No-clean flux is a type of mild rosin core flux with reduced flux activity. It’s less aggressive in nature, designed so that the residue left after soldering contains substances like binders and pH stabilizers to minimize reactivity. The ‘activity’ of solder flux refers to its ability to remove oxidation from pads and pins during the heating process, which is crucial for proper solder wetting. Ideally, flux acts as a heat-activated acid that evaporates after the solder bonds. However, some residue always remains, which can affect the board.
No-clean fluxes, particularly the less active ones, are meant for use with fresh boards and components that have little to no oxidation. These fluxes struggle with older parts where oxidation is heavier, leading to poor wetting. On the other hand, highly active fluxes work wonders for cleaning heavily oxidized surfaces and preventing oxidation for longer periods. However, their downside is that the leftover residue remains reactive and, over time, can attract moisture, leading to corrosion. In one case, a board exposed to ~85°F and 75% humidity over a period of 2-2.5 years lose entire footprints due to highly active flux residues left behind.
Interestingly, cleaning flux residue can sometimes accelerate corrosion, as removing the protective binders can expose active components to the environment. If you want to clean this residue, a mixture of 70%-90% isopropyl alcohol and water works well. You can gently scrub the areas with a toothbrush to remove any thick layers.
The reason cleaning can sometimes accelerate corrosion is due to the nature of flux. Flux residue is a combination of inert substances and more reactive components (like metal halides). When you clean, the easier-to-remove inert substances get scrubbed away, exposing more of the reactive elements. These reactive residues, even in tiny amounts, are more susceptible to humidity, which can lead to corrosion over time if left unprotected.
That said, if you’re using a “no-clean” flux without halides or other highly active components, the risk is lower. However, if you see blackened or “burnt” flux, it suggests improper reflow, meaning the flux wasn’t fully activated. In that case, thoroughly cleaning is important to avoid long-term damage, especially with water-soluble fluxes, which must be completely removed with warm water to prevent corrosion. So, while cleaning isn’t always necessary with “no-clean” flux, if you prefer a clean board, use the right method and tools to ensure all active residues are effectively removed.