What are the symptoms of stale solder paste?

[zapta]

What happens when solder paste gets old and stale?

I am using a 6 months old solder paste, (kept in room temperature) and am having reflow problems (mostly solder shorts on adjacent pins).  Am not sure if this is because of the paste because there are several other parameters that got changed.

[georges80]

typically the flux no long 'works good'...

cheers,
george.

[Alex Eisenhut]

My friend keeps his in the fridge. I would guess that the flux gets less good, and instead of wetting the pads, the solder gloms to itself instead.

I'd try this: get some RMA on a rag and wipe your board down just before screening on the paste. Can't hurt.

[zapta]

My friend keeps his in the fridge. I would guess that the flux gets less good, and instead of wetting the pads, the solder gloms to itself instead.

I'd try this: get some RMA on a rag and wipe your board down just before screening on the paste. Can't hurt.

This my be the problem with my reflows, insufficient wetting. I will get fresh solder paste. Somebody recommended in another thread EP256 so I will give it a try.

[peter.mitchell]

another issue with stale paste is small sections of balls not re-flowing and getting porosity.

[StrangeloveMD]

Solder paste is balls of metal floating in goo.
Some (all?) fluxes are comprised of chemicals that degrade more rapidly at higher temperatures, thus refrigeration prolongs shelf life.  ("Do not freeze!" the manufacturers warn.  Do they give a freezing point?  No, the dolts mean the freezing point of water; roughly.)
Fluxes are what the metal balls float in.
As they degrade  the ability of the flux to hold solder balls in suspension is altered.
It was right when they started; alteration is bad.

Symptoms of "stale" solder paste may be similar to those of a poorly fluxed or highly oxidized solder joint.  This could indeed lead to shorts, especially on tighter pitch components.

Pics, and more details on "changed parameters" may be helpful.  I'd try new paste first; it is likely to work.

[zapta]

Thanks Dr Strangelove.  I fixed the shorts by hands but I will post pictures from the next batch. The main parameter that changed was a stencil from a new source. So far I used 5mil mylar and am using a stainless still one from China. Possibly it leaves thicker paste than before so I may need to reduce the solder paste holes. The problems occurred with 0.65 mm pitch SSOP and micro USB connectors. No problem with 0603 or DFN8. It resulted in shorts between a pair of adjacent pads. The short looks shiny.   I am reflowing with a desktop IR oven T962 and it seems to work very well.  The solder paste I used is Chip Quik Sn63/Pb37 No Clean .

Do all solder pastes have the same amount of solder for a given volume of paste? Will changing the paste change the amount of actual solder my stencil leaves behind?  I look to this kind of parameter but could not find.

[ajb]

I've definitely seen a reduction in the number of bridges and poorly wetted joints when going from a crusty old jar of paste to a fresh jar.  This was with Kester EP256, which tolerates a hell of a lot of abuse but still starts to go south after four or so years in the fridge .

There should be a spec for "% Metal" in the datasheet--Kester EP256 is 90%, which presumably means your stencil should be sized to provide a volume of solder paste equal to 100%/90% = 111% of the volume of solder required for the joint.  A random Chipquik datasheet indicates 85%, so you should need slightly less of the former versus the latter.

[fcb]

I keep mine in the fridge.  Behind the pickles.

[Kjelt]

If the paste gets older it gets drier and makes it more difficult to smear it with stencils and flow through thin nozzles when manually dispensed.
I got the same problem today with one and half year old paste i put enough for four large pcbs on the squeegee the first two pcbs that were done right away were fine the last two i did at the end of the day and the paste at the squeegee dried in had short on the tssop packages. The reason was that due to the dried paste not all holes were covered correctly and ihad to smear it again  which you should not have to do with good paste and can have bad results (too much paste on some spots).
I heard that you can give older paste (not completely dried in ofcourse so still a bit wet but too dry for it self) a second life by adding new flux but you should get the right one (no clean)

[andrew_c]

Just as a fun bit on the side, my parents found this tin of flux in their garage today and thought it might be of use to me.





Looks pretty useable to me! Actually, doesn't look like it's ever been used

[zapta]

I The reason was that due to the dried paste not all holes were covered correctly and ihad to smear it again  which you should not have to do with good paste and can have bad results (too much paste on some spots).

That's exactly what happened to me. May explain why I had extra solder that created bridges.

[Psi]

Looks pretty useable to me! Actually, doesn't look like it's ever been used

Not sure if you're being sarcastic or not, but i'll assume you're not, just for safety...

Dont use it for electronics!

It's acid flux for plumbing and will totally destroy PCB tracks/components and ruin you soldering iron tip by eating holes in it.

Notice how it has two warnings on it
- Avoid prolonged skin contract, flux contains chemicals that will irritate the eyes and skin.
- Wash heating system with hot water to remove flux residue

You know it's serious flux when they're so worried about the stuff being left on plumbing fixtures that they tell you to wash it off after soldering.

And just think, this was produced before OSH/OSHA requirements for product labeling/work safety.
Warning labels on stuff this old should be taken extremely seriously.

[StrangeloveMD]

ajb, The metal load spec of solder paste is given as a percentage of mass.  A 90% load paste is ~1/2 solder by volume.

Zapta, glad the board wasn't a loss.  Your new stencil should come with a thickness as well, or check with the manufacturer.  Assuming uniform application, this will obviously make biggest difference in the amount of paste applied.  But with the footprints you describe there shouldn't really be any problems on that front.

Some fun info on paste spreadableness and effectiveableness:
http://www.ami.ac.uk/courses/topics/0246_spc/

[Kjelt]

I The reason was that due to the dried paste not all holes were covered correctly and ihad to smear it again  which you should not have to do with good paste and can have bad results (too much paste on some spots).

