PCBA Tech - Characteristics and Selection Requirements of Flux in PCBA Production

In the PCBA production and manufacturing process of electronic products, the production of control circuits is nothing more than SMT and PCBA. The choice of auxiliary materials varies with the application of the process. SMT uses solder paste, while PCBA uses flux. A good flux, not only good welding performance, clean board surface, but also good electrical performance.

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; Non clean or clean flux can be used for other types of electronic products (such as communication, industrial equipment, office equipment, computers, etc.); General household appliances and electronic products can use cleaning free flux or RMA (medium active) rosin flux without cleaning.

(1) PCBA function of scaling powder: rosin resin and active agent in the flux produce living reaction at a certain temperature, which can remove the oxide film on the surface of welding metal. At the same time, rosin resin can protect the metal surface from oxidation at high temperature; Flux can reduce the surface tension of molten solder, which is beneficial to the wetting and diffusion of solder.

(2) PCBA requirements for characteristics of scaling powder: 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 scaling powder can be diluted with solvent, generally controlled at 0.82~0.84. The non cleaning scaling powder 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 scaling powder shall be easy to clean after welding. Stable storage at room temperature.

(3) PCBA selection of scaling powder: 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). The choice should be made according to the specific requirements of the product on cleanliness and electrical performance.

Most flux systems use a glue drip device. In order to avoid reliability risk, the flux selected for selective welding should be inert - that is, inactive - when it is in an inactive state.

Applying more scaling powder will lead to the potential risk of penetration into the SMD area and residues. Some important parameters in the welding process will affect the reliability. The most critical is that the inactive part is formed when the scaling powder penetrates into SMD or other processes at a lower temperature. Although it may not have a bad effect on the final welding effect in the process, when the product is used, the combination of the inactive scaling powder part and the humidity will produce electromigration, making the expansion performance of the scaling powder a key parameter.

A new development trend of using scaling powder in selective welding is to increase the solid content of the flux, so that the welding with higher solid content can be formed as long as a small amount of flux is applied. Generally, 500-2000 is required for welding process μ G/in2 of solid scaling powder. Except that the scaling powder can be controlled by adjusting the parameters of the welding equipment, the actual situation may be more complicated. The expansion performance of the flux is critical to its reliability, because the total amount of solid after the scaling powder is dried will affect the welding quality.

In PCBA processing, many engineers are trying to control the amount of scaling powder used. However, in order to obtain good welding performance, more scaling powder is sometimes required. In the selective welding process of PCBA processing, engineers often focus on the welding results rather than the scaling powder residues.