Most manufacturers' installation guides recommend using "no flux" on copper. To meet this requirement a phosphorus/copper/silver braze rod is used. The phosphorus content makes these alloys "self-fluxing" on copper. Products like 15% silver content Stay Silv® 15, or economical Dynaflow® are excellent choices.
When brazing copper to brass or to steel a flux is always required. A flux is also required when using high silver braze alloys like Safety Silv® 45 or 56. Brazing fluxes contain 15 – 35% water so we want to keep excess flux from inside the lines. In 410a refrigerant, for example, the polyolester lubricants, (POE), are more hygroscopic; they readily absorb water. The potential problem is moisture can cause the breakdown of POE oils into organic acids.
Here's a suggestion. Always use a brush to apply a thin coating of flux and don't flux to the end of the tube. Do not apply flux to the fitting socket. When heated, flux liquefies and capillary action will draw it to the fitting.
If flux is used the flux will turn semi-transparent close to brazing temperature. This is a visual indication you are ready to apply the brazing rod. The flux should be the same consistency on both parts. If there's an obvious difference one side may require more heat.
Braze rods are also available flux coated. These braze alloys have a uniform covering of flux on the outside of the rod. While offered primarily as a convenience, (you don't have to carry a separate flux jar), there is another benefit. The coating melts during heating and flows into the joint ahead of the melting braze alloy. This action and the uniform coating thickness minimize unwanted flux inside the connection. NFPA 99 allows using flux coated rod on 7/8" or smaller dissimilar metal joints.
After brazing remove flux residue from parts by washing with a wet rag. If parts are overheated and flux is difficult to remove, mechanical cleaning may be required.