Solder paste expiration

[schmitt trigger]

With SMT components becoming ubiquitous, they find their way into more and more projects.

Although one can solder a SMT component with thin solder wire, once that one has employed solder paste and reflowed it, and seeing how much better the whole process is, one does not want to go back to soldering with solder wire.

The problem with solder paste is of course, its limited shelf life (even with refrigeration) as compared to solder wire. And with the minute quantities that SMT components require, even the smallest solder paste presentation will last a very long time.

So, the question begs: how does one know whether the solder paste is still good?

I found the following very simple test, which I'm reproducing for the benefit of forum members:

In order to gauge the degradation, it is best to put a small amount of paste on a non-wettable substrate, like a piece of ceramic. Reflow the paste and see how well it coalesces. If coalescence is good, the solder paste will reflow into a ball, surrounded by a flux pool that is relatively free of uncoalesced solder particles. If the paste has significantly degraded, the paste will not coalesce well and there will be a significant amount of uncoalesced solder particles in the flux pool.

 

[DirtyLude]

I buy cheap paste from china. dx.com usually and I order a new small container every couple months. I don't refrigerate it, I only have my kitchen fridge and I'm not putting a bottle filled with lead paste in my kitchen fridge with my kids. If you use the same type of paste regularly it's not that hard to see whether it's gone too far based on the consistency. Mine goes hard as the rosin dries up. I will add liquid rosin to soften it up to the consistency that I like. When it gets too old, it tends to melt inconsistently and will break up as it's heated.

 

[KeepItSimpleStupid]

Words of wisdom. First, remotely related.

Sta-silv solder flux used for silver soldering/brazing. That stuff is in the worst container ever made. If you replace it with domed lid containers from "thesciencecompany.com:, the flux will last a long time. It water soluable, so it can be renewed. It's hard to find containers in Qty=1, but I'm always buying methanol from them.

What's been recommended is to use a smaller syringe for what your going to use.

Yea, I do use the food fridge in a tiny zip lock bag, (3"x4") but I should convert to the domed lid containers too.

 

[3v0]

Bad is bad but don't assume solder paste is unusable because it is old. The expiration date is mostly there for the people who use paste on production lines. As hobby types we can often use it past that date. If it does what you need it to do, it is good enough.

I have been using the same paste for several years. I keep it refrigerated or frozen most of the time. When it doesn't do the job I will replace it.

Some years ago I looked at fresh solder paste under magnification. I wonder if old stuff looks any different? Nope, I am not currently setup to have a look. No drivers for the 'toy' scope after windows 7. At some point I want to get a scope with a longer focal length so I can more easily place parts using it.