Flux chemistry

Up to the mid-1970s, the predominant flux chemistry was rosin-based. Today, although rosin fluxes are still in use, great advances have been made in flux chemistries and a large variety of formulations with very different chemistries are being used. 

Biocca, Peter, Flux Chemistries and Thermal Profiling Avoiding Soldering Defects in SMT Assembly, Proceedings ofSMTA International, Chicago, September 30,2001. 

Do not use a saponifer or solvent other than water unless the flux chemistry has been tested and qualified for compatibility and unless saponification or other solvent is in use with standard post-solder board cleaning. 

Very often, completed boards that are prepared with a water-clean flux formulation are repaired using no-clean flux. This strategy precludes the need for another wash cycle. In another situation, there may be parts that are on the board or associated with an assembly that are not compatible with a wash cycle (electromagnetic interference [EMI] shields, enclosure pieces, etc.) in such cases, a no-clean repair is necessary. However, once a board is repaired with a no-clean flux, it is important not to subject the board to an aqueous cleaning process for the reasons previously described. Conversely, aqueous cleaning must be used if a water-wash flux chemistry is chosen for repair. 

No visible residue fromcleanable or any activated fluxes is allowed. Production operations may not be required to remove cleanable residues if qualification testing is performed that demonstrates no need for cleaning the assembly. No-clean or low-residue flux chemistries may be allowed during the soldering process. However, it is critical that the cleanliness of the elements such as the bare boards and components be specified, controlled, and closely monitored otherwise, the contaminant build-up can far exceed allowable limits for the end item s functional performance. 

Ready, W. I, and Turbini, L. I, The Effect of Flux Chemistry, Applied Voltage, Conductor Spacing, and Temperature on Conductive Anodic Filament Formation, Journal of Electronic Materiab, Vol. 31, No. 11,2002, pp. 1208-1224. 

It is important to note that lead-free soldering requires higher processing temperatures. Thus, it is anticipated that flux chemistries are changing to deal with this fact. 

Low Solids Flux. Until the mid-1980s, liquid soldering fluxes were formulated in 25 to 35 wt% solids or non-volatile liquid. Then flux chemistries changed and new formulations. 

When utilizing a lead-free solder in a wave process, the machine and process must be modified to ensure the alloy selected is compatible with the internal parts of a wave machine. Lead-free solders are not drop in replacements , for eutectic Sn-Pb solder. Issues such as lead contamination, flux chemistry compatibility, dross removal equipment, and dissolution of coatings on the surfaces of a wave machines internal parts into the solder alloy must be considered. 

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