Although capillary action basically is the magic behind ensuring proper filler metal distribution into a joint, six basic steps also are necessary to make sure that the design and engineering of the joint helps lead to a good brazed joint.

Step 1: Ensure good fit and proper clearances.
Brazing uses capillary action to distribute molten filler metal between the surfaces of the base metals. So when you're brazing, maintain a clearance between the base metals to allow capillary action to work most effectively. In almost all cases, this means a close clearance. Optimal clearance or joint gap for most filler metals is 0.0015 inch, but typical clearances range from 0.001 to 0.005 in.
In everyday brazing, clearances don't have to be overly precise to obtain a sufficiently strong joint. Capillary action operates over a range of clearances, so you have a certain amount of leeway. In everyday shop practice, an easy slip fit typically will produce an adequate brazed joint between two tubular parts. Keep in mind that generally, as the clearance increases, joint strength decreases. Capillary action stops around 0.012 in. If you're joining two flat parts, you can rest one on top of the other. The metal-to-metal contact is all the clearance you'll usually need, because the average mill finish of metals provides enough surface roughness to create capillary paths for the flow of molten filler metal. Highly polished surfaces, on the other hand, tend to restrict filler metal flow.
When you're planning your joint clearances, remember that brazed joints are made at brazing temperatures, not at room temperature. Take into account the coefficient of thermal expansion of the metals being joined, particularly with tubular assemblies in which dissimilar metals are joined.
How much allowance you should make for expansion and contraction depends on the nature and sizes of the metals being joined and the configuration of the joint. Although many variables are involved in pinpointing exact clearance tolerances for each situation, keep in mind this principle: Different metals expand at different rates when heated.

Step 2: Clean the metals.
Capillary action works properly only with clean metal surfaces. If they're coated with oil, grease, rust, scale, or dirt, you must remove these contaminants or they'll form a barrier between the base metal surfaces and the brazing materials.
Cleaning metal parts seldom is complicated, but you have to do it in the right sequence. Oil and grease should be removed first, because an acid pickle solution aimed to remove rust and scale won't work on a greasy surface. Start by getting rid of oil and grease. In most cases you can do this either by dipping the parts into a suitable degreasing solvent, by vapor degreasing, or by alkaline or aqueous cleaning. If the metal surfaces are coated with oxide or scale, you can remove those contaminants chemically or mechanically. For chemical removal, use an acid pickle treatment. Make sure that the chemicals are compatible with the base metals being cleaned and that no acid traces remain in crevices or blind holes. Mechanical removal calls for abrasive cleaning.
Particularly in repair brazing, where parts may be very dirty or heavily rusted, you can speed the cleaning process by using an emery cloth, grinding wheel, or file or grit blast, followed by a rinsing operation. Once the parts are thoroughly clean, flux and braze them as soon as possible to reduce the chance for recontamination of surfaces by factory dust or body oils deposited through handling.
Note that certain cleaning mediums will leave a residue and embed in the surface, making it unwettable.

Step 3: Flux the parts.
Flux is a chemical compound applied to the joint surfaces before brazing. Its use, with a few exceptions, is essential in the atmospheric brazing process. This is because heating a metal surface accelerates oxide formation, the result of a chemical reaction between the hot metal and oxygen in the air. If you don't prevent these oxides from forming, they'll inhibit the brazing filler metal from wetting and bonding to the surfaces.
A coating of flux on the joint area shields the surfaces from the air, preventing oxide formation. It also dissolves and absorbs any oxides that form during heating or that were not removed completely in the cleaning process.
You can apply flux in any way to the joint as long as you cover the joint surfaces completely. Flux conventionally is made in a paste, so it's usually most convenient to brush it on. But as production quantities increase, it may be more efficient to apply the flux by dipping: dispensing a premeasured deposit of high-viscosity flux from an applicator gun.
Typically, you apply flux just before brazing if possible so it has the least amount of time to dry out and flake off or get knocked off the parts in handling. Choose a flux that's formulated for the specific metals, temperatures, and conditions of your brazing application.