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How to solder a 32L QFN

Discussion in 'Electronic Design' started by TT_Man, Sep 20, 2007.

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  1. TT_Man

    TT_Man Guest

    I'm trying an RF design using the NRF9E5. It's the biz and very, very
    cheap.(<£2). The thing is, it's only available in 32L QFN package 5x5 mm.
    OK in production, but I need a few prototypes.I have been quoted £550 for 10
    off all in.£250-£300 of that is stencil costs..
    Given the pads are on the underside of the chip, is there a way to prototype
    this manually.? The pcb's are cheap <£1 ea @ 1.5" x1". I can tolerate a few
    failures as well. If I made 10 and got 3 I would be happy.
    What are the options? I know about the home brew USA toaster ovens... that's
    not really an option.
    TIA
     
  2. dalai lamah

    dalai lamah Guest

    Un bel giorno TT_Man digitò:
    Completely underside? Are you sure? In the typical footprint patterns
    suggested by the manufacturers for QFN packages, the pads aren't completely
    under the chip, there is a smaller exposed part (some tenths on mm) that
    makes possible to solder the chip manually with a very fine tip. I do it
    quite often.
    You can try to pre-solder all the PCB pads, put the chip in place and heat
    it with hot air. I've done it with some small BGA chip, with good
    percentages of success.
     
  3. Guest

    TTMan

    We have had quite a lot of success with a hot plate (set at150C) a hot
    air soldering iron and solderpaste manually placed on the PCB pads.
    You might be able to forgo the hot plate and just use a modified hot
    air gun and solder paste. I'm not sure quite how you can modify the
    hot air gun, what you need to do is limit the air flow (to make sure
    it doesn't blow the component away) and make sure that the air temp is
    runnning about 220 to 240C.
     
  4. DJ Delorie

    DJ Delorie Guest

    Having done this before for a friend... The hotplate trick works for
    these too. The only hard part is getting the solder paste on the pads
    neatly, since they're so close together. Try an xacto knife to
    "neaten" up the paste, place the chip, and hotplate it. Do it first,
    then do the rest of the parts by hand.

    Even without a hotplate, it helps if the lands on the pcb extend out
    from the chip by 20-40 thou. That gives you something for your iron
    to touch if you need to reheat under the chip.

    Another idea we've tried for QNs with belly pads is to put a big via
    in the middle of the pad, big enough to fit your iron through from the
    other side. Just don't try to make that land much of a heatsink or
    you won't be able to actually solder it.
     
  5. TT_Man

    TT_Man Guest

    You're right.!!! I just got a sample chip and the lands do run up the side
    of the chip a few thou....... so that makes it whole lot easier!. I like the
    big via idea , but the pad size is only 30x10 thou..... And extending the
    pcb tracks 20 thou will help.
    Did I miss something about the big vias?
    Thanks for the answers.
     
  6. DJ Delorie

    DJ Delorie Guest

    If the pad is that small, you're probably out of luck. The via needs
    to be big enough for the tip of your iron to fit through (25 thou in
    my case), or you need to pump enough heat through it to reflow the
    paste on the other side.

    For a pad that small, I'd try the hotplate method. For prototypes,
    you don't need the pad's paste to be as perfect as a production run
    (unless you're testing its thermal limits) and if you do, you can have
    a mylar mask made and still hotplate it. Still extend the lands,
    though, in case you need to fix up a solder joint manually.

    http://www.sparkfun.com/commerce/present.php?p=Reflow Skillet
    http://www.delorie.com/pcb/hotplate/
    http://www.stencilsunlimited.com/solder_products.php
    http://www.smtstencil.com/
     
  7. DJ Delorie

    DJ Delorie Guest

    Note also that extending the lands isn't just so you have something to
    solder *to*. The lands are small enough that they'll conduct heat
    under the pad also, so if you have paste under the pad, heating the
    copper land can reflow the paste without actually having to touch the
    paste or the chip with your iron. That also keeps the solder from
    sticking to your iron and messing up the chip's placement.
     
  8. TT_Man

    TT_Man Guest

    Thanks DJ....... I'm sure that will help a lot of people. It has certainly
    given me the encouragement to have a go.!

    Here's a thought..... I have a large flow solder machine in the garage.
    Could I use that as my hotplate maybe?
    What also amazed me was the way the 132 pin chip had its solder paste
    applied! I would have thought that would have ended up a total mess, but
    clearly that is not the case.
     
  9. Well, every home should have one!
    The older SMT packages were designed to be flow soldered as well as
    reflowed, as I understand it. The components are glued down first.

    I wonder if it would work with the QFN (with extended lands)?
     
  10. DJ Delorie

    DJ Delorie Guest

    If his part has a belly (thermal) pad, I don't think using the flow
    solder machine will work to actually *flow* the solder under there,
    but it might work to heat the PCB enough (on the other side) to reflow
    the paste. Assuming you can hold the board steady long enough for the
    heat to work its way through the board.
     
  11. Yes, I was ignoring the "thermal" pad. I use the "soldering through a
    big hole" technique for that.
     
  12. TT_Man

    TT_Man Guest

    Could be worth a try, so long as the part is located bang on the pads.....
    Yes, this chip does have a big thermal pad underneath.... But the data sheet
    says this mustn't be connected to anything, least of all ground.
    They also say that the gnd via connections on their reference design will
    not contact the belly pad because of the solder mask..... Sounds a bit iffy
    to me... but perhaps I will cover the vias with the mask as well ( some
    peeps do this anyway).
    I can hold the board steady with the PCB carrier that holds the boards for
    flow soldering.
     
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