What will happen if a sloder paste is used after 24hours

We're not sure of the condition of your solder paste. So, afte 24 hours, if it's: * Unused, in a reclosed container, maybe a week of life depending on the environment * Used, scrap

A simple coalescence test can quickly determine the condition of solder paste after prolonged use. Simply print a small disk [around 4-5mm diameter and 0.2mm thick - a business card with a hole from a standard paper hole punch makes a good stencil] of paste onto a non-wettable substrate [eg, glass microscope slide, solder mask on board, ceramic tile], and re-flow as normal. A single solder ball in a clear pool of residue indicates good coalescing ability. Numerous solder balls remaining in the flux residue pool indicates poor coalescing ability and the paste may have become damaged. Note this is not an absolute test of the condition of the paste. It is only a first-line check and should only be used to confirm paste failures. [based on Multicore Solder Paste Handling Guidelines, September 2004]

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Depends on the solder paste. If it has been on the stencil for 24h there is a good chance that the paste is destroyed. You can however check if it has not dried out too much by letting the stencil printer do a few strokes and see if the paste still has good rolling properties and if the printed result is ok. A solderball test, as described in the post above, is also an indicator of the state of the paste. Never mix used and fresh paste to become better results, that does not work.

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