reduced paste mask size for bottom pads of large parts

[fieldeffect]

I am using a microcontroller (Cyan eCOG1X) in a QFN68 package that has a large ground pad underneath the chip. Datasheet advises to limit the solder paste applied to max 30% of the pad size. Otherwise solder will flow from under the chip and create short circuits between the pads.

When I create a shape for QFN68 with bottom pad (first creating a custom pad shape), I'm not able to specify the size of the paste mask separately from the size of the pad. You can only turn paste on or off.

Does anyone know a way to fix this in the design? Or do I have to edit the gerbers manually... 

Thanks

Hans 

[hgregg]

You could turn the paste off for that pad then draw your own. To draw the mask as you want it to be, make sure the paste mask layer is the active layer and then use the shape tools to draw the shape.

Hg

[fieldeffect]

Thanks Hg

It works fine: If I uncheck the paste mask for the pad and draw a rectangle in the paste top layer this paste mask will also switch layers when the part is moved to the bottom layer.

Unfortunately this does not seem to apply to keep-out zones that are part of the shape (this part has corners connected to ground that should be kept free): when you specify these for only the top layer they will remain in the top layer while the part is placed on the bottom side. I'm considering to use extra (not connected) pads covered with solder mask in stead of keep-out zones.

Hans 

[hgregg]

That's an interesting idea. I've also had this issue with keep in/keep out areas and I used the in-place edit to change the keep in/keep out layers as needed. This would be a pain on a large project however and I can see the benefit of using extra pads for the same effect.

Hg