I was at Fry's yesterday, looking for some thin solder to use on an Adafruit kit with through-hole components.

Two of the solders I found were labeled RA Flux and No Clean, respectively. An engineer who was present said that with my use-case I should choose No Clean, as I wouldn't need to clean the board afterwards.

Can anyone clarify how one should select solder based on flux type? I had always been under the impression that you just use some sort of (one size fits all?) rosin-core solder with for basic board work, so I was confused at the different flux labels. I didn't think you had to clean your board (although I do know that after soldering yellow gunk can be left behind, and I suspect this is a flux by-product). I also know that you should avoid solder with an acidic core for electronics work, as that is for pipes + household plumbing.

The wikipedia page for soldering classifies many of the different types of fluxes, most of which require cleaning:

• R (non-activated)
• RMA (mildly-activated)
• RA (activated)
• No Clean

Can someone explain why/how you should choose a flux type for a given application? Most of the answers below answer "what." I'd like to learn a bit of the theory.