Your typical high mix line would consist of, Stencil printer, 1-2 foot conveyor belt, fast PNP machine (say a universal 30 spindle turret lightning head), 1-2 foot conveyor belt, large/heavy part PNP (say a universal flex jet 7 spindle or 4 Spindle), output conveyor belt 1-2 foot.
then you would have a walking space and the entrance to the oven.
now if your doing all small parts 1cm x 1cm x 4mm and smaller there is less risk of parts miss picking, And you can have the output conveyor go right in to the ovens chain conveyor(provided you don't have large boards that would do better flowing on the flat screen than hanging by there edges.
the advantage of requiring someone to manually move the board from conveyor to oven belt or chain(Edge hold), is that person will almost always spend a moment to check for missing solder paste, misplaced parts, train wrecks(1 part gets on anthers pad and it just goes down the line). Sometimes you may have parts that need hand placed as well, because there to big/wide/heavy, or the PNP machine just wants to have a bad day.
The common problems are... Someone loaded the wrong part in to the wrong feeder. The stencil printer ran dry. A aperture was clogged in the stencil printer. A placement location was wrong and the part is 1 of backwards, on its side, no where near the pads it needs to be on. The PNP machine thought it found a fidual and proceeded to pick the whole board shifted down and left 1 inch. The PNP machine had to be palmed down(cycle stop + estop) to fix, reject bin, board on top of another board, clogged nozzle, missing nozzle, any number of jams: and the machine instead of resuming from the correct location decided to repick every part on the board.
My experience is with universal instruments PNP machines, they work well in general with a few bits of character here and there, i doubt theres much better/faster.
If you want more advice you might also consider joining the #robotics and ##electronics channel on irc.freenode.com .
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