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I'm using nozzles from JUKI for example 502 and a 1 size bigger one during testing. Very often the components stay sticked to the nozzle, after placement. I do not use solder paste at this moment.

Is this is normal , you really have to trust on the quality of the solderpaste for placing components? What happens with some older solderpaste, which will stick less, this still works?

The components are light, it does not take very much for them to stick, and I think the forces are very small. In my testing, I use light coat of spray-on post-it glue, and parts never stay on the nozzle, and when I use paste, no problems either.

> What happens with some older solderpaste, which will stick less, this still works?

I have experience only on good paste, but I guess any sticky stuff on board would do the trick, even if not super sticky.

I ran through a handful of boards over this past week and had a similarly frustrating experience when my parts would stick to the nozzle. There was one run where I tried to place a quantity of ten resistors in 0603 package, and I think 8 of them just stayed on my nozzle after hitting the board and solderpaste.

Luckily I solved the issues that I was having and became much more pleased with the results. My issues were a combination of a few things. First, my solder paste application was slightly offset along the x-axis. Then, the tolerances of the tape packaging lead to the placement of the part slightly offset in the other direction. So these two things alone resulted in less sticky paste for the part to land on.

But lastly, I found that using the smallest JUKI nozzle (#1) for these 0603 parts was able to overcome these offsets. Perhaps there's less residual vacuum with the smaller nozzle, so less force required to pull the part off. I was using the #2 nozzle because I had some larger parts (2512) and hoped I could just run the entire board with the #2 nozzle.

Once I modified all the small parts to use the small nozzle, my problem practically went away and I was much more satisfied.

The vacuum off time setting likely also plays a role.

is there any guidance for the vacuum off time setting? I am having the same problem

Not really, I don't have enough experience to recommend a value - other than the obvious, if a part is not released with low setting and the valve works, increase it.