What is "Flux"?
- Tacky Fluxes (rework flux) – These are the consistency of toothpaste and have a number of characteristics that are handy – They stay where they are dispensed. That allows a person with a solder iron to handle the iron with one hand and the board with the other. It is also used for BGA and Flip Chip rework where there is no way to reach under the part while reflowing the solder. It is normally dispensed, but can be printed or dipped. There are 2 major use areas – A. building new assemblies with BGAs and Flip Chips,and B. reworking bad solder joints.
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- Wave Solder Flux – These are normally water or alcohol based, so they flow out like water of alcohol. – They are used for area fluxing. The most common uses are A. In wave solder machines where they are sprayed or bubled on the surface of printed circuit boards B. In flux pens where they soak a pourus wick and is wripped on (like a felt tip pen does with ink),
- Solid Fluxes – These are rarely seen, by are used inside wire. Hollw cored wire has flux in the core. The way the flux gets inside the wire – This solid flux is melted into a hot liquid, the solder is also heated and extruded out through a die into a course wire. In the center of this die is a hole that creates the hollow core, and pures the liguid flux in the core, as the wire cools it solidified the flux. The wire is then drawn down to a smaller size, but the ratio of the hollow core and the wall remains the same as the diameter is gradually reduced
What does Flux do?
Flux cleans the surface of metals, normally pads, leads and the solder powder chemically. This can mean clean the dirt and oils, but it is generally an acid and strips the oxides off the surface. Different acids are used and are called activators. A SAC305 or Sn63/Pb37 solder alloy probably has a different flux that has a different activator set to strip the metal at a temperature around the reflow point. Fluxes also coat the surface and prevent air from oxidizing the surface (protects the surface). The ability of a solder to adhere to the surface is also called wetting. Clean solder surfaces will wet quickly.
Types of Fluxes
There are 3 basic types of flux: A. RMA, B. Water soluble, C. No Clean.
- RMA fluxes are active (meaning the acids are very active and strips the surfaces quickly and powerfully). The acids are not fully neutralized after reflow and need to be cleaned. In the past, these were cleaned with CFCs like MEK or "Freon". There is an international agreement called the Montreal Protocol phasing out the production of the cleaning agents used with RMA fluxes. This forced a shift to No Clean and Water Soluble fluxes. The military and older assemblies still use these fluxes. Also, water with a saponifier (soap) can be used. Still the market has switched away from RMA fluxes.
- Water soluble fluxes. These are about 10% of the North American solder paste market. They were much larger in the 1990s. This is the best choice for Printed Circuit Boards that need to be fully flux residue free (cleaned) for hi reliability reasons.
- No Clean fluxes are the most common. The residues that remain after reflow are normally small and clear. They are hard and non-reactive. The acids in the paste are volatilized or encapsulated so there is no chemical activity. This is often measures with a surface insulation resistance test. If the residue cannot conduct current, it cannot corrode or short out the circuit.