Mold (Fungus) build up on PCB
Pictures are too bad to determine what is this. On second one - whitest "thing" looks like crystal (some salt?), that yellow may be tin sulfate (salt), but I don't know. Maybe you can figure this out with microscope or take sample and try some chemical analysis.

Use some rubber gloves and be careful. If this is made in Asia - they could use solder with lead. Lead salts are very dangerous.



If this is not fungus - this is corrosion caused by acid (from flux that was not cleaned?). Corrosion is salt (result of metal + acid reaction).

You can try to clean everything in 99%+ isopropanol, it will dissolve salts and acids.



If this is fungus 

You can eradicate them by putting your boards in electric oven. Don't use gas oven - burning gas produce water vapor so gas ovens are not "dry".

Do this if you are sure that is not corrosion, because corrosion will speed up and "eat metals" on your boards in high temperature very fast.

If parts on the board are not vulnerable to heat "bake" them in dry air:

140°C for 3 hours or 160°C for 2 hours

This should sterilize boards.

If you have parts that don't like heat (plastic parts, ADC converters, voltage references) - you can try:


99%+ isopropanol bath for 15 minutes
Dry it (not in oven!!!, be careful - isopropanol is flammable and ignites below 400°C)
"Bake" in dry air - 100°C (or a bit above water boiling point) for few hours




I would not recommend to put these boards in your products.