It really goes well beyond that as far as the fabber is concerned.

     Need a little more info to be able to accurately address, but here are
     some factors to consider:
     Width of lead attach pad = Remaining space between pads.

     This is critical, since most LPI soldermasks must have between 3 to 6
     mils in order to maintain a "web" or "dam".

     10 to 12 mil pads means a space between of 8 to 10. You should get a
     web in all cases.

     13 and above means, depending on the mask used by the fab house, they
     may or may not be able to maintain the "web".

     Remember, in order to "guarantee no mask will be on the solder pad, 5
     mil minimum clearance over the pad is required.

     10 pad + 5 clearance = 15 mask opening = 4or5 web (depending on
     whether the pitch is 19 or 20) :)


______________________________ Reply Separator _________________________________
Subject: Re: [TN] Soldermask web at fine pitch components leads
Author:  Matthias Mansfeld <[log in to unmask]> at SMTPLink-Hadco
Date:    7/24/98 3:15 AM


There are two aspects: what needs the fab and what needs the assembly
house.
> I have a question regarding soldermask web surrounding fine pitch components
> pads/leads. In our current fabs, we are finding that not all 20 mil pitch
> components leads had soldermask web so we have decided to provide soldermask
> layer with 1:1 opening for pad, without any clearance.
> I am expecting that fab house will provide some solder mask clearance to
> prevent solder mask running over the pads along with soldermask web
> surrounding fine pitch leads.
The assembly house should specify whether they need soldermask
between the 20 mil pitch pads to avoid solder bridges or not (depends
on their printing, placing, and soldering procedures)
Then you could provide soldermask data 1:1 and the fab should be
able to oversize the data depending on their soldermask
process and its tolerances to achieve exactly that oversize in
soldermask your assembly house specified.
Regards
Matthias Mansfeld
-----------------------------------------------
Matthias Mansfeld Elektronik
* Printed Circuit Board Design and Assembly
Am Langhoelzl 11, D-85540 Haar, GERMANY
Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
E-Mail: [log in to unmask]
Internet: http://home.t-online.de/home/matthias.mansfeld

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