Allowable max interval of solder paste printing -> Reflow -> wash ?| 14 February, 2018
you would be best to get this from your paste/solder manufacture. but here are some rules of thumb: -Solder paste out of fridge before use - get out the night before needed.. don't let your op's warm on the oven!! you will regret that! if not over night verify temp prior to use, use at room temp. -use on stencil, depends on ambient condition of course!! key is only have a 3/8"-1/2" roll on the stencil, dont let it dry out and dont let is slump. add to it as needed. if you are lucky have a way to verify visc, if you don't, dont be cheap.. toss the paste when you feel its out of spec.. saving pennys on the stencil will cost you rework time! -maybe the biggest point you were after?? how long to leave flux on the board. you must be using water sol chemistry, if so i train my team no more than 4 hours with the materials we use. the manufacture says 12 but i'm building product for others so i error on the safe side!! if you leave it on to long it can stain/discolor things, it can become very hard to remove. -the dif with lead free vs lead is the temp, higher temp, sometimes makes it harder to remove the flux residue on the board, most manufactures have overcome this issue now, 10 years ago when this was new it was more of an issue for sure!! but i have my team follow the same rules 4 hrs on, no more! -Wave solder flux,, now that i treat a little different, those i have washed within an hour. keep in mind any topside flux that does not hit the solder pot may not have been fully activated, this means its more aggressive.
hope this helps a little.. but hit your flux and solder manufacture up they will have white papers on this for sure