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I have my soldermask expansion set to "rule" which is .102mm by default. Is it okay to reduce this value to make sure that the pins don't accidentally short during production? enter image description here

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You can also consider reducing pad width. Assuming this is a FFC/FPC connector for example, IPC for example recommends pad width +0.01/-0.04mm to lead width. Using 0.23mm pad width or so, with 0.5mm pitch, is pretty effective in my experience.

Also don't forget to set rounded corners, so the soldermask expansion follows shape. Notice the square corners on square shaped pads, they don't actually expand the outline, they just put in the same shape, blown up!

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Refer to the 'capabilities' of your PCB supplier. It will vary with their capabilities and probably will be greater if the copper thickness is greater than 1oz. 0.102mm is 4 mils which is at the limit for normal (as opposed to premium) processing for some short-turn/proto makers and 1 oz copper.

In some cases you may have to abandon the solder mask between the pins to get it to pass DRC check.

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