Electronics > Manufacturing & Assembly

Expired solder paste - what to expect?

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I have some reflow soldering to do, my solder paste expired in 2017 (I got 5 x 500g, different brands) - was always in the fridge. One is totally dried out and hard. Tried one that appeared normal, but sort of melted on the PCB (means could not hold the shape before soldering).

It solders, but created quite a bit pearls (micro tin balls). The softness is more of a problem than the balling because of the PIC flatpack. Type I use is Sn96.5Ag3.0Cu0.5 (must be lead-free!)

So basically I have to buy a new lot I guess.

Comments are welcome.

If stored properly, 1 year after expiry date is about the latest when  it may perform somewhat OK. No chance that paste which expired 5 years ago will give acceptable results. Adding solvents and fluxes will not fix it.

I've seen pastes last ok stored outside the fridge at room temperature for 2 years. Stored in a sealed syringe, hadn't dried out. Very ugly flux residues after use and a lot of little tin balls too, but all cleaned off ok and left normal seeming solder joints. This was a leaded paste though, lead free might not last so well.

soon metals will  come with expiration dates  :-DD
maybe add fresh flux

Its not at all surprising that solder pastes expire.  The solder component is in the form of fine particles vastly increasing their surface area, and nearly all 'wet' fluxes are to some extent corrosive at temperatures between zero and room temperature.   Therefore as soon as the manufacturer mixes the paste, the flux starts slowly corroding the solder, using up the activators in the flux and turning solder into dross.  The more dross and the less activators left the poorer the wetting ability of the solder eventually leading to a failure of molten solder droplets in the (ex)flux to coalesce.

Additionally, nearly all plastics are to some extent moisture permeable, and seals of reclosable containers are usually not perfect, so even if carefully stored in sealed containers, it will slowly pick up moisture from the air, which will combine with the metal salts resulting from the corrosion or with the activators, and be driven out as steam as you heat the paste causing violent sputtering.

Fresh compatible flux can help if the only thing wrong with the paste is its 'dried up' - loss of volatile solvents from its flux component - but it does nothing for the accumulated dross and moisture, and if the added flux is not compatible may make the sputtering problem worse, so is very much a last resort for hobbyists that pro's wont even contemplate.


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