Shelf life of solder paste

Generally depends on the environment and use. The main issue is when the thinner dries which happens very slowly. A hotter environment will speed up the process.

It's not so much that the paste will become unsuable but that it's quality will generally degrade to a point that it is not usable for high density work.

So the real question is "What do you mean by 'work'"?

Shelf lifes of industrial products are so that any mil contractors that use them use them with correct diligence.

With many chemicals, they break down after long time periods.

Chip capacitor reels have expiration dates because the end terminations oxidize and they pose solderability issues that are costly to address post assembly.

So, there are many reasons why expiry dates are utilized on a product.

It depends on the product.

Fluxes perform by their de-oxidation capacity as it relates to metallic surfaces. It allows the solder to achieve covalent bonds where possible.

There can also be viscosity or specific gravity issues if it is used as a foaming flux for a foam fluxer on a soldering machine, etc.

So if it still performs fine, and cleans fine, there is likely nothing wrong with it at all.

If you are ISO certified, there should be an issue imposed by your quality assurance wording or the like, but nothing *real* is wrong with using it.

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Originally, I had something called Amtech Syntech (leaded but not water soluble). Still have the jar....Manufactured date was 1/06 and expiry date was 1/07. Well, it became useless (hard, no tack)in about

Didn't expect much better in terms of life expectancy so I'm a bit surprised that it is not only still useful but performance seems to be on par with when it was new. Both were refrigerated and handled similarly.

F>

Hence my surprise given a shelf life of >

-- Tony

Right, but they are giving the minimum shelf life for some targeted application. 0.8mm isn't very dense so it might work fine. In any case you have two different companies and so their methods to determine the shelf life will be different. Also two different substances so the chemsitry involed are different.

It simply seems that Kester was very conservative. IIRC, the real reason the paste goes "bad" has to do with the stenciling. For fine pitch the paste won't hold the form. There is a technical term for it but I don't remember it.

Of course, again, it all depends on what their criteria used was and the specific types of paste. There is some standarization of testing but it depends on a lot of factors. If the Kester works for you then go with them next time ;)

Most proper assembly houses utilize a tumbler device to constantly properly 'stir' their paste. It comes out of the fridge, and then goes into the tumbler for a half hour before it goes into the puller.

Well depends what you mean by 'proper' assembly house. I work with Tier1 CEMs for volume manufacture and have never seen such a thing. Mixing will not prevent drying out, although I guess it is probably used to even out the evaporation in low volume production. In volume production the paste is dispensed onto the print screen and worked in with a squeegy blade or injected through a sealed print head. This mixing occurs on the screen or in the head.

Printed paste normally has a life of a hour or so.

-- Tony