Residual removal, solder-flux

Trichlorotrifluoroethane (CFC 113) was widely used in the electronics industry for the removal of soldering flux residues from printed circuit boards, for degreasing components in the precision engineering industry, and for the dry cleaning of garments. 

Is Your Solder Pad Joining "Hairy" As part of a solder/flux/ cleaning procedure, the residue In Figure 10 (Insert) was produced. The residue consists of 6xl0pm leaf-like crystals on a ceramic substrate. The location of the residue prevented Its analysis In-sltu, so extraction replication(5) was used to remove some of the crystals for analysis by several small area techniques, as needed. Electron probe microanalysis showed lead, carbon and oxygen, which could Indicate many possibilities. 

An active flux chemically removes the oxide film, has an acid base, and is highly corrosive. These fluxes are usually hydrochloric acid in which zinc has been dissolved to form zinc chloride, known as killed spirits . Any joint prepared using an active flux must be thoroughly washed in warm water when soldering is completed, to remove any flux residue. For this reason an active flux is not suitable for electrical applications. 

The contamination that must be removed after the soldering process is predominantly from flux residues. The removal of flux residues from the soldered assemblies (6) is generally the most critical application. Thus it is the important fliat the assemblies are cleaned. A subsequent coating process demands a very clean and residue-free surface to assure long term stability of the coating against environmental stresses such as humidity. ... 

The eomplete proeess in based on a closed-loop principle. Consequently, the cleaning process merely liberates those substances that are to be removed from the pe-boards (i.e., soldering flux residues). Closed-loop operation is not only advisable for eeologieal reasons, but is also a very sound economic proposition as it significantly minimizes process costs. 

The final step in the soldering process is the removal of flux residues from the completed circuit board. No-clean fluxes, as the name implies, produce residues that need not be removed from the assembly after soldering. The corrosive activators are locked up in a hard, polymerized residue. However, no-clean flux residues can potentially interfere with in-drcuit testing (probes) and inspection, degrade cosmetic appearance, inhibit rework activities, and lessen the adhesion of conformal coatings. In the event that any one of these factors becomes critical, it is best to switch to a cleanable flux than to develop a cleaning process for a no-clean flux. 

The impact of Pb-free technology on wave soldering has largely occurred in the equipment performance. It has been determined that the same solder bath temperatures that are used for Sn-Pb processes (250 to 270°C) are suitable for the Sn-Ag-XCu Pb-free alloys. Therefore, excessive dross formation and flux residue removal have not become a significant problem during equipment operation. The lack of shiny fillets with the Sn-Ag-XCu alloys has been addressed by modified alloys having Ni and Ge additions that alter the solidification process, which leads to shinier fillet surfaces. 

No-clean solder flux formulations can be used for rework and repair even if the PCA was manufactnred with an aqueous clean chemistry, although the reverse is not recommended. Sometimes when a no-clean board is subjected to an aqueous cleaning, the no-clean flux residue takes on a white, gnmmy characteristic that is conducive to dendritic corrosion, which can result in soft or hard electrical shorting. Saponified aqueous cleaning can be nsed, but must be tested for effectiveness in removing the polymerized flux residue from the no-clean process and for compatibihty with the selected no-clean flux to avoid generation of corrosive byproducts. This is particularly important under connectors, area-array devices, and other components with low headroom between the underside of the body and the PWB surface. 

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