Cyrus Ringle wrote:
> David Cary wrote about local fiducials:
> This doesn't interfere with most routing, but isn't visible after parts are
> placed, so it's impossible to check placement automatically.
>
> Just curious what placement you are checking for? We place them under components
> because the Pick-n-place machines need them to locate the position to place a
> fine pitch component. After it is placed visual inspection is done with
> machines to confirm the correct parts have been placed by scanning for component
> markings with color imaging vision equipment. This helps to also determine if
> parts are really skewed but the local area fiducials are never used after that
> to determine fine pitch component locations. I am a designer and am familiar
> with our vendors processes and this is what I understand them to be but hay I
> could be wrong...
>
> Cyrus Ringle CID
> IPC Certified Interconnect Designer
> CAD Engineer
> Inter-Tel, Inc. http://www.inter-tel.com
> 7300 W. Boston St.
> Chandler, AZ 85226
> (480) 961-2263
> mailto:[log in to unmask]
> President of the Greater Phoenix Designers Council
>
> David Cary <[log in to unmask]> on 01/26/2001 01:05:48 PM
>
> Please respond to "DesignerCouncil E-Mail Forum." <[log in to unmask]>;
> Please respond to David Cary <[log in to unmask]>
>
> To: [log in to unmask]
> cc: (bcc: Cyrus Ringle/Inter-Tel)
>
> Subject: Re: [DC] DC IPC Design rules for Fiducial clearances
>
> Dear designers,
>
> All the information one could want about "global" fiducials are in
>
> The SMEMA Fiducial Mark Standard 3.1
> http://www.ipc.org/html/smema3.1.pdf
> http://www.smema.org/
>
> Protel fiducials
> http://www.protel.com/earticles/maskexpan.htm
>
> Hollomon p. 291 recommends a "BMW-style" fiducial over the simple round dot, but
> notes that some vision systems will not recognize anything but the round dot.
> I put one of these "bow-tie" fiducials on my board, but I'm pretty sure no one
> uses it for alignment purposes -- we just use it to check for over-etch or
> under-etch.
>
> That SMEMA standard seems a little vague about local fiducials. The SMEMA
> standard mentions putting extra local fiducials centered underneath fine-pitch
> components (less than 0.05 inch pin pitch). This doesn't interfere with most
> routing, but isn't visible after parts are placed, so it's impossible to check
> placement automatically. But I've seen many more boards have a fiducial next to
> the part, near pin 1, and another next to the diagonal opposite corner. They are
> visible after parts are placed, but a little more difficult to route traces
> around them. These boards seem to conflict with the above SMEMA standard -- has
> a improved "best practices" come out ? How old is the above document ? (I'm
> annoyed when documents leave out important metadata like the publication date).
>
> --
> David Cary
>
> Charlene Dwyer <[log in to unmask]> on 2001-01-25 05:01:15 PM wondered:
>
> ...
> Hi! Does anyone know of the standards for board
> fiducials?
> ...
> -Charlene Dwyer Lee
>
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I do not quite agree with your statement that the fiducial does mot interference with most
routing. In our case, almost all our boards are very dense, to have them on, we will have to
detour our routing. To the assembly plant, the presence of fiducial is critical as it ensures
placement accuracy. However, I agree if you have thousands of such boards to assemble and
many of them have high IOs, then it make a difference to have them. As we seldom have more
than 5 of the components >250 IOs on a boards, manual soldering suit us fine. In addition,
why bother with fiducial of our operators can easily placed them on accurately, unless
BGAs. However, I agree that to have them inside the component is more practical than to
have them outside the component perimeter.
Regards - Wee Mei
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