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October 2002

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Subject:
From:
"Barmuta, Mike" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 22 Oct 2002 12:56:59 -0700
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Bill:
1. Since this board has been made for over 10 years, did you change
fabricators? If so the new fab shop obviously was not aware of your SMOBC
desire. Although a good board supplier should have reviewed your drawing and
questioned you as to what style of fab and final finish you wanted. Never
assume what someone's default process is.

2. From your description this sounds like an electroplated fused Sn/Pb board
with soldermask. At least I hope it's fused. If the fab shop is set up for
this process it's cheaper to build than an SMOBC board. Did somebody try to
get a lower cost on this board?

3. Soldermask over fused Sn/Pb is not necessarily a bad thing. If done
properly it should have better solderability and shelf life than HASL. The
issue of wrinkling mask in the wavesolder process could be a problem.
However it depends on the board design. If you don't have large features or
groundplanes on the bottom wavesolder side of the board you should not see
much, if any loss of mask.




Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076



-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Tuesday, October 22, 2002 12:20 PM
To: [log in to unmask]
Subject: Re: [TN] Solder UNDER Soldermask Accept/Reject Criteria?


I guess I should clarify also that this is a board that has been made for
years. Without the soldermask coming in over solder. The board has been
around more than 10 years and this was a shock to the manufacturing folks
when it happened... we just don't see this sort of thing happen anymore....
I think the last time I saw this was in 1985.  We all learned to call out
SMOBC to get the vendors to avoid making the boards with solder under the
soldermask. In fact, it took some convincing of some engineers that the
board would be fine without a solder coat over the entire copper surface,
and that they only needed the solder where the component lead was...
I'm sure there are some of you guys that are my age (46) that remember this
stuff... its just so surprising to see it in this day and age... :)


Bill Brooks



-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Tuesday, October 22, 2002 12:08 PM
To: [log in to unmask]
Subject: Re: [TN] Solder UNDER Soldermask Accept/Reject Criteria?


The Print did not specify SMOBC. But it also didn't specify SOS (Soldermask
over solder) either! The implication is that the order of the notes IMPLIES
that we want soldermask after the solder coating process.  Don't all PCB
vendors know that the solder under the soldermask will reflow, wrinkle and
damage the soldermask in assembly?  If we have soldermask on a thru hole
technology board.... we are going to wave solder the board. Why would a
vendor interpret the drawing in such a way as to compromise the performance
of the board? I can't believe this is still happening in 2002. I thought we
learned our lesson back in 1980.....the first time I ever saw anyone DO
that...

 Doesn't IPC have a spec that tells us not to do it that way?

Bill Brooks



-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Tuesday, October 22, 2002 11:44 AM
To: [log in to unmask]
Subject: Re: [TN] Solder UNDER Soldermask Accept/Reject Criteria?


Hi Bill!

Was that called out on the fab drawing? I've ran across a few boards here
that still call that out on the drawing, and TRY and get that changed before
the boards are fabbed...most of the time I've been lucky, but there's been a
few that slipped by me.

Bottomline, if it's called out on the drawing, you don't have much
recourse...

-Steve Gregory-




Anyone aware of a spec that sets the accept/reject criteria for boards that
come in with solder UNDER the soldermask? Is this actually done as a
legitimate practice? WHY would a vendor make the boards this way? Shouldn't
we be making all boards to be SMOBC when there is a soldermask?

Bill Brooks




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