Fiducials
Fiducials are used by automatic printing machines that have a video camera
or cameras, which look at the fiducials on the ECB and those on the stencil.
The computer captures these images and determines how to adjust the stencil
to assure that it perfectly lines up with the ECB.
On the ECB the fiducial is typically a round pad of about 40 mil with no
other metal nearby. On the stencil you need a high contrast feature of about
the same size and shape. With etched stencils you often end up with an
aperture that has the cross-section of an hourglass and the ridge in the
middle works fine to hold the epoxy in place. Or the aperture was only half
etched and filled with epoxy.
The epoxy recommended by some printer manufacturers seems to shrink when it
sets up and the result is that it is not very well attached to the aperture
wall. Under a microscope with light behind the stencil that fiducial looks
like an eclipsed sun, a dark dot with a bright edge. Some alternate epoxies
have more appropriate properties and do stick well to the metal.
With laser-cut stencils the choices to produce a fiducial are cutting it
through the metal and filling it with epoxy (similar potential problems as
with chemically made stencils) or engraving it in the metal with the laser
beam. The engraving method causes a "burnt" spot in the metal which provides
the high contrast required by the camera. This has proven to be quite
durable.
Whether or not the fiducial can be detected by the vision system depends to
a large degree on the camera, its adjustments and the illumination. Tests
have shown that by changing the incident angle of the light a seemingly
perfectly black fiducial can be made invisible to the camera.
Most present day printers have a camera system that looks down to the board
and up to the stencil. That means the fiducial has to be visible from the
contact side of the stencil - to engrave this fiducial fits easily in the
laser-cutting process. Some printers still use dual camera systems where one
camera looks down on the board and the other looks down on the stencil. For
these it is necessary to have the fiducial on the squeegee side of the
stencil - a little extra work when engraving it with a laser system.

More questions, clarifications??

Regards, Ahne Oosterhof,
        A-Laser, Inc
        (503)641-9428

-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of Matthew Lamkin
Sent:   Thursday, September 23, 1999 1:32 AM
To:     [log in to unmask]
Subject:        Re: [TN] Fiducials

Can someone explain to me why this is done please?
Is it something to do with optical recognition on automatic pasting
machines or something?
(We still only have hand operated pasting).


>-----Original Message-----
>From:  Doug Black [SMTP:[log in to unmask]]
>Sent:  Thursday, September 23, 1999 9:06 AM
>To:    [log in to unmask]
>Subject:       Re: [TN] Fiducials
>
>We currently half cut our fiducials with our Laser's and gain a black
enough
>contrast that there is no need for these to be filled with black adhesive
>after cutting. They stay black through all cleaning process's.
>
>Doug Black
>Technical  Manager
>Dek Precsion Screens Division
>Weymouth,England
>Tel: (01305) 760909
>[log in to unmask]
> ----------
>From: Taheri, Kamran
>To: [log in to unmask]
>Subject: [TN] Fiducials
>Date: 22 September 1999 21:28
>
>Is there any other methods of putting fiducials on the stencil? Our vender
>Laser cuts the fiducials half the thickness of the stencil then fills it
>with black adhesive. After a while the black adhesive starts to deteriorate
>and my phone rings of the hook. I usually get good results filling the
>fiducials on the thicker stencils with Loctite 410 black adhesive. But not
>much luck when dealing with 5 or 6 mils thick stencils. Any comments
>appreciated.
>
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