How long can I leave paste before ovening

[Simon]

I am placing a board and have found one part to be the wrong package. Can I leave in 24 hours until the replacements arrive or should I reflow now and then try to rework the parts in?

[mikeselectricstuff]

Look at the paste datasheet, it will usually spec the working life. ISTR 24hrs is typical, but keep it cool.

[Simon]

hm they are not too specific: http://www.chipquik.com/datasheets/SMD291SNL10.pdf

[Simon]

Parts are on order and the boards are in the fridge with all parts but the missing ones.

[wraper]

Depends on the solder paste. For example I left GC10 for 3-4 days and it still was tacky and reflowed without noticeable issues. On the other hand some other pastes become dry and non sucky in a few hours.

[wraper]

Parts are on order and the boards are in the fridge with all parts but the missing ones.

Leave them at room temperature for at least a few hours before reflowing to avoid issues with moisture.
http://www.indium.com/blog/humidity-and-solder-paste-do-not-mix.php

[Simon]

OK will do, fortunately this is all in the office and humidity is at 37% in the room. I'll also do a prolonged warm up in the oven, do a spot of 50-80C before bringing to 100 and then the reflow temperature.

[mikeselectricstuff]

I think the main concern is screen life, once printed, the flux is still there even if the solvent has evapotared

[wraper]

I think the main concern is screen life, once printed, the flux is still there even if the solvent has evapotared

Once solder paste is no longer tacky, you will have tombstoning problem or parts could even move off from the pads completely.