Where the solder comes from.

A solder-ring fitting has a measured quantity of solder already formed into a groove inside the socket. Heat the joint and that solder melts and fills the capillary gap on its own. An end-feed fitting is a plain socket with no solder at all; the installer feeds solder wire to the joint mouth once it is up to temperature.

The consequences are cost and control. End-feed fittings are significantly cheaper, which matters on a large job. Solder-ring fittings are faster and deliver a consistent, correct quantity of solder every time regardless of who makes the joint, which matters where the workforce is mixed or the work is being audited.

Both produce an identical joint when made correctly. The difference is entirely in how much the result depends on the installer's judgement, and in what the fittings cost.

Enough to see a continuous ring appear all the way around the mouth of the fitting, and no more.

Capillary action draws molten solder into the narrow gap between tube and socket and holds it there. The joint is full when solder appears right around the circumference at the mouth - that is the visual confirmation that the capillary space has filled from the back of the socket forward.

A useful rule of thumb is that the length of solder wire required is roughly the diameter of the tube, but the visual check is what matters. Feeding more solder after the ring has formed simply produces a large external blob, which looks worse, wastes solder and hides the joint rather than improving it.

Check all the way round. A ring visible on the side facing you but not on the far side means the joint has filled unevenly, usually because the heat was applied from one side only.

Because a partly filled capillary gap can still hold pressure initially, and then fail as the system cycles.

If the joint is under-filled, solder occupies only part of the overlap. That is often enough to seal against a static pressure test, particularly a short one. But the joint has a much smaller bonded area than it should, and thermal cycling, vibration and pressure fluctuation work on that reduced area until it opens.

The usual causes are heating the joint from one side so solder is drawn toward the heat and away from the far side, insufficient heat so the solder chills before reaching the back of the socket, poor cleaning so flux cannot do its job, or moving the joint before the solder has solidified.

The defence is procedure rather than testing: heat evenly around the fitting, confirm the full ring, and do not disturb the joint while it cools. A test proves a joint is not leaking today.

Square cut, deburred, cleaned bright, and fluxed - and the cleaning is the step that gets rushed.

Cut the tube square with a rotary cutter; a saw leaves an angled end that does not seat fully in the socket. Ream the burr from the bore, because a burr left inside creates turbulence and, in a water system, a site for erosion corrosion.

Clean the outside of the tube for the full insertion depth and the inside of the socket, using wire wool, an abrasive pad or a fitting brush, until the copper is bright. Oxide film prevents the solder wetting the surface, and copper oxidises visibly within days.

Apply flux sparingly to both surfaces immediately after cleaning, assemble, and make the joint promptly. Flux is aggressive - excess left inside the pipe continues to attack the copper and causes pinholing later, so wipe the outside clean after soldering and flush the system thoroughly before use.

Lead-free solder for anything carrying drinking water, with the flux matched to the solder and to the service.

Lead solder is prohibited on potable water systems throughout the world, and lead-free tin-copper or tin-silver alloys are used instead. These melt at slightly higher temperatures than the old lead alloys and are less forgiving, which is part of why joint preparation matters more than it used to.

Fluxes range from mild water-soluble types to aggressive self-cleaning pastes. Aggressive fluxes make a poor joint easier to achieve and are correspondingly damaging if residue is left in the system. For potable water, the flux must carry the local drinking-water approval as well as the solder.

Gas installations have their own requirements and in many markets require brazing rather than soft soldering for certain applications. Check the governing standard for the service and market rather than carrying practice across from one to another.