Thankyou, I was just a bit unsure as to how they are used on paste
screens.
It has been suggested before that we have our P&P fiducials cut out of
our screens to aid hand alignment
but we've not tried it yet.


>-----Original Message-----
>From:  Ahne Oosterhof [SMTP:[log in to unmask]]
>Sent:  Thursday, September 23, 1999 10:17 PM
>To:    Matthew Lamkin; 'TechNet E-Mail Forum.'
>Subject:       RE: [TN] Fiducials
>
>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).
>
>

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################