That's exactly what happened to me. May explain why I had extra solder that created bridges.

For the extreme small footprints with small distance you really should only apply once. My experience was that the other components 0603, SO etc. were still ok so it really depends on the foot print. Btw an extremely small diode tombstoned half on me that time due to too much paste.
So you have to Inspect the paste thickness under a microscope after the application to see if it is ok, thats experience. Also how deoes the paste hold, is it nice and sturdy or is it too hot and oozes over the footprint to other pins?
If the layer is too thick best thing to do is clean the entire print and  restart from scratch. If only a few footprints are affected and it is for hobby anyway you can choose to clean only those footprints and reapply manually through needle or solder it on in a later stage. I usually have a manual needle applicator standing by to patch up some spots where there was too little paste or where i removed access paste and reapply it smaller dose. That is for small manual batches suited only.

[StrangeloveMD]

One should only apply solder paste once; sometimes one fucks up, and must reapply.
End product is generally not affected by how many times you "apply solder paste".
If "valid" solder paste is properly applied the only factor affecting the load is stencil thickness. (heh)
Visual inspection of paste applied to a "new" board via a "new" stencil could only be called prudent; though it is usually unnecessary. 
If you must do patchwork after stencil pasting a board, be wary of air pockets.
Tombstoning is a function of proximate thermal masses. 

[zapta]

So you have to Inspect the paste thickness under a microscope after the application to see if it is ok, thats experience. Also how deoes the paste hold, is it nice and sturdy or is it too hot and oozes over the footprint to other pins?

Now that I think about it, the old paste stays on the board like small cement bricks, maintaining its shape but when it was new after 15-30 minutes it was flowing 'oozing over' and was more fluid and more shinny.  I can use this as an indicator for the staleness of the paste.

BTW, if solder paste is solder mixed with flux, how come normal tacky flux can last years in room temperature and solder paste doesn't?

[zapta]

I reflowed today four more boards of the same design and with the same paste and stencil and the results were very good. The main process change was following the advise in this thread to do only one swipe of the paste. I got a good quality 4" putty knife at the local hardware store (before that I used a credit card as a squeegee and it is too flexible), pressed it hard against the stencil at 45 degrees and moved it very slowly to let the old paste to drop into the stencil holes. All four boards worked out of the box with nice soldering and no shorts.

I have fresh solder paste on order but even this old one is still useable with with correct process.

[calexanian]

generally the flux does not "Go Bad" what happens is the liquid carrier dries out. Depending on what you flux is soluble by will determine what you add to get it going again.. I find in my no clean solder if I want it to flow better I put in a drop or two of Dr. Marty's flux. Its mostly IPA. Makes it squeegee better. Otherwise tin/lead/whatever is in your solder is just metal and does not really go bad. Just the flux dries out.

[Kjelt]

BTW, if solder paste is solder mixed with flux, how come normal tacky flux can last years in room temperature and solder paste doesn't?

Good question, i don't have an answer for that one.
I can only do a best guess that the higher the temperature the faster the seperation process starts and that you have to restir to get the correct "rheological performance" again (as they call it).  In a syringe that will be a no go since you can not stirr it.
That is when the 90% metal/10% flux stays the same.
Perhaps the flux evaporates so if you have paste with 90% metal, 10% flux and if 30% of this flux evaporates this changes to a mixture of 93% metal and 7% flux which might be noticable in rheological performance.
While if you have a 100% flux in a jar/syringe and 30% evaporates you still have 100% flux but only 70% of it  left.
Just a wild guess from my part, could also be a combination between the metal and flux that changes over time,  i don't know, if there are experts here I would also like to know the answer.

[Kjelt]

I reflowed today four more boards of the same design and with the same paste and stencil and the results were very good. The main process change was following the advise in this thread to do only one swipe of the paste. I got a good quality 4" putty knife at the local hardware store (before that I used a credit card as a squeegee and it is too flexible), pressed it hard against the stencil at 45 degrees and moved it very slowly to let the old paste to drop into the stencil holes. All four boards worked out of the box with nice soldering and no shorts.
I have fresh solder paste on order but even this old one is still useable with with correct process.

Good to hear 

[StrangeloveMD]

Glad this paste is giving good boards again!  A process improvement is a lot more fun than simple restock work. 

Sorry for double linking, but this is the subsection for the "slump" you describe new paste having.  Cold slump characteristics are indeed a visual indicator of "the state of all these things here".  http://www.ami.ac.uk/courses/topics/0246_spc/#3

As for what solder paste "flux" is?  I know it varies, and would like some more info on this.  There seem to be some good folk out there still using pure rosin.  (Though they call it colophony, the goofs.)  In pastes the flux acts to hold the solder balls in physical suspension as much as anything.

calexanian is right; you can generally solvent up some old paste into something nice.  (water in crystallized honey)  Do be warned, though; there are paste MSDS that list chemicals that will degrade over time, rather than simply buggering off via volatility.

For people into reflow; blogs.indium.com is a happenin' place.
Now somebody in turn give me more info on exotic flux compositions!   

[Kjelt]

calexanian is right; you can generally solvent up some old paste into something nice.  (water in crystallized honey)  Do be warned, though; there are paste MSDS that list chemicals that will degrade over time, rather than simply buggering off via volatility.

But is it somewhere 100% confirmed that this solvent is indeed IPA? That would (for the hobbieist) be great knowledge to prolong the usage of their paste.

[StrangeloveMD]

Kjelt, it seems to widely vary by manufacturer and paste type.  Hexylene Glycol seems common in "Water clean up" pastes.

[Kjelt]

Ah ok that is somewhat more difficult to get I guess.