Expired solder paste - what to expect?

[Yellofriend]

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.

[wraper]

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.

[Infraviolet]

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.

[strawberry]

soon metals will  come with expiration dates 
maybe add fresh flux

[Ian.M]

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.

[thm_w]

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. 

OK, but if its no-clean solder paste is it corrosive before activation?
I would assume not.

[Ian.M]

Many so-called 'no-clean' fluxes are corrosive if not heated to reflow temperatures (which deactivates them when they cool again) so are known to be problematic if used as additional flux for manual rework without subsequent cleaning.  A 'no-clean' solder paste based on such flux will have similar shelf-life problems.

[mikeselectricstuff]

It varies a lot by brand - I once had some EFD paste that was fine at least 5 years expired.

[tooki]

Indeed. The paste I have at home is a syringe of MG Chemicals no-clean 63/37 that I bought in 2015. Just used it the other day and it’s still like it was when I started it. Stored in the fridge of course.

[rooppoorali]

Here are some opinions regarding expired solder pastes:
https://www.circuitinsight.com/programs/54171.html
https://www.indium.com/blog/solder-paste-expiration-shelf-life.php

[Calder]

If you have been careful with it (properly stored) everything should be fine. We have used paste 2 years over date of expiration and everything went fine.

[tomgat]

You can pretty much always get solder paste back to life if you want to spend enough time on it.  You can just add a little RMA-218 flux to it to get it back to the consistency of new flux.  There are other flux pastes you can use, however.  Just add it slowly and mix relentlessly because its extremely easy to add too much.   The downside(s) to doing this is are several.  First, its really tough to get it smoothly mixed again (unless you have a spin mixer), and it can leave more flux residue on your reflowed board if you add too much.  Next, it will leave a little more flux on your boards, which means you probably will need to alcohol rinse them.  Finally, if you do too much of this and run allot of boards doing this, you end up seeing burnt flux residue on your reflow oven which is a royal pain to clean later.   

This trick is also useful when your in a pinch and seeing allot of solder bridging on fine pitch leads.  I will intentionally added a little more flux to paste when I didnt think through my solder paste stencil and I am seeing bridging on boards.       

[james_s]

I used some expired paste for years, I would just squeeze some out onto a scrap of shiny printed cardboard and add a few drops of liquid flux and stir it up. I wouldn't try that for automated production but for hand assembly it worked fine.

[tomgat]

People overthink pcb manufacturing.  Unless your running massive volumes where extremely high yields are therefore extremely important, it’s rarely an issue.  Solder pastes that look right are very forgiving.  I’ve ran even a couple hundred boards with older paste, including double sided component boards.  Sn63/Pb37 paste is more forgiving than no lead high temp solder like SAC305.  Worst case it just decreases board yield slightly, but unless your running 10s of thousands of boards daily, it’s typically not noticeable.  Other variables like proper stencil design is much more important. 

[wraper]

People who say that 5 years old paste worked fine most likely did not inspect PCBs under microscope. It will probably solder, but it will have tiny solder balls all around and under the parts. Neither flux does any good to a bad paste. If you stored it not properly sealed and liquid evaporated, maybe you can make it somewhat work by diluting. But if it has soldering problems, adding flux will do nothing.

[tomgat]

Solder paste life isn’t indefinite due to inevitable clumping (even when spun), but people throw away too much paste that is perfectly good.  Tiny balls around solder pads occurs equally as much with new paste on a poorly designed stencil.  Same with bridging on the fine pitch leads.  Just my 2 cents.  Add the fiducials to your stencil for positioning.  Use 80% sizing on the fine pitch leads prone to bridging, and use either baseball mound or tear drops if you want to limit solder balls.  Give me a well designed paste stencil that is properly positioned on scrape, and I will run older paste all day long without solder balls at very high yields. 

[Berni]

Yep old solder paste leaves tiny solder ball sand around pads. It is much more visible under magnification.

