IPC-A-600 2.9.3 allows (unless otherwise specified) for encroachment by solder mask onto the pad of (roughly) up to 1 mil for <40 mil pitch, up to 2 mil for > 40 mil pitch.  That's for Class I, II ,and III.  Point being, reasonable is in the eye of the beholder.

We found that our fab couldn't hold to a 12 mil pad and still get the web to stick...they have to go to 10 mil pads with 2 mil relief (6 mil web) on 20 mil pitch if we don't want them ganged.

-----Original Message-----
From: Greg Scott [mailto:[log in to unmask]]
Sent: Friday, June 21, 2002 3:19 PM
To: [log in to unmask]
Subject: Re: [TN] Registration of soldermask


Loan,

What we do is design with 2mil per side opening with a solder mask note:
     No finished solder mask over pad edge.  Annular ring of exposed board material around pad edge to
be .001 min, .003 max.
     Maintain  a solder mask bridge of .004 min. wide between fine pitch solder pads.

Also the pad can not be wider then .012 with this approach.  Do not allow for ganged soldermask
relief.  We had all
rework problems with that approach.
Have not had problems since this note.

Greg Scott
Cray Inc.
Seattle, WA.

"Tempea, Ioan" wrote:

> Hi technos,
>
> what would be a reasonable requirement for the registration of the solder
> mask on boards with 20 mils fine pitch having solder mask dams. We asked for
> 0.002" and it almost comes over the edges of the pads.
>
> Is 0.001" doable, with no impact on the price?
>
> Thanks,
> Ioan
>
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