On Tue, May 11, 2004 at 12:42:17PM -0700, David Koski wrote:
Also, I have only done prototypes without solder masks so far. What
is the
"right" amount of mask? I noticed that even library footprints vary
as can be
seen in the file referenced above.
There isn't really a "right" amount. A conservative rule would be to
have a 5 mil
soldermask relief around the pads, so for example a 20 x 20 mil square
pad would
have a (5 + 20 + 5) x (5 + 20 + 5) square soldermask opening. Another
rule
would be to ensure at least 5 mils of soldermask (not the relief, but
where the
soldermask is) between pads. Now, clearly on a fine pitch part with a
20 mil
pin pitch, you can't have 15 of those 20 mils taken up with soldermask
opening
and soldermask. In those cases, I've used a 5 mil clearance and
removed the
(now less than 5 mils) of any remaining soldermask between pads.
Dan has a good point here. It depends how your part is being used.