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QFN SMD Question — Parallax Forums

QFN SMD Question

ElectronegativityElectronegativity Posts: 311
edited 2009-11-27 07:31 in Propeller 1
Is the central square under the QFN Propeller a ground plane?

Should I solder it to the board?

If so I would have to connect it to ground by extending the Vss traces under the chip.
Is that the right thing to do?

Happy Thanksgiving!

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I wonder if this wire is hot...

Comments

  • LeonLeon Posts: 7,620
    edited 2009-11-26 19:33
    It should always be soldered - you need a central pad of the correct size. If it isn't soldered, the chip could shear off if the board was mishandled. For manual soldering I use a largish via and feed solder into it from the underside of the board with a soldering iron tip. For reflow soldering, you need a stencil with a grid pattern for the pad, otherwise the chip floats on the molten solder. There is a suitable footprint in the data sheet.

    It should be grounded.

    Leon

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    Amateur radio callsign: G1HSM

    Post Edited (Leon) : 11/26/2009 7:40:17 PM GMT
  • ElectronegativityElectronegativity Posts: 311
    edited 2009-11-26 20:22
    Thanks Leon.

    I have never soldered a QFN before, but I have a plan.

    What I intend is to etch a solder mask of the lands in a sheet of 5 mil copper, then squeegee in solder paste, place the components, and pan fry it on an electric hot plate.

    The pan fry method works well for SX28/SS and 1206 SMD components, but this is really going to be pushing it.
    Since I was feeling outlandishly optimistic I bought EEPOM in 8MSOP and voltage regulators in 6MLP as well.

    The fallback plan is to reconfigure the whole thing for the TQFP package, but I bought 5 QFN propellers and I am going to give this a try.

    How do you solder QFN or other no-lead packages?

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    I wonder if this wire is hot...
  • LeonLeon Posts: 7,620
    edited 2009-11-26 21:01
    Drag-soldering with plenty of gel flux. I extend the pads slightly.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
  • ElectronegativityElectronegativity Posts: 311
    edited 2009-11-26 21:31
    Wow, you must have good hands.

    I will report back if there is any progress.

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    I wonder if this wire is hot...
  • LeonLeon Posts: 7,620
    edited 2009-11-26 21:39
    Drag-soldering doesn't need a particularly steady hand, except when tacking down the two opposite corner pads. I use a microscope.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
  • ElectronegativityElectronegativity Posts: 311
    edited 2009-11-26 21:44
    What kind of microscope?

    I use a 10X jeweler's loupe or 3.5 diopter lens.

    Good microscopes are not cheap.
    The one I use at work costs about $100,000.

    I'm pertty sure they would frown upon turning it into a soldering station. tongue.gif

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I wonder if this wire is hot...
  • LeonLeon Posts: 7,620
    edited 2009-11-26 21:49
    A cheap stereo dissecting microscope. It's made in China and the optical and mechanical quality are surprisingly good. IIRC, it cost me under £60 (GBP). The boom version would be a lot nicer for PCB work, but costs about 2x as much.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
  • william chanwilliam chan Posts: 1,326
    edited 2009-11-26 23:51
    Which iron and what tip you use?

    Can I use a beautician's lens? The one with a circular fluorescent lamp?

    The lens seems to restrict the movement of my iron, making soldering difficult.

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    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2009-11-26 23:59
    I use a Metcal STSS power unit with an MX-500 handpiece and MX-500 cartridges. I use a mini-hoof cartridge for drag-soldering,

    Whether that magnifier works depends on your eyesight. I can't manage without the microscope.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
  • william chanwilliam chan Posts: 1,326
    edited 2009-11-27 00:05
    Any links to the Metcal units?
    Does your iron accidentally touch the lens or other parts of the microscope?

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    www.fd.com.my
    www.mercedes.com.my
  • william chanwilliam chan Posts: 1,326
    edited 2009-11-27 00:12
    Leon,

    Can I use this microscope for PCB work?

    my.farnell.com/duratool/59-020-040/microscope-stereo/dp/7222294

    Is the lens too close to the board?

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    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2009-11-27 01:26
    The objective is too close. There is plenty of clearance under mine - about 6".

    Here is Metcal:

    www.metcal.com/

    They aren't expensive if bought second-hand. Mine cost me £125 (GBP) with a new handpiece and some new cartridges.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
  • Nick MuellerNick Mueller Posts: 815
    edited 2009-11-27 07:31
    You'll find a lot of videos on YouTube about soldering.

    I do use a Weller soldering station, a PU81 with a WSP80 pencil. It's not very cheap, but it will last for decades. I do have a second station, a WECP20 that is almost 30 years old (still getting parts, no problem).
    For magnification, I do use lenses like these: <http://www.conrad.de/goto.php?artikel=814160&gt; but without LED.
    I'm not using solder paste, but 0.5 mm wire.


    HTH,
    Nick

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    Never use force, just go for a bigger hammer!

    The DIY Digital-Readout for mills, lathes etc.:
    YADRO
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