Moisture Protection PCB with soldering flux

Throughout the year, various experiments have been carried out with gel soldering flux Diamond Flux FN231 and liquid soldering flux Diamond Flux FN232.
The purpose of this experiment was to demonstrate the protective properties of soldering fluxes in an aggressive salty environment. The environment was not chosen by chance - strong humid winds, rain, 40 degrees Celsius during the day and about 15 degrees Celsius at night. This test PCB was exposed to outdoor conditions for 30 days without any protection. PCB parameters - FR4 with polar open contours with a width between tracks of 0.5 mm. Manufactory Tinned Copper on Sample 2 - new board.

The Sample 1 left bottom board of the PCB was tinned with FN231 gel flux using a soldering iron at 320 degrees Celsius.

The Sample 1 right bottom board of the PCB was tinned with FN232 gel flux using a soldering iron at 320 degrees Celsius.

The top boards of PCB on the Sample 1 did not undergo any processing and were left as is, an example of a clean board in Sample 2.

As can be seen in Top boards of 1 and 4 pictures, the top untreated parts of the PCB, including the circular connection pads, were completely covered in tin oxidize and corrosion. This happened in humid, salty ocean air over 30 days. Both lower areas did not show any reaction and remained under a layer of flux. For engineers dealing with marine or coastal electronics, cleaning flux residues and applying a conformal coating ensures reliable long-term performance according to industry best practices and IPC guidelines. --> Is it possible to evaluate the hygroscopic properties of the tested fluxes? Partly yes, if there is high hygroscopicity, one could notice oxides on the lower parts of the printed circuit board, but they are not there. This coating should not be considered a full-fledged varnish; it is just unwashed flux residues. At the same time, we see dust adhering to the bottom of the printed circuit board. During the time that the unhardened flux residues were in an open environment, any atmospheric dust that we see perfectly adhered to them. These residues can conduct electricity and absorb moisture. Residues from professional soldering flux may slow down corrosion, but they are not a substitute for proper PCB protection. Flux removal and the use of proper moisture-resistant coatings are critical to the long-term reliability of PCBs in salt air environments.

Anticipating your question - why not dry the board after soldering so that atmospheric dust does not stick to it and send the printed circuit board to life? The flux residue will be stickier than the waterproof varnish. These materials are nevertheless created for different tasks - for soldering and for protecting printed circuit boards. Remember, to ensure 100% corrosion and oxidation free operation, it is necessary to remove flux residue and apply a special, proven protective layer, which will ensure long life of the PCB, especially in harsh coastal or humid conditions.

Thank you for reading!