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QFN 0.4mm Pitch

jatins

Hi 

I am designing a 0.4mm pitch 32 pin QFN Package(.dra). So far i have used only 0.5mm pitch packages.

I read that for these fine pitch packages solder mask needs to be designed which looks like this on the right:

 

So question is: this large solder mask opening should be defined in the .pad file or

Should it be done in the .dra file and if yes, what is the common procedure for this 

Thanks 

  • The fabricator can cause this as part of their CAM process.  Swell the mask larger than the pad, then trim the mask between pads because what's left won't be wide enough to avoid flaking off.   Mask is usually epoxy, so you end up contributing FOD (foreign object debris) which we don't want.  I believe there is a DFM DRC for mask slivers built into Orcad and Allegro.

    I would add the mask void to the footprint because swelling the mask in the padstack will have to be wide enough to cover the gap between pads, but it will extend beyond the first and last pad, which will look weird.

    I believe you want to include a pour void to protect the size of those pads relative to the paste artwork.  I think you want to make sure solder paste doesn't escape during reflow from under the pin.  (avoids intermittent open joints, like collapsed balls in a BGA).

    Fabricators have problems where the mask artwork may not be perfectly the same size as the board after outer etch, so they swell pads to prevent rejects from mask on pads.  Humidity causes problems.

  • in reply to RFinley

    Thanks.

    OK I got the point that it is practical to add solder mask in the footprint. (Maybe Board Geometry> Solder Mask on all four sides would do) and include it in Solder Mask artwork.

    My other choice is to define Solder Mask individually for the pads i.e. with Pad size of 0.2x0.7mm having Solder Mask Size 0.22X0.72mm

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