Classification and selection of fluxes for electronic components

Flux is a process material used during the soldering process to clean the surface being welded and prevent it from oxidizing again. According to the form, it can be divided into three types: solid, liquid, and paste.

In terms of flux classification, the international main basis is IPC-J-STD-004 "Flux Requirements". According to the main chemical composition of solids content (non-volatile), fluxes can be divided into four categories: rosin type (RO), resin type (RE), organic type (OR) and inorganic type (IN). It is further subdivided according to the corrosiveness or conductivity of flux or flux residue, which can be divided into low activity (L), medium activity (M) and strong activity (H). The 0s and 1s in the flux type name indicate that the flux is halide-free (<0.05%) and halide-contained, respectively. A certain type of flux can only be classified as a certain flux if it meets all the test requirements. In the practical application of flux, there are different requirements for flux in different soldering scenarios. For example, in the precision welding of electronic equipment, due to the small solder joint spacing and high electrical performance requirements, it is necessary to use fluxes with moderate solid content, reasonable acid number and low halide content, which can not only ensure good welding results, but also avoid electrical safety hazards caused by excessive residues or halide conductivity.

Classification test requirements forflux activity Flux is a typical formula product, and its technical content is fully reflected in the selection and ratio of ingredients, which is also the core technology of flux production units. Therefore, the user can only understand the basic properties of the flux through various indicators. The technical specifications of flux are usually divided into three categories, and the test method can be based on IPC-TM-650: one is related to the basic physical properties, such as color, specific gravity, solid content (non-volatile), etc.; the second is related to performance, such as acid value, wetting ability, spreading ability, etc.; The third is related to corrosiveness and electrical safety, such as water extraction conductivity, halogen content, copper mirror corrosion, surface insulation resistance, electrochemical migration, mold, etc. Generally speaking, the higher the solids content of the flux, the better the solder mask, but the flux with high solids content also has relatively more residue after soldering, so to find the balance between solder mask and residue, the mainstream solids content is between 2.0% - 7.0%. The acid in the flux removes the oxide film, and the higher the acid number, the stronger the solder mask. The acid number of low solids fluxes is typically 17 - 35 mgKOH/g, and the acid number of high solids fluxes is usually above 50 mgKOH/g. The conductivity of water extraction reflects the surface electrical insulation resistance of the fluxfrom the side, and the resistivity is small when the conductivity is high. Halides are chloride equivalents (%) in the solids (non-volatile) composition of fluxes.

In mass production, such as batch soldering of PCB boards, in addition to the six aspects of evaluating the quality of solder joints, it is also necessary to pay attention to the cost and efficiency of flux. Some fluxes with high reactivity, high solids content may perform well in soldering, but may not be preferred if the cost is too high or the process of use on the production line is too complex. For some special soldering materials, such as some superalloys or metals that are more sensitive to corrosion, more care must be taken when choosing a flux. The flux needs to be specifically tested to ensure that it does not corrode the material being soldered and that it achieves good soldering results under special soldering conditions. The total halide content in flux is the sum of Cl-, Br-, F-, and I- measurements. As an active agent in flux, halides were found in early flux formulations, but the conductivity of halogen ions also brought hidden dangers to the electrical safety after soldering, so cleaning after soldering was an essential link. When it comes to flux selection, users mostly evaluate based on the quality of the solder joint. The key points of flux evaluation are mainly carried out from six aspects: (1) bridging defect rate; (2) through-hole tin permeability; (3) The fullness of tin on the pad; (4) PCB surface cleanliness after soldering; (5) In - Circuit Test first-pass rate; (6) Surface insulation resistance of flux residue. There are many reasons for the bridging phenomenon, such as PCB transfer direction, pin density, pin material characteristics, etc., and the main thing for flux is to evaluate its coverage and solder mask ability. Through-hole tin penetration, aside from design reasons, is mainly to evaluate the wetting ability of the flux. The fullness of solder on the pad is mainly evaluated according to the coverage area of the solder on the pad and the amount of tin, such as IPC610 stipulates that 75% of the pad coverage is the minimum requirement. The surface cleanliness of the PCB after soldering mainly evaluates the resistivity of the extraction solution of the flux residue on the board surface after soldering, GJB5807 criterion requirement for tertiary electrons is that the ionic residue should not be greater than 1.56μg NaCl/cm². In - Circuit Test (ICT) first-pass rate is to use the test between the probe and the solder joint to determine whether the flux residue has an impact on the test, the general impact should be less than 5%, and the first-pass rate is closely related to the formula of the flux. The surface insulation resistance of the flux residue can indicate whether the quality of the materials and processes used in the production meets the requirements, GJB5807 the surface insulation resistance is not less than 100MΩ.

In addition, with the increasingly stringent environmental protection requirements, the research and development and application of halogen-free fluxes have also received more and more attention. Halogen-free fluxes can reduce the environmental impact caused by halogenated emissions while meeting the welding requirements, which has also become an important direction for the development of fluxes. Both manufacturers and users need to pay close attention to the development trend of fluxtechnical indicators in order to meet their own needs while adapting to changing market and environmental requirements.