Fiducial Recognition

We only clean ones that are rejected (typically 1 out of 250 or more). Very well could be an issue with the boards supplier if your having a inconsistency in plating/hasl process, under a scope do the fiducials look the same between boards? More likely that your lighting or fiducial acceptance score on the machine could use some tweaking, try adjusting it to produce a better contrast since that all the vision algorithm works on.

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I clean all fids with a fiberglass brush pencil so that I know there will be no rejects (it doesn't take too long). The problem is the machine lighting. My Essemtec machine uses a light ring (red LEDs) around the cam.
Here's one of many documents that may help explain the differences: http://www.graftek.com/pdf/Marketing/MachineVisionLighting.pdf (google "machine vision lighting filetype:pdf")

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I think, each engineer had this situation. Try to change something - pattern of fiducial, finish cover on your PCBs (HASL to flash gold) or solder new LEDs in to your mounter.

On my Mirae's mounters we have two rings of LEDs - red LEDs and blue LEDs - and few times per year I have problems with fiducial recognition, and only if PCB made in my country. If I order PCB in Europe or in Asia, I assemble it without any problems.

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When fiducials on pcba's have poor plating and the vision cant find them repeatably there are a couple of things you can try.
1. Use the WAVI light and turn off (setting of 1) the main bright LED ring on the downward camera. After doing this in teach fiducials...you need to go into extended paramaters and adjust your grey value scale down very low to get a good contrast between the fiducial and board.
2. You can also lower your acceptance tolerance. You can only do this if the fiducials are isolated from anything else the vision can find.

I have only had a few different boards over many years that the machine had a major problem with finding the fiducials. Usually the ones that had the problems had the fiducials surrounded with no color difference between the board and the fiducial.

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We have a Quad QSV-1 and have had some trouble with it reading 0.040" fiducials. Our work is very low volume so we just manually spot the ones it can't read. We do not clean the fiducials. Instead we increased the fiducial size to 0.060" and it has been working far better. I know it is not an overnight solution and you may not have the freedom to modify the artwork if you are a CEM. Perhaps you at least have the freedom to change the panel fiducials.

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I have some suggestions. We have been fighting bad fids for years. Some things we have tried that have helped is:

1) Look at your relief area around your fid. Does it butt up against the solder mask. We allow 20 mil around the fid as a relief area. This helps with the contrast.

2) Is your fid thickness within spec? Check QC and see what the specs are. The board house might be able to put a thinner HASL finish on the fids, which will give you more consistent results under the camera.

3) How do you handle the pcbs when they come in? Are they left wrapped up, or do you unpackage them and put them on the shelf? This is OK for a little while, but the HASL finish will oxidize, which will effect your inspection score (crummy image).

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We clean a good number of fiducials, mostly where HASL PCBs are used. Since the pad is not exactly flat, our SONY 1500 MKII reflects the light from what is essentially, a small 'mound'...

Many times our operators use an eraser to 'dull' the finish. However, tweaking the recognition algorithm always manages to set things right... Gold Flash PCBs have absolutely no problem at all...

Still get the odd one on HASL though...

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