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SMT by hand

Discussion in 'Electronic Design' started by Jon Slaughter, Dec 27, 2009.

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  1. Any have any tips for QFN packages by hand without a stencil? The main
    problem is that there seems to be no easy way to check/fix to make sure the
    leads are soldered since they are completely under the package. I was
    thinking of using puting solder paste on the pads by hand and attempting to
    properly align the IC. The problem I see is that most likely the solder
    paste will end up between the pads rather than on them and I'll probably end
    up with shorted pads. Does the solder on the solder mask tend to "run" to
    the copper? (after all, isn't that what the solder resist is for?)

    If I have the right amount of paste in the right places and the ic is
    somewhat closely aligned will it align itself?
     
  2. krw

    krw Guest

    The problem with QFNs is the center pad and getting just the right
    amount of solder on it. There isn't a lot of room for error here. Too
    much and the part will "float" on the center pad so it won't align
    properly.
    It should, though it's a lot harder to get right than a BGA, for
    instance. The center pad works against you, where in a BGA all the
    balls work in your favor. Since the pad is huge, in proportion to the
    pins, it can get dicey. Even with automatic PnP, we have far more
    problems with QFNs than BGAs. Of course the crappy RoHS processes
    make things far worse.
     
  3. Nico Coesel

    Nico Coesel Guest

    That is usually solved by putting several uncapped vias underneath the
    QFN. The vias also allow to solder the center pad manually. Preheat
    the board using a heat-gun and put solder in one of the vias. Wait
    until the solder comes up from the other via. Then you'll know the
    center pad is soldered.

    Nope. If you make the pads a bit larger you can hand solder the part
    with a very fine soldering tip. I usually wrap a piece of thin solid
    copper wire around the soldering tip to solder the pins. Copper is
    still the best material for soldering tips.

    Ofcourse, you'll need flux as well.
     
  4. IF the pad is the problem I can just leave it off then. At this point it is
    not an issue(chips have thermal shutdown and I don't plan on running them
    hot). I have vias that I can inject some solder or thermal paste if I do
    have some issues with heat.
     
  5. krw

    krw Guest

    Be sure to check that the pad isn't (the only) ground. I've seen this
    more than once.

    QFNs are a great idea but what a PITA. QFNs and interboard connectors
    are our two largest problems.
     
  6. qrk

    qrk Guest

    For non-production type work (e.g. repair and prototype), I tin each
    PCB pad so there's a small bump of solder. I make sure the bumps are
    approximately the same height. If you have a thermal pad in the
    center, only apply enough solder to raise the part slightly. Too much
    solder on the center thermal pad will cause problems. I use leaded
    solder since it's easier to handle and has a lower melting point than
    unleaded solder.

    If you're reworking a board that used unleaded solder, I flood the
    pads with leaded solder which dilutes the unleaded solder enough where
    it isn't a problem. If you have leisured time, you can remove the
    unleaded solder with solder wick or a vacuum desoldering station.

    Next step, coat the bottom of the IC with flux.

    Place the part.

    Preheat the board to 150 deg C. I use the Madell QK853 hot air plate
    to preheat boards.

    Once the flux has thickened enough (I use RMA flux) use hot air
    soldering station to solder the part. You will see the part self-align
    when the solder melts. If you use water-based fluxes, be sure to wash
    the board as soon as possible as water based fluxes are highly
    corrosive. Water based fluxes are also conductive.

    Hot air systems can be had for under $200 for cheap Chinese equipment
    (See Madell QK857D). They are fine for occasional repair work or
    prototyping.

    Other ways of soldering are toaster ovens and, my favorite, a waffle
    iron with flippable plates (waffle or flat).

    I have used this technique down to 8-pin parts in 1.6 x 1.6 mm
    packages.
     
  7. Thanks guys. I'll have to get some paste and see how it turns out. The main
    problem is that a simple toaster oven or waffle iron is too small for my
    boards. Maybe with some type of flat pan iron it will be possible or using a
    hot air gun(after all, it takes them off pretty easily)?

    Anyone know where I can get some solder paste on the cheap and easy and
    possibly non RoHS(i.e., leaded)?
     
  8. Nico Coesel

    Nico Coesel Guest

    I wouldn't design those packages in. They are hard to get right by the
    assemblers and you can't check the joints visually.
     
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