How to Select Flux for PCBA Processing

In general, military and life support electronic products (such as satellites, aircraft instruments, submarine communications, life support medical devices, weak signal testing instruments, etc.) must use cleaning type flux; Other types of electronic products (such as communication, industrial equipment, office equipment, computers, etc.) can use cleaning free or cleaning flux; The general household electrical appliances and electronic products can use non cleaning flux or RMA (medium active) rosin flux, or they can be cleaned.

(1) Function of flux.

The rosin resin and the active agent in the flux produce living reaction at a certain temperature, which can remove the oxide film on the surface of the welding metal. In addition, the rosin resin can protect the metal surface from oxidation at high temperatures; Flux can reduce the surface tension of molten solder, which is beneficial to the wetting and diffusion of solder.

(2) Requirements for characteristics of flux.

The melting point is lower than that of solder, and the expansion rate is more than 85%; The viscosity and specific gravity are smaller than those of molten solder, which is easy to be replaced and does not produce toxic gas. The specific gravity of flux can be diluted with solvent, generally controlled at 0.82~0.84; The non cleaning flux shall have a solid content of less than 2.0wt%, no halide, less residue after welding, no corrosion and good insulation performance; Water cleaning, semi water cleaning and solvent cleaning fluxes shall be easy to clean after welding; Stable storage at room temperature.

(3) Selection of flux.

According to the cleaning requirements, the flux can be divided into four types: no cleaning, water cleaning, semi water cleaning and solvent cleaning. According to the activity of rosin, it can be divided into three types: R (non active), RMA (medium active) and RA (full active). It should be selected according to the specific requirements of the product on cleanliness and electrical performance.