Interesting thread...
Different companies have different needs.
One thing that has been said by manufacturers of bare boards is that the
silk screen and soldermask cause them more trouble than any other part of
the manufacturing process.
A common way to indicate to the board manufacturer that you want them to
quote the extra labor required to composite the silkscreen with the
soldermask and clip the legend anywhere it overlaps the soldermask is to add
a note to your fab drawing indicating that you want the legend clipped and
that you do not want any legend ink to be on any solderable surface.
The note I use looks like this...
23. APPLY COMPONENT LEGEND(S) USING WHITE NON-CONDUCTIVE EPOXY BASED INK
AS REQUIRED BY CAD DATA. LEGEND INK MUST NOT BE PRESENT ON ANY SOLDERABLE
SURFACE OR LAND AREA.
Now that pretty much gives them license to do what ever it takes within
their existing processes to keep the ink off the pads... I don't care HOW
they do it... I only set the criteria for receiving inspection's acceptance
of the boards when they come in.
I'm sure there are other ways to describe what is wanted... maybe someone
else can post their note or 'call out' on the fab drawing that indicates the
same sort of thing in better language.
Matt, not all companies are okay with not having reference designators or
part outlines on their boards... I can name a few benefits from having them,
and usually we can find a way to get them in there if we work at it...
One benefit from having them is it helps the inspectors to verify if the
part orientation is correct, pin one or a notch in the corner of the part
body matching the silkscreen legend is a quick way to visually inspect...
The reference designators help in troubleshooting and rework too... if you
ever had to troubleshoot a dysfunctional board before you would understand
what a time saver it is to have those component identifiers there so you
don't have to constantly refer back to a drawing just to locate the part...
Of course these graphical aids don't add any functionality to the operation
of the circuit electrically and they take up room, room you may need to fit
the parts on... so if you are designing high density circuits like cell
phones, there just isn't any room for all the components and legends and the
first thing you sacrifice is the silkscreen legend... I think that is
understood. But realize you are giving up the benefits to the assembly and
test folks by not having a legend on the board.
And... I don't think 'weasels' is a fair thing to call those of us who are
communicating that we want the board house to clean up any errant reference
designators or text or part bodies that violate the solderable lands that we
may not have caught in check or would have had us delay the prototype build
because we needed to break apart a component and edit it to clear the
silkscreen legend in it that overlapped a pad or solderable land...
I don't think designers are intentionally causing the problems with the
silk-screens... Many of the CAD libraries that ship with CAD tools fail to
address the proper silkscreen construction rules... And, many times you run
out of room to put reference designators near the parts and need to get
creative to make them fit on the board... and not every designer has the
luxury of spending the required time it takes to get the silkscreen perfect
... so I look at adding the note to the drawing as being extra insurance,
that of course you pay for... hopefully it gets fixed before you make
production quantities... That way you don't have to pay the vendor to do the
extra step... but some vendors do this anyway just as good practice... any
ink on the pads will just move if it is printed on HASL (solder) bit it will
really be bad if the surface finish is a non-melting material that the
solderpaste must bridge to make the part adhere to the pad, so it really is
a contaminant and should not be present on the pads at all.
I believe you can avoid most of the issues with legends by following some
simple guidelines when building parts for your CAD library... and also being
careful about where and how you locate the reference designators and text or
graphics on the board. Good discipline and attention to detail makes for
better board designs, and saves your company money every time they have to
build another board that you designed.
Best regards,
Bill Brooks
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California
-----Original Message-----
From: Matthew Lamkin [mailto:[log in to unmask]]
Sent: Wednesday, February 07, 2007 1:05 AM
To: [log in to unmask]
Subject: Re: [DC] Silk Screen Verification
I tend to agree with John here, I do not have silkscreen outlines or names
for 99% of my SMT components.
They are all dealt with by SMT assembly drawings.
IMO there is no reason for most of them to be there, why would a SMT
component need any outlines etc?
It's a machine that is placing them so it does not need to see where they
go, boards are usually too
dense to be able to put component names on for them all & if you cannot get
them all on then there is no point in putting just what can fit as a drawing
of the others will be needed anyway.
Only PTH components that are manually added or are changed during the boards
life need legend, board idents and connector etc do too.
Although legend on pads is really the PCB designers problem/fault, not the
board houses.
If the person that does the board leaves legend on pads & does not check for
it then it's their fault
and no one else's, and putting a disclaimer on a document is just a way of
weaselling out of doing it right in the first way.
You can easily just take the Gerber layer for the SMT side solder resist and
copy that to be a copper scratch layer though.
Matthew Lamkin
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of John
Parsons
Sent: Tuesday, February 06, 2007 11:07 PM
To: [log in to unmask]
Subject: Re: [DC] Silk Screen Verification
Foolproof! Not sure if there is such a thing;
- Eliminate the use of screen printed legend unless you really, really need
it. Most boards are so dense these days that it is next to useless anyway.
- Train your CAD librarian or whomever to design proper footprints/outlines
to begin with.
- If it really is an ongoing issue with your fab shop, find another
- Legend on SMD pads is an IPC violation so if you are spec'ing a build to
IPC requirements then, as with the note suggested below, you have a case for
an RMA but this is the last course of action you would want to take as it
costs everyone involved time and money.
Good luck,
John
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