| We are using .040" diameter fiducials (global and local) on our boards. They are solder covered and HASL. I get batches of boards where the p&p vision system has trouble reading the fiducial. The fiducial looks dark on the screen. The board vendor says it is the uneven solder plating. My questions are: | | Does anybody else have this problem? | Does anybody use bare copper fiducials? If so, how has that worked out as far as contrast against the laminate and potential oxidation of the copper surface?
| Would using a larger diameter circle ensure flatter plating (although I think this is going to be directly related to how well the fabricator controls his HASL process)? | | Thanks for all inputs. | | Bob | Bob,
I have went through this same problem. First off I would not suggest copper fiducials unless they are treated with an OSP (Organic Solder Preservative). If you use bare copper fidicials they will oxidize. Bare copper fiducials will also cost you more from your PCB vendor. Fiducials will need to be masked before HASL. When I started with the company I am presently working for they were using bare copper fiducials. Operators would pretreat every fiducial with a tarnish remover before pick and place operation.
Making the fiducials bigger will allow for a thinner deposit of solder on the fiducial. You should determine what type of Hot Air Leveler your PCB vendor is using. If your ckt brd is double sided and you have a fiducial recogition problem on one side only, then your PCB vendor is using a Horizontal Hot Air Leveler. The air knives, which blow the solder off the PCB, thinning the solder on the PCB, are above and below the PCB. The bottom of the PCB, or the side of the PCB closest to the bottom lower air knife on the hot air leveler, will always have more solder than the top of the PCB. This is due to gravity. Never the less, your PCB vendor should be able to control the solder volume to eliminate your vision problem.
I assume you know the good old erasure trick. This method has been used for years as a temporary fix to infrequent fiducial vision problems. Just rub a pencil erasure on the problem fiducial. This will take the shine off fiducial.
Thanks,
Chris
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