Here's my 2 cents.......
I worked for a small company that manufactures pick and place machines and had the task of testing/troubleshooting and the dreaded calibration. These machines used vision for part orentation and have what I considered awesome encoders so I would expect that part placement would have been perfect every time. Not so....
We initially were placing real QFP's on a board covered with double sided tape and would place the part, make a visual inspection, make a correction, put the part back in the tray and do it again. This went on for every head/spindle at various rotations all day long. Sometimes into the next day. Oh yes, it was fun to tweek, as long as the machine responded and placed that part beautifully right through final inspection. But what to do when things didn't work out?
The first thing I looked at was the QFP. We would run several different sized ones once we got the first one to place correctly and I believe if the parts were perfect they would have placed the same all the time. Bent leads due to their manufacture or operator handling were a big killer. Glass slugs were next tried, but after the first couple broke cost became an issue. We eventually started using "perfect" parts made out of plastic and milar. They were cheap and no bent leads.
The next thing was the test board. Double sided tape isn't the greatest thing to hold parts onto a board and I found that parts would actually move and rotate after a while. An airisol spray can also be used but after a while more parts will stick to your hands then the board. I also found that all boards are not created equal. They call it board streach and skew, and if I had it my way they would have to put local fiducials for every part from 1206's to fine pitch. I stuck with one "perfect" board until it was so beat up that fidutials were not detectable. I also retaped the board a couple times a day.
The next thing that was an issue for me was the conveyor system. You can have awesome placements and then the board is transported to the oven and it is violently dumped from one machine to another. I think of it as someone shipping glassware and writing "Fragile" on the box. This word is motivation to a handler to throw it harder. What I'm getting at is solder paste and glue won't hold a part over a poorly aligned conveyor system.
The last thing I question is the oven. Parts move big time. Normally it's a good move as the solder "sucks" the leads over the pads.
So in defense of the machine if all outside varibles could be kept consistant tweeking would be a thing of the past. Tweeking may not be a good thing for the people who maintain the machine but for those responsible for pushing out product it keeps them employed. So I say tweek away but be responsible.
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