Please ID this solder.

[Raymond P.]

I just received notice my Crack+Speedball order has shipped and am super excited. In the meantime, I'm gathering tools for the build. I found this spool of Kester solder (see image) in the garage, which I know nothing about. So I'm wondering if this is appropriate. More specifically, does it have a rosin core, as I understand from these forums, an acid core is a no-no?

I googled the code "QQ-S-571" found on the side of the spool in hopes of finding useful information, but didn't. If anyone can confirm the usability of this solder, much appreciated. If not, I'll just have to buy some, which is fine.

Oh yeah, it also looks very old, in case that matters.

Thx,
Raymond

[Paul Joppa]

Edited with further info: I kept googling for a while - it appears that 331-type fluxes are conductive. The flux is more active - makes a better joint - but must be cleaned off. Rosin core is less aggressive and does not need to be removed after soldering. Kester 44 has been widely recommended for decades. It's the inorganic acid fluxes that are most aggressive and are widely said to be not suitable for electronics. Here's a reasonable tutorial: https://www.lifewire.com/types-of-solder-flux-818849

My original post is below:

Google "kester flux 331" to find some information. Looks to me like a highly suitable solder to. The very small diameter gives you a lot of control over how much you feed the joint. The 63/37 alloy is eutectic and preferred by many to the more common 60/40.

[Raymond P.]

Thanks, Paul, for the search hint. It turns out, from Kester's website, 331 flux is a water soluble type. The data sheet for this flux states that flux residue should be cleaned away (with water) within 48 hours because it can cause metal corrosion over time. Oh well, looks like I'll be buying some solder with Rosin core flux.


-Raymond

[Paul Joppa]

I have edited my post after a bit more searching. Thanks for posting, I learned some things while searching for an answer!