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Typically no-clean fluxes are especially sensitive to process conditions, which may make them undesirable in some applications. [6] Other kinds of flux, however, require a cleaning stage, in which the PCB is washed with solvents and/or deionized water to remove flux residue.
Flux residues also tend to outgas in vacuum and space applications, and traces of water, ions and organic compounds may adversely affect long-term reliability of non-hermetic packages. Trapped flux residues are also the cause of most voids in the joints. Flux-less techniques are therefore desirable there. [13]
No-clean flux residue is acceptable on all 3 classes of PCBs as defined by IPC-610 provided it does not inhibit visual inspection, access to test points, or have a wet, tacky or excessive residue that may spread onto other areas. Connector mating surfaces must also be free of flux residue. Fingerprints in no-clean residue are a class 3 defect [19]
There is a wide variety of cleaning agents for these fluxes. A no-clean flux is designed to leave only small amounts of inert flux residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. However, these pastes need a very clean assembly environment and may need an inert reflow environment.
Three mechanisms allow the disruption of the protective deposits. The most well known, although now the least common, is the presence of carbon films on the bore. Stagnation and flux residues are the most common initiation mechanisms that have led to Type 1 pitting failures in the last ten years.
Non-metallic inclusions are chemical compounds and nonmetals that are present in steel and other alloys.They are the product of chemical reactions, physical effects, and contamination that occurs during the melting and pouring process.
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Welding flux is a combination of carbonate and silicate materials used in welding processes to shield the weld from atmospheric gases. When the heat of the weld zone reaches the flux, the flux melts and outgasses. The gases produced push the atmospheric gas back, preventing oxidation (and reactions with nitrogen).