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how to solder QFN/DFN chips?

Discussion in 'Electronic Design' started by Michael Noone, Jul 14, 2005.

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  1. Hi - I'm going to need to solder a DFN chip in a couple of weeks for a
    prototype. This is the chip: Sheets/KXP74%
    20Series.pdf. I just got them in the mail today and am now a bit frightened
    of them... The pads are entirely on the bottom of the chip - thus are not
    exposed at all when placed on a board. How does one solder such a chip? I'm
    thinking I'll make the pads long enough that they stick out from underneath
    the chip (I haven't sent the board off to be made yet) - then I'll place
    the chip on the board and touch my iron to each pad on the board
    individually. Does this sound like a good plan?

    Would a hot air station be helpful in soldering a chip like this? I've been
    looking at some of the inexpensive Hakko knockoff hot air stations (ie Or is it
    best just to stick with an iron? (I have a Hakko 936-12 along with a couple
    fine tips)

    Thanks for your help!

    -Michael J. Noone
  2. Leon

    Leon Guest

    I've managed to solder a similar device using your proposed technique.
    It is fiddly, though, and I needed to feed some solder in to get a
    decent joint. Now I have a stereo microscope it should be a lot easier.
    Hot air and solder paste is another way that should work.

  3. PeteS

    PeteS Guest

    I see (in common with a lot of these) that the device has a pad
    underneath, although there is no guidance on where that's connected.

    Anyhow, with such a device, hot air makes the most sense. One guy I
    have do a lot of work for me uses a simple hot air gun (you know, the
    industrial type) for these things. Does the job and a whole lot cheaper
    than a pro hot air station.

    Get some solder on the pads (if you have paste that's even better) and
    then heat it gently (but not TOO slowly) from an angle of about 45
    degrees, moving around the device. It should get hot and reflow in a
    very short time.


  4. That is why I built a little setup:
    It is untested till now.

  5. Leon

    Leon Guest

    For prototyping the pad can be soldered by putting a via in the centre
    and heating it up with the soldering iron tip whilst feeding solder
    into it.

  6. Al Clark

    Al Clark Guest

    QFNs can usually be soldered just like leaded SMT parts if you extend the
    pads outside the body of the part. We use lots of flux, a microscope and a
    soldering iron. If the part has a large center pad, we have used solder and
    a heatgun effectively for this connection. We also started placing a large
    thru hole in the center of the pad so that we can access the pad from the
  7. JeffM

    JeffM Guest

    Leon wrote (without context):
    FYI: It's easy to get context when posting from Google Groups:
    Instead of clicking the Reply link that is in plain sight,
    click the **show options** link, then click THAT Reply link.

    I hope PeteS read this; he's dong it too.
  8. How does one solder a part with hot air and solder paste? I've always
    thought that it seemed like the hot air would just blow the part away...
    And how/where does one apply solder paste?


  9. JeffM

    JeffM Guest

    That is why I built a little setup: :Some fearlessness from electricity is required at this point.
    :I might have to look for some high temperature insulation at a later
    Heh...or new employees.
    Beside that no overheating occures at any locations.
    Besides that, no overheating occurs at any locations.
  10. I've done it by preheating the board to ~100°C, then melting the
    solder with hot air, putting the chip on there (no preheat on the
    chip, hey, it's just a prototype), and waiting until the chip settles
    into place with the pads all nice and watery. A proper layout should
    draw the chip into position from surface tension. You might want to
    practice a bit first. A flux pen doesn't hurt either.

    Best regards,
    Spehro Pefhany
  11. Pooh Bear

    Pooh Bear Guest

    A flux pen ? Never come across one of those.

  12. Just paint across all the pads.

    Best regards,
    Spehro Pefhany
  13. Wim Lewis

    Wim Lewis Guest

    I've "hot air reflowed" SOICs over a small paraffin lamp, mostly to
    see if it would work. It did work, though I had to touch up a couple
    of pins with an iron (not enough paste on those pads). Single-sided
    boards only, of course!
  14. Terry Given

    Terry Given Guest

    quite the opposite - excess flux prevents bridging. Wide wedge-shaped
    tips can be used to solder all the legs on, say, a tqfp by:

    - applying lots of flux
    - lying a piece of thin solder across all the pins
    - applying wide iron tip to all pins simultaneously, melting solder then
    dragging tip away from chip

    voila, all pins soldered.

    Soldering station salesmen show similar setups at trade shows. They say
    the iron/tips are the magic part, but the copious quantities of flux are
    what really helps, along with lots of practice.

    then you need to clean off the flux.

    IMO a (binocular) microscope + soldering iron with reasonably fine tip +
    thin solder is all thats required. At a minimum you need to be able to
    see between pads, to find shorts.

  15. I seem to recall an article (in Circuit Cellar) a year or so ago about
    using a toaster oven to do hot air soldering.
  16. JeffM

    JeffM Guest

  17. So is pre-tinning the pads or pins typically done before using hot air, but
    not before using an iron?

    I have a (monocular) microscope and I've used it to repair PQFP pins, but I
    could only use it near the board edge. I'd love to have a swing arm if I
    can find a cheap one.
  18. Terry Given

    Terry Given Guest

    all the PCBs I work with are HASL'd, so yes. Sometimes I add a little
    bit of solder to one corner pin, to tack the device in place. In
    practice I dont have one of the nice drag-soldering style wide tips, so
    solder pins one at a time. Its easy when you can see.....

    And just like Spehro said, I use a flux pen.
    The swing arm on my microscope is a 2" steel bar about 3' long,
    connected to a base so heavy I need both hands to lift it (and I'm a big
    guy :)

    OTBS, it helps hold the house down when the winds pick up. A tree
    snapped in half yesterday.....

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