When soldering, you often need to use a flux to create proper joints. There are many types of flux, and they can be found in liquid, spray, or pen form, as well as inside solder wire itself. Below is a brief overview of a few types of fluxes. As always, if you have any questions on the application of a specific product, the manufacturer is the best place to go for up to date information.
No-clean fluxes are rather weak, and the residue that they leave behind will typically not harm your devices. This is why they are called no-clean; the residue it creates can be left on the board and it will not corrode your parts. If you are applying a conformal coating to your PCB then you will need to clean off the residue from no-clean fluxes so you can ensure even protection.
Mildly Activated Rosin
Mildly activated rosin fluxes are a bit stronger than no-clean, but also typically leave a non-corrosive and non-conductive residue. These fluxes vary from manufacturer to manufacturer, so be sure to read the directions before leaving the residue to sit on a sensitive component. Mildly activated rosin fluxes also have a high thermal stability, ensuring that preheating does not degrade its solubility.
Activated rosin fluxes are the next step up from the mildly activated rosins. The added power in the activated rosin can remove oxides that no-clean and mildly activated fluxes cannot. The residue left from activated rosins may be corrosive, so check out the data sheet or talk to the manufacturer to see if your particular flux needs to be cleaned off the board after soldering.
Inorganic acid fluxes are also very strong, perfect to help solder difficult metals. This can be extremely helpful for heavily oxidized metals, or for materials that have a low solderability. Again, a corrosive residue is left behind, so a hot water or solvent rinse is required to protect your board.
These fluxes are made to be easily washed off, especially during wave soldering processes. Water soluble fluxes are some of the strongest, and will definitely corrode your PCB if left on, so be sure to thoroughly clean off any residues left behind by this flux.
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