I have many needles I used for smd which still have soldering paste inside them but I guess it dried and hardened and there's no way to get it out with syringe.
What is the way to remove that paste and clean the needle passage so it can be reused?
Warm it up or put to some dissolvant?
Once it has hardened up, the only way is to find a drill bit that fits the needle and drill out the dried solder paste. Next time you use the solder paste syringe, clean all the solder paste out and use IPA to loosen the residue.
Do you keep the unused paste in a refrigerator? That keeps the particles from coalessing, but that only works for a few weeks. Solder paste has a limited shelf life.
Use it regularly if only to keep the needle from gumming up - typically I'm squeegying the stuff
through a solder stencil so I just unscrew the needle anyway as I need a sizable blob to do this,
so I often find the needle is fairly dried out, which usually clears with a lot of force applied and a
bit of patience.
Fridges are often suggested, which may be OK for small amounts, but you should have a healthy
respect for the rule of never putting anything containing solvents in a fridge or freezer. Firstly
they are poisonous and should not be near food, and secondly many solvents evaporate even
when very cold, causing an explosive atmosphere in a box with an ignition source (the light
I use liquid flux in a bottle with a needle tip. Whenever the needle is clogged I heat up the needle a little bit by stroking it against the soldering iron for a few seconds. That seems to liquify the flux inside the needle again. Works every time
That might work for you too but don't heat it to much. You don't want to melt your syringe or whatever dispenser you use.