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QFN Center PAD? — Parallax Forums

QFN Center PAD?

jazzedjazzed Posts: 11,803
edited 2011-02-21 15:12 in Propeller 1
What is the Propeller QFN package center PAD?

Nothing is in the datasheet.
Is it ground/vss? If not what?
Is it not connected?
If not connected can it be connected to ground/vss?

Thanks.
--Steve

Comments

  • Nick McClickNick McClick Posts: 1,003
    edited 2011-02-15 19:11
    I don't use the QFN, but I believe it's not connected - just there for thermal.
  • WBA ConsultingWBA Consulting Posts: 2,936
    edited 2011-02-15 20:07
    It's not declared in any of Parallax's documentation, but as "clarification" the Propstick USB ties the center pad to VSS. It's a pretty safe bet to tie the center pad of a QFN to ground when it's use is undeclared. Of every QFN I have seen on all product at work, I have not seen one tied to anything other than ground. I tied it to ground on my M44D40+ module as well.
  • Tracy AllenTracy Allen Posts: 6,667
    edited 2011-02-16 10:32
    There are several good app notes available about how to solder QFNs.
    -- Texas Instruments: sloa122.pdf
    -- Freescale: an1902.pdf
    -- Intersil: tb389.pdf

    They do not say much about the electrical connection to the exposed thermal pad. It is more about thermal performance, which would of course be most important in situations where you expect to run the chip hot. In those cases, the recommendation is that 0.3mm (12 mil) vias on 1mm centers be placed through from the thermal pad to a large thermal (ground) plane. There are specific recommendations on how to implement the vias, the solder mask and the stencil pattern.

    I don't run the chip hot, so I don't place any such vias, which simplifies the masks. But I do attach the pad to ground, and I have a matching pad on the bottom of the circuit board attached to Vdd, set off from a large ground plane. The idea of that is to make a nice high frequency bypass capacitor right under the chip. It would actually be easier to attach the thermal pad to Vdd, and have an unbroken ground plane on the bottom layer. Also 0402 0.1µF bypass capacitors beside the power supply pins.
  • LeonLeon Posts: 7,620
    edited 2011-02-16 12:53
    The stencil should have a grid pattern, otherwise the chip can float during reflow soldering.
  • RaymanRayman Posts: 15,000
    edited 2011-02-16 12:59
    I'd guess it's not connected too. And, the only hot Prop I've seen is a dead one...
    So, I bet it could be left floating...
  • Dr. MarioDr. Mario Posts: 331
    edited 2011-02-20 19:41
    I usually prefer to ground it (there are billions favor of QFN chips that asks for it to be grounded - unless explicitly asked to be tied to Vdd) so it's the habit I made to tie it to ground - ground plane's already huge so why not...

    About the datasheet - I usually check with digital multimeter if I am not 100% comfortable about the lack of information. If not connected at all, I just ground it. That's it.
  • RaymanRayman Posts: 15,000
    edited 2011-02-21 04:29
    Grounding it has the additional advantage that you can just run the ground leads to the center pad...
  • SapiehaSapieha Posts: 2,964
    edited 2011-02-21 06:25
    Hi Rayman.

    It even give better cooling to IC,


    Rayman wrote: »
    Grounding it has the additional advantage that you can just run the ground leads to the center pad...
  • Dr. MarioDr. Mario Posts: 331
    edited 2011-02-21 15:12
    Yeah. That was one of the reason it's a habit (at least for me) to ground the center pad, whether it's GND or No Connect. Very few QFN chips are a exception, though - they want to be tied to Vss.
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