Just the solder paste, it's tacky enough to hold components in their place, the only time we see problems is with components with high mass, such as big surface mount inductors.

You get round that problem by setting the "group" (think mount priority) higher than normal components, that way the machine knows to place those at the end. You then tell it that the acceleration on the X & Y needs to be slower than normal.

You have to change the group as well, because once a component has been placed that has lower accelerations, those become the maximum accelerations that it'll use, and if the machine decides to plonk that component down first, then everything after it will be slow.

It's a neat system, the machine itself self optimises as it goes to try to increase throughput.

You can use glue, but that tends to be for double sided reflow on high mass components - so that they don't fall off in the oven. It's also messy and not want you really want in your expensive pick and place machine. All that happens is that there's a special nozzle on the machine that dispenses a blob of glue where needed before it does the pick and place.

Adrian