That's a really good question, I have asked the same question of my customers for years.
In my career I have built a lot of boards without mask , ( pads only outerlayers ) , and a lot of boards with mask only on the traces . (There are a hell of a lot of land mines and fuze boards out there with only mask on the traces.)
Yes it can be problematic, however if you have a good supplier , it is no problem. Since a large number of soldermask failures occur at the knee of the circuit , then having mask on the whole board or just covering the traces won't make much of a difference. It has become among other things esthetically pleasing .
The reason we used mask over the board was to give the board better electricals , and hide all the issues with the laminate, It was hard to process the early materials without losing or damaging the "butter coat". Anyone remember the call for black mask to hide pink ring ? .Or the massive measles problems that dark green mask covered so well!
The mask is green because of the green color of the laminate so long ago.( or SO I was told by a learned chap involved in the formulation of PC401).There are also rumors persisting that the early makers of mask liked green because it reminded them of all the money they made every time they saw a board was coated.
Also back then it was only screened on ,with a stencil , there were no Photoimagable systems, no aqueous developable systems. thus you only put mask where you wanted it .Not without,it's own set of issues ,most notorious was board to board registration .
From a purely technical point Green is the worst color to choose for a photoimageble solder mask. Think about it green is made from yellow and blue, and you use yellow to block UV light , thus blocking the light needed to polymerize the mask. Blue or red based colors are a much better choice. With mentioning is that the second largest source of Halogen in a mask ,behind bromine in the resin is the yellow pigment. A hint at why mask suppliers halogen free mask was blue , or blue green. most have since found out what we knew that there are halogen free yellow pigments )
Technology has come a long way , and with the flood screened systems in use today , to screen only the circuits is not economically sound , since you either have to prepare a stencil for each part number and side, or was off a lot of mask . Either choice has it's problems.
A better question to ask is why do we still use so much legend, when hand assembly of the boards is a rarity , and the troubleshooting of the boards, (when it occurs) can be automated to the point of locating the device , or at the least highlighting it on a viewer. And we still use thermal legend inks and make stencils , and have bleed , and all the other issues.
Russell Smith
Vantico Inc.
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Thursday, February 06, 2003 4:54 PM
To: [log in to unmask]
Subject: Re: [TN] Soldermask
Good question, Siggi - no answer! (Sorry, I'm feeling facetious this
morning).
I would guess that it's a question of adhesion of the mask to the board. If
only the traces are masked then there would be miles of 'edges' where the
mask ends and the board surface resumes - edges that can be damaged,
delaminated, etc.
Or it may just be tradition, or aesthetics.
Peter
"Zweigart, Siegmund" <[log in to unmask]> 06/02/2003 09:58 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to "Zweigart,
Siegmund"
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Soldermask
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
>
>---------------------------------------------------
>Technet Mail List provided as a free service by IPC using LISTSERV
1.8e
>To unsubscribe, send a message to [log in to unmask] with following
text in
>the BODY (NOT the subject field): SIGNOFF Technet
>To temporarily halt or (re-start) delivery of Technet send e-mail
to
>[log in to unmask]: SET Technet NOMAIL or (MAIL)
>To receive ONE mailing per day of all the posts: send e-mail to
>[log in to unmask]: SET Technet Digest
>Search the archives of previous posts at:
http://listserv.ipc.org/archives
>Please visit IPC web site http://www.ipc.org/html/forum.htm for
additional
>information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700
>ext.5315
>-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV
1.8e
To unsubscribe, send a message to [log in to unmask] with following
text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail
to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for
additional
information, or contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
-----------------------------------------------------
[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
person. Thank you.]
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------
|
