Solder Paste Expiery date?

[Geektronix]

Why would solder paste have an expiry date (and just to be clear, I'm referring to actual solder paste, not flux)?

[ataradov]

Solder paste is solder powder suspended in flux. Over time flux spoils and the powder clumps and settles down.

[RinusB]

I got some solder paste from 30 years ago. I think under some regulation it needs an expiry date or it should say "best before" like foods. Of course in marketing we should tell people to get new stuff to stay in business.

[ataradov]

Expiry date does not mean it will not longer work at all. For manual soldering it makes no difference unless it was stored in the heat.

But if you try to use that in automated assembly, you are going to have a bad time. That expiration date is for mass manufacturing, not for hobbyists.

[Berni]

Solder paste definitely goes bad over time.

Obviously nothing in the solder itself can happen, but the flux that is mixed in looses its potency over time, making the paste perform a lot worse.

If this is just some plumbers solder paste you use for soldering together some copper piping, that stuff will last forever. The flux there is much more aggressive and you don't need that fine of a solder joint, as long as it mostly fills the joint it will hold pressure.

But when you are talking about solder paste used to solder tiny SMD components on a PCB, you do NOT want to use old flux. The stuff does actually start to perform way worse after a few years (or even a year if the container is regularly opened and used). It still solders but it leaves a whole pile of stray solder balls around and in between pins, potentially causing shorts. It doesn't wet nearly as well so you get resistors floating up on top of a solder joint rather than grabbing it, resistors thombstoning etc. All in all it becomes a total shitshow.

[Psi]

Paste application and reflow relies on the solder paste having very specific properties.
So the expiry date just means when the manufacturer no longer guarantees that it will behave how SMT machines expect it too.

The expiry date usually has a temp component too.
So it might be ok for months at 0C but only weeks at 15C etc.. 

[coromonadalix]

as previously written,  the flux part  may dry,   we slowly add liquid flux in it to get a good consistency,   and put it in to a fridge when not used,  that way it stay good way longer

and let it warm up for 1 hr before use

[wraper]

Solder paste definitely goes bad over time.

Obviously nothing in the solder itself can happen, but the flux that is mixed in looses its potency over time, making the paste perform a lot worse.

As if. Since when metals are excluded from chemical reactions? There are tiny solder particles suspended in the flux, as they chemically react over time, they will no longer melt regardless of how much fresh flux you add. You normally don't see this problem with solder wire as surface layer is negligeble compared to amount of solder underneath it.