This is caused by the flux in the paste boiling off or going bad. How many years it lasts depends on the exact formulation of the paste and how it was stored but usually after a few years they all go bad.

You can sometimes revive a tub of expired solder paste by adding some extra flux and mixing it in, but it only works up to a point. You can still keep the older paste around for some large pitch non important jobs, but if you do fine pitch SMDs you better buy a new one. At some point the paste gets so old that it not only leaves a mess around pads but also stops properly wetting component pins.

The solder ball sand around the pads can be removed using a ultrasonic cleaner very nicely, so you can still use marginally expired paste.

[tomgat]

Just my 2 cents, but respectfully what your describing is microscopic clumping.  If you don’t have a spinner machine, then yes always use new paste.  However, I never run paste (even when new) unless I’ve spun it for a while.  When the balls are evenly distributed in the paste, my experience is that it’s extremely forgiving.  .

[Yellofriend]

Meanwhile I bought 200g new lead-free solder paste from Taobao and despite the 1 week or so travel time without cooling during the hot summer it works very well.

The old solder paste (it's still a paste, not hard yet, it tends to have more ball spreading) I can use for some touch up. I works better than solder wire.

[gaganchd2000]

We had similar situation. Best we could do was to little quantity of old paste into new paste when opened. This way we could start consuming little by little.

[wraper]

We had similar situation. Best we could do was to little quantity of old paste into new paste when opened. This way we could start consuming little by little.

What? You were destroying new solder paste by adding old garbage?

[Berni]

We had similar situation. Best we could do was to little quantity of old paste into new paste when opened. This way we could start consuming little by little.

Don't do this. It just makes new paste perform worse.

If you do want to use up old paste then use it on boards with very large components since there the paste performance does not matter nearly as much. Also adding in some fresh flux helps revive old paste for a while longer. Once even that doesn't work just get rid of it.

The cost of solder paste in small quantity is cheaper than the cost of your time troubleshooting badly soldered boards.

[tomgat]

No offense, but I have used older solder pastes perfectly for a very long time.  I also blend them together routinely, and make hundreds of boards a day without any issues.  The key difference, however, is that the paste must always have the proper consistency (ie.  enough flux), and I ALWAYS let it spin in a solder paste mixer for a good 5 to 10 mins right before I use it.  I even spin new paste because I dont always know how long it has been sitting around.  Expired paste?  Same thing.  If you dont have a solder paste mixer (which are expensive), then sure, always use new paste because it starts clumping pretty quickly, especially on the surface.  This is especially the case if you put left overs back into the container which, once again, you should never do unless you have a mixer.

Also, allot of the issues people are discussing about "issues with older solder paste" such as solder balls, bridging, etc. is best fixed by properly designing their solder stencil.  Solder balls have more to do with a too large aperture ratio than anything else.  Baseball mount patterns also help allot on the passives.  My humble suggestion is to refer to the Printed Circuits Handbook, which is the defacto Bible for everything PCB manufacturing. 

[tooki]

Also, allot of the issues people are discussing about "issues with older solder paste" such as solder balls, bridging, etc. is best fixed by properly designing their solder stencil.  Solder balls have more to do with a too large aperture ratio than anything else.  Baseball mount patterns also help allot on the passives.  My humble suggestion is to refer to the Printed Circuits Handbook, which is the defacto Bible for everything PCB manufacturing.

Can you show a picture or describe it in non-sports terminology? I have no idea what a “baseball mount” is, much less what it looks like! 

[tomgat]

Also, allot of the issues people are discussing about "issues with older solder paste" such as solder balls, bridging, etc. is best fixed by properly designing their solder stencil.  Solder balls have more to do with a too large aperture ratio than anything else.  Baseball mount patterns also help allot on the passives.  My humble suggestion is to refer to the Printed Circuits Handbook, which is the defacto Bible for everything PCB manufacturing.

Can you show a picture or describe it in non-sports terminology? I have no idea what a “baseball mount” is, much less what it looks like! 

https://www.surfacemountprocess.com/a-guide-to-effective-stencil-design.html