You need to think first about what type of solder mask you are
specifying. Screen printing a two part epoxy mask needs a solder mask
opening of about .12mm (5 mil) while a liquid photoimagable will need an
opening of 0.05 to 0.075mm (2 to 3 mils). You need to talk to your board
fab about what their production tolerance is. Be aware that's production
tolerance - if you're making hundreds of thousands of boards it makes a
big difference from making a couple of thousand.

Of course the big issue is the track to pad spacing. On a 5 mil
track/space board, you can't have a 5 mil trace to pad space. That 5 mil
solder mask opening around the pad is the nominal space - it's possible
that the opening will range from zero to 10 mils depending the on the
solder mask to copper features registration. If your fab can hold a 2
mil solder mask opening tolerance (common these days with LPI) then you
can use a 5 mil trace to pad clearance - the mask opening should never
be greater than 4 mils and so your trace will never be exposed.


Andy K.
Sychip Inc
Office +61 2 9456 7984

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Fred
Dark
Sent: Wednesday, October 27, 2004 12:21 AM
To: [log in to unmask]
Subject: [DC] Recommended oversize and tol. reg.

Dear DesignerCouncil; I would like to know on a design with 5mil spacing
what is the recommended oversize and tol. registration for soldermask
with 5mil spacing... Pad to trace, etc. We are about to place a big
order on a board with a 5mil spacing, but we want to make sure we are
within "DFM" spec's... Please respond A.S.A.P... Thank you...

Regard's, Fred Dark Jr.

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