Aperture swelling necessary?

Hi All…

I am building a board with a 100-TQFP part (an Atmet ATMega 640). It has a pitch of 0.50mm and the width of a leg is as low as 0.17mm to 0.27mm. People have suggested I do a negative swell to reduce the size of the stencil aperture, so the paste does not so easily spread to the adjacent pin. Also, I realize that a thinner stencil is probably preferable in this case.

Is there a rule of thumb about how much to swell the openings? Too narrow, and I don’t get good paste laydown. Too little, the paste spills over. I have never worked with a component this small before.

Also, i have not been able to figure out how to get Eagle cad to do this swelling. I have tried the recommended method of modifying the DRC but it didn’t work. Short of hand editing the gerber file, how is this done?



I cannot help you on the Eagle aspect of it since we do not use it here. However, I think you should not think of the paste mask expansion as separate from the rest of the pad. Usually, I see around 0.3mm for the pad width, and the stencil aperture size is the same.

By the way, it’s not as if the stencils we cut will come out exactly to what you specify. 0.5mm is right at the limit of what we can do in the sense that if you have approximately the 0.3mm aperture, what we will cut will have holes for each pad and a bit of webbing between each pad. The details of what you do with the pad designs matter more when you are talking about optimizing yields in big production runs. If you’re just doing this yourself with a heat gun or toaster, it won’t matter that much; if you’re having some expensive board done by a contract assembler, you might talk to them about footprint recommendations for their process.

- Jan