I have a semi-Auto paste stencil machine and use laser cut stainless stencil with T4 paste.
When I start pasting boards the paste covers the pads and is well defined. Throughout the job we have been having some issues with feeder on pick and place and stop sometimes for up to 15 minutes. After a while the paste does not cover the pads well and I am thinking that the paste has room temperature life of hours but the small amounts in apertures maybe dry and clog them? Cleaning the stencil during the run is a hassle.
I am considering it might be best to paste all the boards in a batch instead of stopping and starting.
Is this what some of you do, paste a batch and place them in a rack till needed by pick machine?
(We manually place pasted boards onto conveyor but once in here the machine is connected to re flow oven and carries straight through ok.
I batch print boards, we're talking maybe 1/2 days worth at a time into a magazine. Its not a unique approach, especially if you don't run a fully inline process. However to do that you need a paste that has a decent tolerance of being treated that way. In the UK a good option is BLT LFS-UFP-T4, it's a little bit like Loctite GC10 that is very popular with small manufacturers these days worldwide.
You can store it at room temp, the tub is good for 30 days after you open it, it has very low slumping so the print stays well defined on the PCB and it has good "stencil time". You can even partially build a board, rack it and finish it off the next day as it can in theory manage about 3(IIRC) days on the board before reflow which is useful if you encounter an issue mid-run or have a known shortage at the beginning of a job but simply need to get started and can then pan to add that part with tweezers just before reflow. Thats not to say abusing it that much is a good idea, but it lets you get away with it if needs be. Other pastes would be a dry non-tacky deposit and components might start bouncing off your boards during placement.