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qfn center pad in eagle? — Parallax Forums

qfn center pad in eagle?

Fred HawkinsFred Hawkins Posts: 997
edited 2009-02-02 18:33 in Propeller 1
I'm an eagle rookie, so I am just looking at things and trying to understand what I am seeing.· When I look at the details for the qfn version, the center pad seems not connected to anything. Neither ground nor vdd.

The library is called Parallax_Propeller. It has all three versions.

If it's not connected, can I split it with another trace?

Comments

  • mctriviamctrivia Posts: 3,772
    edited 2009-02-02 02:18
    looks like the package designer placed a ground plane below the chip. Should not be a problem to go through it but you may need to edit the package to get eagle to build
  • mctriviamctrivia Posts: 3,772
    edited 2009-02-02 02:29
    here use this one prop.aigamez.net/parallax_propeller.lbr
  • Fred HawkinsFred Hawkins Posts: 997
    edited 2009-02-02 02:54
    Thanks. Though to be fair the center pad is on the datasheet (page 35), though there isn't any connection details. In other chips with a pad, there's usually four globs of solder paste as well.

    Is the qfn bottom just a thermal patch unconnected to the chip or is it part of the chip's·ground?

    Post Edited (Fred Hawkins) : 2/2/2009 3:05:48 AM GMT
  • mctriviamctrivia Posts: 3,772
    edited 2009-02-02 03:12
    in that case I better look into it more.
  • mctriviamctrivia Posts: 3,772
    edited 2009-02-02 03:21
    ok so the data sheet does show this. Has anyone purchased the chip is there something there?

    I would like to use this package myself but have no expierence with them. BGA self aligns with vias bellow but this i think may be difficult for hand work.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2009-02-02 08:30
    Ground the pad, if you are sinking alot of current out the I/O pins there should be multiple vias to aid in thermal disapation.

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    Paul Baker


    Post Edited (Paul Baker) : 2/2/2009 8:37:21 AM GMT
  • LeonLeon Posts: 7,620
    edited 2009-02-02 10:12
    For prototype boards with QFN I put a single large via under the pad, solder the leads first, then feed solder into the via to solder the pad.

    Leon

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  • william chanwilliam chan Posts: 1,326
    edited 2009-02-02 12:20
    Can the QFN package be soldered using a soldering iron?

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  • LeonLeon Posts: 7,620
    edited 2009-02-02 15:52
    I don't have any problems - I tack down two opposite corners making sure it is positioned correctly, apply plenty of gel flux and drag solder each row of contacts, then solder the central pad. A good soldering station like the Metcal system I use helps, of course.

    Leon

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  • grasshoppergrasshopper Posts: 438
    edited 2009-02-02 18:33
    Well Leon stated that he can solder the QFN with an iron but I am not able to do such.

    I could use a heat gun and my experience to solder the IC using solder paste. Realistically I send it to a PCB assembly house and typically they let me use the equipment for free. Its nice to learn how the PCB board assembly works so that when I design I can use what I learned from an assemblers point of view.

    maybe you can call a local PCB assembly shop and ask for a tour and or what it would cost to make you're prototype.

    Also I ground the pads on the propeller QFN.
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