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38 SMT007 MAGAZINE I JULY 2021 Types of Fluxes ere are four major types of fluxes and each one further subdivides into six different cat- egories. In other words, we have 24 different types of fluxes to choose from. However, the last category, IN (inorganic flux), is not used in the electronics industry since they are too aggressive for electronics products. In effect we have only 18 types of fluxes to choose from instead of a total of 24—still a daunting task. Please see Table 1 for a quick summary of all 24 different categories of fluxes. e major four categories of fluxes are: rosin (RO), resin (RE), organic (OR), and inor- ganic (IN). Each flux type has three activ- ity levels to choose from (low, medium, and high). ese levels—L, M, and H—come with or without halides. When you do the math, there are a total of 24 different flux categories to choose from. Fluxes without halides have 0 at the end of their designations, while fluxes with halides have 1 at the end. For example, rosin flux without halide will be called ROL0 and rosin flux with halide will be called ROL1. ese designations repeat for RE, OR, and IN as well. Halides in L1, M1 and H1 are less than 0.5%, 0.5 to 2%, and more than 2%, respec- tively. e activity of the halide-free fluxes comes from naturally occurring acids. e higher the flux activity, the better the soldering results. However, more active fluxes must be cleaned properly to prevent corrosion in the field. No-clean fluxes can be RO or RE with or without halides. But OR fluxes must be with- out halides (ORL0) to be classified as no-clean. Due to high activity levels, inorganic (IN) fluxes, commonly used by a plumber, are not used in the electronics industry. Types of Cleaning Materials and Processes It is generally thought that cleaning surface mount assemblies is very difficult because, for example, stand-off heights between the sur- face mount components and the board are small, creating a tight gap that may entrap flux Fluxes do a few other things at soldering temperatures. ey reduce the surface ten- sion of solder causing it to spread and promote wetting, which in turn makes it possible to form strong and reliable solder joints. In addi- tion, they protect solder from further oxida- tion during soldering. For additional oxidation protection, at times we also use nitrogen. But nitrogen plays no role in the formation of inter- metallic bond while it is essential for interme- tallic bond. Consequences of Using Flux We need flux but there is a downside to using it. As we just discussed, we need flux to accom- plish good solder joints. However, once we are done with soldering, what to do with those flux residues? Do we leave them on the board or get rid of them? e answer is: it depends. You can leave them alone or you must remove them depending on how harmful those residues are. e types of flux residues or contaminants that require cleaning are determined primarily by the type of flux used. Halides, oxides, and var- ious other contaminants are introduced during storage and handling as well. e use of aggres- sive fluxes makes soldering easier even if com- ponents and boards are slightly oxidized and contaminated. e cleaning process to be used is selected based on type of flux, types of contaminants, and type of assembly. For example, mixed assem- blies using both SMT and through-hole com- ponents may need one cleaning process aer reflow soldering and another one aer wave sol- dering, but a two-sided full SMT assembly may need only one cleaning process aer the second side is reflowed. When no-clean fluxes are used, boards may not require cleaning. In no-clean fluxes, chem- icals such as carboxylic acids activate and per- form their deoxidizing function, then burn off and leave no active chemicals on the surface. But no-clean fluxes require perfect surfaces to solder. Otherwise, solder defects will be too high.