Choosing Solder Mask Expansion Value

The solder mask expansion value is the size of the gap between a copper pad and the dielectric material that usually coats circuit boards. Thus, the material of the gap is bare fiberglass.

What is the purpose of this gap, and how do I determine the minimum gap size? For a 64-pin chip, I usually set it to something small like

1mil or 2mil since a greater solder mask sliver between the small pads prevents solder bridges.

But, if I am not limited in the size of this gap, how big should I make it? Should the gap increase in size proportional to pad size? The specific footprint that I am working on now is a 700 x 250 mil pad. My first thought was to give it a solder mask expansion of 5 mil, but I would really appreciate a more scientific way of calculating it.

Reply to
Dan D.
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Please specify the package of this 64 pin chip. It could be anything, QFP, QFN, BGA, PLCC, TSOP???

The gap is to allow for tolerance of the placement of the soldermask over the copper. You don't want soldermask on a pad (usually). The tolerance has gotten quite good and you can certainly use 1 or 2 mils. However if you can be happy with a larger opening it does make the board easier to manufacture, thus cheaper. I only use tight tolerances if the footprint demands it. ie there's nothing to be gained specifying molecular accuracy on a DIP 14...

However you need to keep some sane value so the board can be soldered normally.

The second thing to look out for is the minimum web, that's the bit of mask left over between pins. If it's below 3 mils, usually that's not manufacturable and the vendor will remove them. Or they might charge a premium. Depends. You can proably tell your CAD package how to handle soldermasks, it should be able to generate voids between pins if necessary.

It sounds like you might be soldering this manually? No paste stencil?

You need to contact the manufacturer and ask for their capacities. Usually they publish it on their site. Set the gap to whatever they specify is their standard capacity so you don't get dinged in price or lead time. Of course, some parts do require some bizarre soldermask magic so YMMV with my advice.

Don't forget to keep a clear and open comm channel with your board vendor. Specify EVERYTHING in WRITING at least twice, once in your PO and once on your fabrication drawing. (Make sure they are the same instructions)

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Thanks.

Reply to
Dan D.

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