Title pretty much explains the problem, but a little background will probably help. Installed a copper water line about a year ago. The valve has been off, so no water has been in the pipe for the last year. My question -- did the flux I used cause the leak (the leak occurred at a joint) since it wasn't being constantly flushed by water, or is it more likely that I used too much flux initially and/or didn't properly wipe it off after the joint was soldered. I don't want to make the same mistake again. Thanks!

"too much flux initially and/or didn't properly wipe it off " probably neither is the problem. Not enough heat or not enough solder. Flux totally flushes with the right heat and wiping is more cosmetic than a sealing feature. Did you heat the pipe or heat the joint?

My technique was to heat the pipe for 7-10 sec and then heat the fitting for 7-10 sec.

I think you may need some help with your technique. YouTube - Building Tools : How to Sweat Copper Pipe Fittings

Okay, thanks, I'll give that a shot.

If you drain the water you may be able to just reheat the bad sweat job and add more solder. The "plumber way" would be to throw away the old joint, clean the pipe and niw joint and re-sweat using more heat until the flux boils

Since it's going to eventually be covered, I'd rather go the plumber route and not worry about it possibly failing later.

Wrong technique. Only heat the fitting and until the solder melts. Hope you used lead free solder!

Too much flux - not flushed soon enough. Track homes were particularly vulnerable to this issue. They would do the rough piping, but the water mains weren't hooked up - sometimes for months. There are crystals in the flux, I want to say aluminimum hydroxide? that will eat right through the copper over time, or at least start the process. Cold water lines are always a source of new contaminants, particularly oxygen, which helps the process along.

I sat on a committee group in NY approx 30 years ago. We had the CDA (copper development association) a, copper tube Mfg, Utility Manufacturing ( a chemical company that makes flux ), a plumbing wholesaler (me), Embassy Industries ( a manufacturer of baseboard, and a couple of track plumbers. We researched, tested and discussed for several months.

The baseboard pin holes seemed to happen first and were more prevalent because the wall thickness was thinner than even Type M copper. After several meetings/months, the conclusion was excessive flux use, that was not flushed out with water soon enough. During the building boom of the 70's / 80's, the water mains in these homes/apartments were not even connected up right away.

Utility changed the usage directions on the flux container to say apply sparingly. It used to say apply liberally - helped them sell more flux, but also contributed to the problem. While some of the other comments may contribute or speed the process - the pin hole starts with the chemical crystals in the flux which are corrosive in nature.

I never had a problem with flux causing pinhole leaks. The pinhole leaks I've worked on were caused by poor grounding, (electrolysis). Of course we always cleaned off any excess flux and flushed our water lines. No flux problems in any of our units. Regards, Tom