Rob,
I would be similarly confused if your vendor was actually printing solder paste
onto the
fiducials during assembly.  Here's my view of how fiducials should be used
during
assembly.  Fiducials are features that are etched onto the board during the
same
process that creates the SMT features where you are placing components.
These will
allow you to compensate for any individual dimensional error when printing
paste or
placing components by adjusting the offset from "virgin" cad data on a board
by
board basis.  If you are using a screen printer that has a stencil alignment
system,
you should half etch the fiducials into the solder paste stencil, so that you can
align the stencil to the board.  Here's where I see a problem starting with the
scenario you describe.  If you print solder paste onto the global fiducials, your
pick and place equipment will be aligning to the error of your solder paste
printing operation, NOT to the error of your board fab process.
Sorry for the long winded response, that didn't directly answer your questions,
but I don't think the solder paste should be printed onto the fids in the first
place!

Hal Winslow
Sr Mfg Engineer
SCI Systems
Hooksett, NH

    HI,
        WE HAVE AN ISSUE HERE THAT NEEDS SOME CLARIFICATION.  WE'VE BEEN
     BUILDING FIDUCIALS FOR 4 YEARS NOW THAT HAVE NOT HAD SOLDER PASTE
ON
     THEM (NO RELIEFS IN THE PASTE MASK).  NOW OUR VENDORS, (YES THE
SAME
     ONES WHO HAVE BEEN BUILDING THE BOARDS FOR 4 YEARS), ARE SAYING
THAT
     THEY'VE ALWAYS PUT SOLDER PASTE ON THE FIDUCIALS.  THE FIDUCIALS IN
     QUESTION ARE GLOBAL BOARD FIDUCIALS.
        MY QUESTIONS ARE;
     1. WON'T THE SOLDER PASTE, (PRE-REFLOW), LACK
CONTRAST/REFLECTIVITY?
     2. WHAT PURPOSE DOES SOLDER PASTE SERVE ON THE FIDUCIAL?


                                        THANKS,
                                                ROB


     ROBERT BUTTERWORTH
     CAD DESIGN ENGINEER<-----WHAT THE HELL IS THIS?
     ADC BROADBAND COMMUNICATIONS
     11 FAIRFIELD BLVD
     WALLINGFORD, CT 06492

     203-630-5755

     [log in to unmask]

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