ATMega QFN-28 pattern

[ion]

I've been making a pattern for the ATMega 28 pin QFN package from the datasheet and came across a dimension I can't make sense of.

Page 20 from this pdf:

http://www.farnell.com/datasheets/1130840.pdf

Can anyone tell me what the "0.4 Ref" dimension cutting across the corner pins is?  0.4mm?  0.4 multiplied by some reference value (perhaps dimension C?)?
Those were my guesses, and they give significantly different results, so any clarification would be appreciated.


On a related note, does anyone know if it is possible to subtract one shape from another in the Diptrace pattern editor?  Making that notch on the centre pad was more trouble than it should have been.

[rx8pilot]

That is not the pad layout, it is the physical dimensions of the device. If you make the pads to those dimension, you will likely struggle until your last days to get it soldered. I created my own layout in in Eagle for this device, not sure why they do not hat a recommended pad reference in the data sheet. Maybe it is a separate document.

[Farley]

You might try searching google for "28 pin QFN footprint" and see what turns up. My search yielded this:
http://cds.linear.com/docs/en/packaging/QFN_28_05-08-1721.pdf
as the first hit.

The short answer to your question is that the ".4 Ref 4X" dimension would indicate an uncontrolled dimension (Ref, short for reference) of .4mm that occurs in 4 places on the drawing (4X).

As far as I know DipTrace cannot perform boolean operations on shapes.

[ion]

That is not the pad layout, it is the physical dimensions of the device. If you make the pads to those dimension, you will likely struggle until your last days to get it soldered. I created my own layout in in Eagle for this device, not sure why they do not hat a recommended pad reference in the data sheet. Maybe it is a separate document.

Thanks for pointing that out!  Sort of noticed it wasn't a normal footprint since it was mirrored, but didn't think of the consequences of using those dimensions for the pattern.

Strangely, even the full 650-page datasheet lacks layout information.

I did find a separate document eventually, but it's almost as if Atmel don't want you to find it - must have been searching for over an hour.

It seems to contain the patterns for quite a few of Atmel's packages, so here's the link in case someone else finds it useful:


http://www.atmel.com/images/atmel-8826-seeprom-pcb-mounting-guidelines-surface-mount-packages-applicationnote.pdf

[ion]

You might try searching google for "28 pin QFN footprint" and see what turns up. My search yielded this:
http://cds.linear.com/docs/en/packaging/QFN_28_05-08-1721.pdf
as the first hit.

The short answer to your question is that the ".4 Ref 4X" dimension would indicate an uncontrolled dimension (Ref, short for reference) of .4mm that occurs in 4 places on the drawing (4X).

I did come across that link fairly easily, but the pin pitch and the pad width are both too small.

Thanks for explaining the ".4 Ref 4X"

As far as I know DipTrace cannot perform boolean operations on shapes.

Thanks, I was wondering if I missed something after I got fed up of fine tuning the polygon.

[Araho]

Atmel generally refer you to the JEDEC standards for footprints of their packages. All TQFP, QFN etc I've seen from them these last few years simple have a physical dimension-drawing in the datasheet, and 1 line about which JEDEC standard you should search for. Check out http://www.jedec.org/committees/jc-1110-0.

Membership to find the standards are free. Their website is somewhat crappy when looking for the correct document, but it's all there.

I also think that when they actually spend time developing a standard for footprint, they could give us all the useful dimensions. Making us do the mini-math to for example give us a center-to-center of pad instead of edge-to-edge creates more room for error.

But hey, at least it's a standard

[zapta]

Try this

http://www.atmel.com/images/atmel-8826-seeprom-pcb-mounting-guidelines-surface-mount-packages-applicationnote.pdf