Bill,
I'm glad your understanding has increased. Many of us have seen wrinkled or
lifted mask caused by reflowed solder. It's not pretty, and can cause other
problems including open solder joints. One thing to keep in mind is that
you are not the fabricator's only customer. They probably have other
customers with requirements for legacy designs with plated/reflowed solder.
It is bad practice to assume that your fabricator will automatically
question a valid specification for reflowed solder under mask.
If you work closely with a fabricator, they will learn your preferences and
know when to question a specification that appears valid when it varies from
your norm. If you shop for low price and don't have significant business
with a single fabricator, they never have the opportunity to learn your
preferences.
Don Vischulis
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Brooks,Bill
Sent: Tuesday, October 22, 2002 5:58 PM
To: [log in to unmask]
Subject: Re: [TN] Solder UNDER Soldermask Accept/Reject Criteria?
Thanks, all of you for shedding light on the notes... Actually, I wanted
your unbiased opinions to bounce my own take on the situation against...
You see, my take was,
1) if the notes are ambiguous... Call me.
2) The notes are ambiguous...
3) they didn't call, they just made them the way they interpreted the notes.
Now, the logic of plating the copper vs. HASL makes sense... I think I can
see how that would have influenced the interpretation. And, of course, I
believe the arbitrator in all of this is the Fab drawing notes.
Our solution to the problem was to rev the board and change the notes.
And... In the interim, add a note to the P.O. indicating our preference for
SMOBC.
I can see now how the shop might have been influenced by the wording of the
notes. On the other hand.... I hope that any shop that sees a design come in
with solder UNDER the soldermask, would question it... The end product is
not a great result to end up with. I wish I had a picture of the boards to
show the wrinkled appearance of the underside of the board after they wave
soldered it. Very ugly... It really should not be an acceptable way to do a
board, in my humble opinion.
Bill Brooks
-----Original Message-----
From: Don Vischulis [mailto:[log in to unmask]]
Sent: Tuesday, October 22, 2002 2:17 PM
To: TechNet E-Mail Forum.; Brooks,Bill
Subject: RE: [TN] Solder UNDER Soldermask Accept/Reject Criteria?
Bill,
You received exactly what you specified -- reflowed (I hope) solder plate
under mask. If your supplier was really on the ball, the exposed copper
might have been questioned because solder plating is done before etch and
masking. SMOBC is a solder coating applied after solder mask. Also SMOBC
normally does not have a thickness specification.
IMO it's an oops for both parties. Your notes resemble the call out for
reflowed solder plate under the mask. Selective solder coating, hot air
leveled solder coating, or SMOBC don't appear in the notes. The fabricator
missed the hint that you wanted with the solder coat exposed copper (BTW
that note could be intrepreted to mean complete solder coverage including
edges (really old requirement) instead of SMOBC.)
Don Vischulis
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Brooks,Bill
Sent: Tuesday, October 22, 2002 3:20 PM
To: [log in to unmask]
Subject: Re: [TN] Solder UNDER Soldermask Accept/Reject Criteria?
I raised the issue in the open forum for a reason. (OBTW ... It's not my
design)
One, I may be able to locate someone else who has had this problem before
and knew of a spec that would enlighten the folks involved in reviewing
this.
Two, I may find that there is no spec and we are at fault for not specifying
SMOBC and that we have been enormously lucky all these years that the vendor
we were using interpreted our preference for assemblies that don't short out
under the soldermask after wave soldering....
I went to the IPC specs looking for guidance on this and found little that
I could interpret to be related to this scenario.
Here are the notes off the actual drawing... Maybe you guys can shed some
light on the results we received... (it was a new vendor)
NOTES: UNLESS OTHERWISE SPECIFIED
1. Material: .062 FR-4 C2/C2 Finished
2. Solder Plate exposed copper .0003 to .0005
3. Hole sizes are after plating, Min. wall .001 thk
4. Finished board to comply with IPC-A-600, Class II
5. Component side shown
6. Solder Mask Both Sides
7. Silkscreen component side with white epoxy ink
8. Tolerances: Front to Back: +/- .005
Hole location: +/- .003
Hole Diameter: +/- .003
That's all there was... I didn't create the notes, I have only been here 2
years, it was created before 1996 by another designer. So any speculation on
why one vendor used SMOBC and another used SOS... ?
- BB
Bill Brooks
-----Original Message-----
From: Barmuta, Mike [mailto:[log in to unmask]]
Sent: Tuesday, October 22, 2002 12:57 PM
To: 'TechNet E-Mail Forum.'; 'Brooks,Bill'
Cc: Barmuta, Mike
Subject: RE: [TN] Solder UNDER Soldermask Accept/Reject Criteria?
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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