Random rotations?

[SteveW]

It's been a long and mostly successful day of placement - many boards, many thousands of parts.
Final board of the day, and the RV1S pulls this - two parts off by 180 degrees. Physically near each other, placed at very different times.

They went down straight, just off by 180 degrees. The other 4 channels of this on the same board went OK, and the other 5 of these boards went OK.

So - WTF? Did it just have a bit of a moment? Should I worry much? Fortunately, these are easy enough to spot, but if it's going to make a habit of this sort of thing...
I've had a few (maybe 5 in total) of the 'I can't be bothered to turn the vacuum pump on' component mis-picks today, which I think is more than average, but doesn't feel very related.
Anyone else seen similar weirdness? I thought I was running out of ways to be surprised by this machine.

Also, are there any cheap ways to AOI?

[phonoplug]

I get this very occasionally. I don't think the RV itself is to blame here. I think it tends to be down to the part bouncing in the carrier tape between the point where the cover tape is peeled off and its picked. SOT23's are great at that. RV will square it up but doesn't specifically recognise the leads so 180 degrees out will pass vision.

[SteveW]

Yeah, that might do it - I wasn't watching it by the end (everything was going so well), so a bit of unlucky tape bounce could do it.
I thought that SOT23s drew a triangle around the shape during vision - but I can well believe it then turns that into a square and bangs it down regardless...
Strange that it was adjacent parts - but I'll shelve my paranoia for a while!

[Gopher]

Most affordable 'AOI' I have come across are the machines they sell as First Article Inspection. They are essentially based around a large flatbed scanner with some software. They are capable of catching things like this but can't match the multiple roving camera approach that proper AOI machines have that spot poor wetting, bent leads etc.
FAI machines cost about £10k new last time we looked and are no doubt rapidly improving just what the software can extract from the scanned image.

http://www.blundell.co.uk/product/extra-eye-now-aoi-fai/

http://www.aoisystems.com/Scanner-Based-In-Line-Desktop-AOI.html

http://www.blakell-europlacer-distribution.co.uk/Quins_First_Article_Inspection

You should also be able to find older AOI machines 2nd hand fairly easily, the rapid improvement in technology means those who rely on them often upgrade. I get the impression some of the earlier models involve all sort of profile tweaks etc to get tuned in.

[Mike]

I'd agree that it may be tape bounce or part swivelling on the nozzle.
As regards AOI on the cheap, maybe a quick & dirty solution would be a flicker comparator - simple setup with a camera and jig to place PCB at a consistent place (or maybe just mount an overhead camera & light on the RV)
Store an image of a good board, then alternate the display between this and a live camera image.
Any changes would be easy to spot as the eye is sensitive to movement.

[spiyda]

If it is indeed parts bounce, I had an issue with small zener diodes (SOD-723) being placed upside down

I narrowed it down to carrier tape bounce, they were flipping over while still in the tape, it doesn't take much bounce as they are nearly square in cross section

Since the feeder was already adjusted to pick the diode as close to the cover plate as possible, it wasn't that one part was flipping while the precious was being picked, so it must be at the very moment the tape advances and the cover is stripped.

hanging a weight on the end of the tape reduced the problem, but it still happened randomly.

I reasoned that the feeder was designed to take the  paper carrier which is much thicker than the plastic tape, so there was excessive vertical freedom of movement of the tape.

My temporary bodge was to stick a strip of thin card to the underside of the stripper plate with double sided tape, just wide enough to fit the slot in the feeder.

Worked like Magic, no more bounce !

for a more permanent solution I considered modifying a whole feeder to suit plastic tape by machining the top of the main ally block off,

but my temporary bodge is still working, and if it ain't broke, I tend not to fix it.