A question on SMT soldering paste

Thread Starter

cmartinez

Joined Jan 17, 2007
6,935
I have a small syringe of SMT soldering paste that I bought at least six years ago, and have used very little of it. Well, today I had to use it again on a small circuit that I've been working on (see pic) and found that it had turned a little sandy and dry, and therefore not easy to apply through a stencil that I made for that specific circuit.

1590011926786.png

Question, is there a definite shelve time for soldering paste? Or does it change from brand to brand? Also, is there a way to "rejuvenate" the one that I have, or will I just have to dispose of it and buy a new one?
 
Last edited:

MrSoftware

Joined Oct 29, 2013
1,775
Yes there is a shelf life, yes it differs between solder paste but off the top of my head I can't say which brand or part number lasts the longest. Storing it in the refrigerator does help the shelf life. I suspect it's the flux that dries up, so completely guessing, I wonder if mixing in some new flux might help. Surely not as good as new, but maybe it would become usable again?
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,935
Here's something that I've just noticed a moment ago, it seems that the flux has become separated from the paste!:

e674c7af-4bb9-4ff7-993d-fb0a9db000a5.jpg

And yes, I've been keeping it refrigerated. Except for a few weeks because I moved to another house and hadn't yet connected the small fridge where I had been keeping it.

I guess a possible solution would be to take it out, mix it again thoroughly, and put it back in ... kind of a real drag, if you ask me.
 

SamR

Joined Mar 19, 2019
2,096
The biggest problem for any non-organic material is Oxygen, for which reactions are accelerated by heat. So keep it in an airtight cool environment.
 

kubeek

Joined Sep 20, 2005
5,724
First, I would heat the paste to say 30-40C (say in a water bath) to get it to flow, remove if from the syringe and mix it well. If you cant then make it work, then add very slowly some liquid flux, like future rework jelly etc, mix and try again. Or just isopropyl could do it too, if you don´t have anything adequate. Don´t forget to test the reflow, someone told me that a bad mixture, with say water present or glycerol could start popping like food in a microwave.
I doubt the tin itself would be oxidised, so you should be good on that front.
 

SamR

Joined Mar 19, 2019
2,096
Those shouldn't react with each other but I'm sure there are other ingredients that might. They will all react with Oxygen. They would have been mixed as powders with some carrier material that has at least dried out some due to loss of volatile vapors. Remixing it with some liquid flux doesn't sound like a bad idea. A little at a time to avoid it becoming "soupy".
 

jpanhalt

Joined Jan 18, 2008
9,348
Yes, separation of solder paste from the active flux is one thing that is recognized to happen. Second, several entities, including SparkFun and Adafruit, recommend adding a no-clean compatible liquid flux (e.g., Kester 959T) and mixing to rejuvenate old paste. They also recommend a slightly more fluid paste than commercial products.

The Kester paste I got a couple of weeks ago expires late Summer. I plan to keep some in the refrigerator and restore as necessary with liquid flux. I also plan as an experiment to keep some in a deep freezer. Advice against freezing seems to be based on typical refrigerator freezers. I suspect they do not apply to LN2 or even -70°C freezers. My "deep" freezer is at least -20°C and is not self defrosting.

Some people equate "freezing" to being at less than 0°C. Of course, in the biomedical arena, everyone knows that serum doesn't freeze at that temperature. There are lots of papers about stuff being less stable at "freezer" temperatures than at refrigerator temperatures. The reasons for that are three-fold: 1) Self defrosting freezers were used; 2) Materials were in a protein matrix and not frozen solid, and 3) As water crystallizes out, the concentration of nucleopliles or other reactants in the remaining solution increases.. I do not have a -70°C freezer anymore, but I hope to find out whether the deep freezer I have will help preserve my paste. Of course, one must always be aware of the effects of water condensate. Be sure stuff is well protected (e.g., in glass, not thin plastic) and is allowed to equilibrate to RT before opening. If under vacuum, that time can be a lot longer than you may think it is.
 

SamR

Joined Mar 19, 2019
2,096
The only problem with freezing is possible crystallization which is active separation. Great if you are trying to separate compounds but not if you are trying to keep them homogeneous. YMMV
 

Wolframore

Joined Jan 21, 2019
1,746
I’m not entirely certain your flux evporates, rather it’s dries out. I would add a solvent. Not sure what type... the flux is still there just dessicated.
 
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