Eagle and Aperture Size

Hi All...

I am using Eagle to make some gerber files. One file will be used to make a solder paste stencil. On two of the chips I would like to make the aperture size smaller than the actual pad, because the stencil I have let too much paste through and I got one big solder bridge. Can anyone tell me how to accomplish this? I have been Googling for an hour and still no joy :-(

If it's a critical chip it's best to draw the part yourself in the library editor, turn off the automatically-generated paste opening (Change->Cream->Off) then draw the rectangle (or whatever shape) yourself on the tCream layer for each pad.

Simpler (but not as flexible) is to set this distance in the DRC (DRC->Masks->Cream text fields). Setting the minimum to 2mil, for example, will make tCream openings smaller by 2mil on each side of the pad than the pad itself.

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Wow, that was fast, thank you!

if I go the DRC route, can I apply the change to just the parts of interest or will it be a global change? Sorry if that's a dumb question. Eagle is not my best skill :-(

Since you seem very knowledgeable, I wonder if you have run accross this paste issue before? How much reduction should I consider? 25% More?

That's why I said "not as flexible" because the DRC route does indeed affect all parts on the board (except for ones that have the cream layer off when they were drawn with the library editor). So, global change.

The paste issue is a complex one. It depends on the thickness of your stencil and to some degree the paste itself. Are you using stainless steel laser stencils? Or mylar? Are you using a paste designed for stencil printing?

If you Google "stencil aperture ratio" you will get more reading material than you'll know what to do with.

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Okay thanks again. Apparently this is not going to be as easy a change as I had hoped. The paste was fine for all the parts except two chips, and even then, one of them I could live with. So maybe I need to do some drawing. Alternatively I could edit the GTP file that gets produced.

The stencil is made from kapton and is .0035 inches. Its an Ohararp stencil. I bought some kestor Easy 256 paste, but the paste I used was the stuff sold by SparkFun, which I am sure is not as good a quality. I thought I would use it up on prototype boards before I opened the good stuff.

Sometimes it's a good idea to thin out the cheap solder paste a bit, using 99% alcohol. Especially when using the plastic stencils. You want it easily spreadable, but not so runny that it squeezes under the stencil a lot. Aim for a consistency close to toothpaste.

Hm, its already pretty thin.