I'm new to PCB soldering. I just bought a cheap soldering iron, solder wire and flux to try something. I have no previous experience with soldering.

I'm well aware of the potential risks of prolonged exposure to flux fumes, and I have already taken precautions to avoid it.

I was all excited and ready to start soldering, when I saw this on the flux box:

enter image description here

I'm OK with not putting that into my eyes and not tossing it into the waters, but why does it say "CAUSES SEVERE SKIN BURNS"?

All the dudes on YouTube that I saw soldering touched rosin with their bare hands.

While that doesn't necessarily means it is a safe practice, it surely didn't cause severe skin burns! I would rate something as severely burning if it burns like sulfuric acid, for example.

The only explanation I came up with was that maybe this product is made of really cheap dangerous stuff instead of rosin.

The product is this:

enter image description here

However, looking online I saw the same warnings even on boxes clearly marked as containing rosin flux.

If all those people on YouTube can handle this stuff barehanded, I'm clearly overreacting.

So I ask you, why is this stuff depicted as extremely dangerous on the box?

  • Can I safely touch it?
  • Are those warnings put on the box only for legal reasons?
  • Are rosin based and different fluxes more or less dangerous?
  • 3
    \$\begingroup\$ You haven't bought rosin (Kolophonium in German) but Lötfett. The latter is meant for plumbing. Kolophonium will do nothing to the amount of oxides on old copper pipes, so you can't use it for that. \$\endgroup\$
    – Janka
    Nov 23, 2020 at 11:57
  • \$\begingroup\$ They do have a MSDS in German: wentronic.com/media/perfion/perfion-product/45238_MSDS.pdf \$\endgroup\$
    – pjc50
    Nov 23, 2020 at 11:57
  • \$\begingroup\$ I think that you are overreacting. All the skin burns that I've had (and I had some in 30+ years as hobbyst) occurred when I mistakenly touched the hot solder iron. I've never had any occurrence of chemical burning from flux. This is not "extremely dangerous". Try to use it normally, as the Youtube folks did; and keep some tissue paper near you; but if you feel some skin rash, stitch, heating, stop immediately and clean your hands. EDIT: Please note that I'm talking about rosin, as I'm not German and I'm not aware of difference of "Löftett" and "Kolophonium", as @Janka explained. \$\endgroup\$
    – mguima
    Nov 23, 2020 at 11:58
  • 2
    \$\begingroup\$ @mars, your question makes me think in an answer that is more philosophical than practical. I think that the fact that some consummers blame vendors for every kind of failure has made vendors to overreact, until the point that the packages have labels advising about very unprobable risks. Maybe this rosin can be absorbed by skin, and maybe it could cause a little harm... But, just to be safe, just to be able to say "the consumer was warned about this, the vendors put those things in the labels... \$\endgroup\$
    – mguima
    Nov 23, 2020 at 12:20
  • 1
    \$\begingroup\$ @user263983: Just being "natural" doesn't make it harmless. Aconite and bella donna are natural, but you don't want to be fooling around with either one. Maybe you'd like some hemlock tea - natural to be sure, and it'll kill you stone cold dead. \$\endgroup\$
    – JRE
    Nov 23, 2020 at 12:21

1 Answer 1


There seems to be a translation issue, and it's a good job you asked the question; this is not the roisin flux commonly used in electronics, but Zinc Chloride (if my machine translation of the German datasheet is correct).

Zinc chloride is mildly corrosive - indeed, that's partly how it works, it's intended for soldering bulk copper in plumbing where removing the copper oxide layer is important.

I wouldn't panic about it but it's in the same category as kitchen cleaning chemicals where mere contact will sting a bit and you should wash it off promptly.

(Labelling it as "soldering paste" is even more confusing in English, because "solder paste" normally refers to tin or tin-lead particles in suspension used for automated soldering in ovens)

  • \$\begingroup\$ Lötfett is the correct term in German. The English term is wrong, yes. \$\endgroup\$
    – Janka
    Nov 23, 2020 at 12:08
  • \$\begingroup\$ Thank you. I already bought a box of SRA rosin fluxant, as this clearly is the wrong product. If I can safely touch this, I would suppose touching rosing is even safer, despite what the packaging may warn me about? \$\endgroup\$
    – mars
    Nov 23, 2020 at 12:09
  • 2
    \$\begingroup\$ Soldering compounds for plumbing may work great for soldering electronics that only need to last a few minutes, but they will cause quickly essentially irreparable damage to the components and boards with which they are used. \$\endgroup\$
    – supercat
    Nov 23, 2020 at 20:15
  • \$\begingroup\$ The English name is not really confusing, because it is much more widely used in plumbing than in electronics. You are not likely to find automatic soldering oven consumables for sale in your local hardware store. \$\endgroup\$
    – alephzero
    Nov 23, 2020 at 22:18

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