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I'm trying to select a solder flux and so many references (and one or two SE questions) discuss the types and differences. They mention "activity", such as "Rosin Activated (RA) flux has a higher activity than Rosin Mildly Activated (RMA) flux."

What is "activity"?
What is it that RA flux does a lot of, that RMA flux does a little of and R flux does barely at all?

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Flux "eats" oxidation. You can't make a soldered connection to oxidized metal - it has to be bare, clean metal.

Flux removes the oxidized layer, and prevents the bare metal from oxidizing for a little while. The melted flux layer keeps air (and therefore oxygen) away from the cleaned surface.

The "activity" says how aggressive the flux is. Higher activated = removes oxidation faster.

The downside is that a more aggressive flux also "eats" more metal from the surface and from the soldering iron tip.

At an extreme, acid flux as is typically used in plumbing will eat the traces right off the board.

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    \$\begingroup\$ More active electronics flux will typically damage the board if not cleaned off, too. That's why they make flux remover. \$\endgroup\$
    – Hearth
    Nov 20, 2020 at 15:33

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