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I grew up as a boy doing soldering kits in the 80s. I wanted to do the same for my daughter so bought a soldering iron online.

It came with a box of orange resin with Chinese writing on it. I'd never seen something like this to do with soldering. Running it through Google translate comes with something like 'solder additive'.

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When I google search online for 'solder additive' I get vague descriptions of better joints, or coping better with dusty joints. I don't understand how that would be solved by resin.

My question is: What does solder additive do?

EDIT: Thanks for all those that contributed to this question and the EE SE community. I asked my Dad what we had done for resin when I was growing up. He responded, "We used resin-core solder, that's why you never saw the resin."

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    \$\begingroup\$ Beware of Chinese chemicals - the health and safety rules are not necessarily adhered to, and that packaging looks distinctly dodgy. \$\endgroup\$
    – Chu
    Commented Jan 5, 2018 at 9:35
  • \$\begingroup\$ Dispose of that "solder additive" and use a good rosin core solder. Problem solved. \$\endgroup\$
    – JRE
    Commented Jan 5, 2018 at 10:02
  • \$\begingroup\$ Resin (basically, pine or spruce tree sap) in alcohol solution is a common soldering flux. I think this is a mix-your-own kit. \$\endgroup\$
    – Whit3rd
    Commented Jan 5, 2018 at 11:45
  • \$\begingroup\$ More reading: en.wikipedia.org/wiki/Flux_(metallurgy) \$\endgroup\$
    – Dampmaskin
    Commented Jan 5, 2018 at 12:05
  • \$\begingroup\$ With leaded solder, a rosin core is usually enough to provide sufficient fluxing action for moderately clean joints. But with lead free solders (which are generally required for any non-aerospace products that will be sold), this is typically not enough unless everything is perfectly "never even breathed on" clean. Additionally, surface mount parts put the old "let the iron heat the work and the work heat the solder" advice out the window - you're often delivering solder from the iron. So additional flux is a lot more common now than it was in preceding decades. \$\endgroup\$
    – Chris Stratton
    Commented Jan 6, 2018 at 22:34

2 Answers 2

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Resin may allow a dirty joint to work where otherwise it would not., but that is not its purpose. You should always clean the components you are soldering together prior to making the joint, they should be bright and shiny. Remove corrosion prior to soldering.

That said, the purpose of resin is to combine with and displace Oxygen. Without resin, air surrounds the components being soldered together. The heat from soldering accelerates the corrosion process where the solder forms bonds between the components being soldered, making for poor bonds between the solder and components. When you use resin, it burns thus depleting the Oxygen (by combining with the resin) and temporarily displacing more Oxygen from getting to the joint.

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I would guess your solder additive is flux, usually rosin, which keeps the metals joined clean and prevents oxidisation during the soldering process, to allow for a good connection.

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