I just finished making probably 30 connections while replacing my boiler. Luckily, the vast majority came out fine. Unfortunately, two in an almost inaccessible space leaked. They were next to the wall, behind the air handler and indirect tank at arms length, and I really had to do that by feel since to reach it with the torch and solder, I couldn't get my head into a position where I could see...very annoying. So, I didn't feel too bad that it didn't work! I ended up cutting that section out, redoing most of those with soldered fittings, and left one for a Sharkbite as I did not feel confident trying to redo that, again, where I really couldn't see what was happening. Ever had to try to get enough leverage to seat a Sharkbite when it's so far away you can barely get your hand around it? That was a bear, but it's in and isn't leaking. I really didn't want to cut out the indirect, move it out of the way, and fix that one joint, then requiring a huge amount of work putting it all back together. This is all leading up to what did go right.
When you don't solder pipes every day, I really prefer to use Oately #95 tinning flux (there may be other tinning fluxes out there, but that's what I've found and use). This has powdered solder mixed in it. SPread a good coat on things, and it gives you a visual of when the joint is hot enough to add your solder because you can already see that solder is melting. You need to keep moving your torch, and primarily heating the fitting rather than the pipe - you do need to heat both, but get the fitting hot. You can tell that a joint is overheated by the color...copper will turn sort of a purplish as you get it hotter than necessary. I feel that the required water based fluxes are easier to burn than the old (no longer approved) acid based ones. The tinning flux is a good crutch for someone that doesn't do it all of the time, but is more expensive. When you add up the time and inconvenience of an incorrectly done joint, it may come out cheaper. If you do this often enough, you'll learn.
Anyway, heat the joint and periodically pull the flame and see if the solder will melt. If it doesn't, keep heating. Once it's hot enough to melt, get the solder all around the joint. If you've evenly heated it up, the solder will flow and wick into the full joint. When I pulled that failed joint apart, it was more the fact that I wasn't able to properly heat all around the fitting, as it wasn't actually overheated, just lacking enough solder. An overheated one, if you take it apart, will have blackened voids/scorch marks where you've burned out the flux and created oxides. The copper must be clean for the solder to flow, and the flux, before you burn it out, helps to prevent oxygen enabling the corrosion.
