Most soldermask is photoimageable. You apply it to all areas of the board 
and then remove it where you don't want it... solderable pads and features, 
connectors etc.

In my opinion,
1) since it is applied everywhere, it is technically cheaper to leave as 
much on as possible... saves minor amounts of developer chemistry.
2) to get fancy, determining the minimum locations soldermask is required 
and creating the soldermask artworks, would require greater cad time than 
just blowing it off all surface mount pads, ICT test points, fiducials, and 
component through-holes.
3) soldermask is prettier than raw laminate. To put it some places and not 
others creates an inconsistent-looking product, cosmetically inferior.

At 05:58 AM 2/6/03, you wrote:
>I have a simple question but never got an answer.
>
>Why is solder mask on the complete board, not just on the traces?
>Because of:
>Screen printing
>Tradition
>??
>
>
>Any Input?
>
>Siggi
>----------------------------------------------------------------------------
>-
>Kind regards / Mit freundlichen Grüßen
>
>Dr. Siegmund Zweigart
>New Technology Manager
>
>Solectron GmbH
>Solectronstrasse 2              Fon. ++49 7032 998 194
>D - 71083 Herrenberg         Fax  ++49 7032 998 222
>
>e-mail: [log in to unmask] <mailto:[log in to unmask]>
>internet: www.solectron.com <http://www.solectron.com>
>----------------------------------------------------------------------------
>-
>
>         ----------
>         From:  Martyn Gaudion [SMTP:[log in to unmask]]
>         Sent:  Donnerstag, 6. Februar 2003 14:21
>         To:  [log in to unmask]
>         Subject:  Re: [TN] Here is a Primer on PWB fab
>
>         Dear Technetters,
>
>         After Jim's question on a PCB fabrication primer, I was
>         swamped with requests for Polar's introductory
>         presentation - which goes through the basic steps
>         involved in building a 6 layer board.. So if you
>         would like a copy  (your customers in the PCB assembly
>         industry may like to see it )... you can find the
>         presentation as AP166 in the What's New section
>         on this page:
>
>         www.polarinstruments.com/support/cits/cits_index.html
>
>         Kind regards
>         Martyn Gaudion
>         www.polarinstruments.com
>
>
>         At 17:54 05/02/2003, you wrote:
>         >Anyone know of a good primer (for a student intern) on how PWBs are
>built;
>         >say a web based one from a board show so that she can understand
>how PWBs
>         >are buitl?
>         >
>         >One on assembly/soldering would be good too.
>         >
>         >Jim
>         >
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