Floor life pcb assembly

Floor life of a PCB is rated in years. Are you asking about the solder paste or something else?

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The need to prebake boards to remove moisture from the board [to prevent voiding] prior to wave soldering is driven by the thickness of through hole plating. * 1 thou [25um] or greater requires no prebaking * LT 1 thou [25um] requires prebaking

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It depends on the laminate but a PWB is like any other MSL component. Some fab shops are only waranting standard FR4 laminates for 45-60 days when processed at lead free temperatures.

The best thing I can say is to try some weight gain/loss experiments. This will also help you with your bake out profiles.

Take a fresh board and weigh it right out of the sealed and dried packaging (or bake one out fully, e.g. 24 hrs @110C). Then weigh it again the next day, and so on until the weight stabilizes. You now know the rate of moisture absorbtion. Then do the same experiment in reverse. Bake it at 110C and measure every hour or so until it stops losing weight (i.e. all the moisture is driven back out) Now you know what is really necessary for bake out. Not 'some more is better' committee decision that only serves to oxidize the pads.

Somewhere between 30 and 50% of the max moisture absorbtion should be your limit depending on your preheat abilities and how conservative you want to be.

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