Discussion in 'Electronic Basics' started by [email protected], Nov 28, 2005.

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  1. Guest

    Hi,

    I'm not sure if this is a problem or not. When I desolder resistors
    from a PCB, the desoldering wick sucks up enough solder to remove the
    resistors, but leaves a thin layer of solder on the copper pads that is
    _extremely_ difficult to remove. Is this a normal desoldering
    experience? Do I have to remove all of the solder before inserting a
    new component, and if the answer is 'yes' do you have any
    suggestions on how to do this?

    The materials I'm using are:

    Desoldering braid dipped in liquid rosin flux
    25-watt Weller soldering iron
    97/03 tin/silver solder

    Thank you very much.


    P.S. Also, I would like to thank the people that responded to my
    voltage polarity question a couple of weeks ago. You guys really
    helped a lot.
     
  2. Yes. What is left is called a tinned copper surface and is perfectly
    fine. It will help the new solder to coat the copper easily.
    No. Once enough solder has been removed to easily remove the original
    component and insert the new one, you have removed enough solder.
    (snip)

    That solder has a high melting point that will easily damage many
    circuit boards, separating the copper from the substrate. Must you
    use lead free solder? 63% tin, 37% lead solder melts at a much lower
    temperature. 62% tin, 36% lead, 2% silver melts almost as easily, and
    gives a fine looking joint.
     
  3. Guest

    I've tried 60/40 tin/lead solder and noticed a huge difference between
    the solder with lead and the stuff without, especially when I was using
    a 15-watt iron from Radio Shack. That iron could barely melt the
    lead-free stuff. Most of my solder joints looked like crumpled up tin
    foil.

    The main reason I was using lead-free solder was to make it safer for
    people/pets, because I'm working at home. I'm not sure if that's a
    good reason or not. I just wasn't crazy about having lots of little
    pieces of lead laying around for unsuspecting critters to walk in and
    track around the house, or the best way to clean the mess up, so I
    avoided the problem entirely. :)

    With that said, I may switch back now, as long as I can keep my work
    area squeaky clean. What sort of steps to you take to keep everything
    tidy? Just a decent broom? Or do you even worry about it?

    Thanks again for the help.
     
  4. I do a lot of soldering on the kitchen table and I try to remember to
    wipe it clean before making a sandwich there, but I don't worry too
    much about it. The lead in 63 37 solder is pretty well locked up in
    the alloy as far as absorbing it in the body, compared to lead oxides
    used in old paint, etc. I think there is very little risk from
    ingesting an occasional tiny solder ball.

    Make sure you have a good quality, activated rosin core solder
    designed for electrical work. For hand soldering, I have less success
    with the no clean solder. The flux just doesn't do as good a job at
    helping the solder wet the board and component leads. Never use
    plumbing solder paste flux on a circuit board. It leaves a residue
    that conducts electricity and corrodes metals it contacts.

    If the flux and temperature are right, the joints should have a bright
    shine, especially when eutectic alloys are used. These do not go
    through a pasty stage (crystals of the excess metal growing in the
    eutectic liquid before the freezing temperature is reached) but
    solidify very suddenly.

    Here is an interesting document on some flux choices:
    http://rocky.digikey.com/WebLib/Kester/Web Data/285 RMA Solder.pdf
     
  5. wrote
    I do a lot of soldering on the kitchen table and I try to remember to
    wipe it clean before making a sandwich there, but I don't worry too
    much about it. The lead in 63 37 solder is pretty well locked up in
    the alloy as far as absorbing it in the body, compared to lead oxides
    used in old paint, etc. I think there is very little risk from
    ingesting an occasional tiny solder ball

    Make sure you have a good quality, activated rosin core solder
    designed for electrical work. For hand soldering, I have less succes

    with the no clean solder. The flux just doesn't do as good a job at
    helping the solder wet the board and component leads. Never use
    plumbing solder paste flux on a circuit board. It leaves a residue
    that conducts electricity and corrodes metals it contacts

    If the flux and temperature are right, the joints should have a brigh

    shine, especially when eutectic alloys are used. These do not go
    through a pasty stage (crystals of the excess metal growing in the
    eutectic liquid before the freezing temperature is reached) but
    solidify very suddenly

    Here is an interesting document on some flux choices
    http://rocky.digikey.com/WebLib/Kester/Web Data/285 RMA Solder.pd
     
  6. Jasen Betts

    Jasen Betts Guest

    no, only enough to get the new component in, if you're experimenting with
    different components you can often temporarily mount them on the underside,
    which can be easier, but through-hole mounting is mechanically stronger
    and therfore reccomended for permanent replacements.

    Bye.
    Jasen
     
  7. Rich Grise

    Rich Grise Guest

    I have a suggestion for a thought experiment: If you've removed enough
    solder that the old component falls out of the hole, what will happen
    if you try to insert a new component lead into the same hole that the
    old one has just fallen out of? :)

    Then, part 2: Once the new component is in place, what gets put back
    into the hole? :)

    Should I leave spoiler space? ;-)

    That's as clean as it gets. To get any more solder off of it, you'd
    have to use sandpaper. The copper is _tinned_, that means "coated with
    solder". If, after wicking, there are no more big huge lumps, then
    your wicking job is done.

    I apologize if I sound condescending and patronizing here; after all,
    this is the basics group, and any question is acceptable.

    But, I _do_ wonder, being a tech myself, I have to ask, haven't you just
    tried it and see?

    If, in fact, the question was, "am I leaving too much solder?", then the
    answer is, "If the lead fits, no." :)

    On the other hand, if you're having trouble getting solder out from the
    inside of the plated-through hole, then the answer could be a solder
    sucker:
    http://www.google.com/search?q=solder-sucker

    Sorry it took this whole tirade to get to it! Shoulda signed it "Rich
    the stoner." %-}

    Good Luck!
    Rich
     
  8. Rich Grise

    Rich Grise Guest

    It's not. People have been using 63/37 eutectic solder in their home labs
    for ever since electronics was discovered, and nobody's reported any ill
    effects yet.
    Just don't leave little pieces of _solder_ (you're not melting down car
    battery plates, are you?) lying around where pets and children can lick
    them up off the floor. And even then, the dead epidermis cells and dust
    mite feces are probably more hazardous. ;-p
    Just do your normal, usual, day-to-day, boring, mundane, uninteresting,
    tedious, dull, dreary, mind-numbing, tiresome, lackluster, unexciting,
    monotonous, repetitive, wearisome, humdrum, uninspiring housekeeping as
    usual. Stuff that falls on the floor, even little droplets of solder, is
    just dirt. Just treat it like ordinary dirt, and you'll be perfectly fine.
    :)
    You're more than welcome. :)

    Have Fun!
    Rich
     
  9. Guest

    I'm sure I could get the component to stick if I put it in there, but I
    was mostly wondering if doing so was "bad practice," or would cause
    poor connections/dry joints or something. I guess the answer is "no"
    on both counts. I should have been more straightforward.
     
  10. Pooh Bear

    Pooh Bear Guest

    It's not. The whole lead free thing is a hyped up mess.

    You need specialist soldering gear to use lead free reliably too.
    Specifically, maintaing the correct soldering temperature is far more
    critical.

    Graham
     
  11. Rich Grise

    Rich Grise Guest

    Not a problem. Actually, it was one of our better questions. ;-)

    But yeah, it's not bad practice. There might be some oxide flakes left
    over from the old solder, but the new solder and its flux will clean
    that right up, and the solder will pull itself into the hold by
    capillary action and any crud will float out and get fluxed away. ;-)

    Cheers!
    Rich
     
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