Can the motherboard be affected after it was touch by rainwater? Accidentally left the the whole computer below water rain drops... Even though without power connection to it...
Of course it can.
Depending on how the board was handled after getting wet is key to its survival or lack thereof. Rainwater is not likely to be contaminate free and could leave semi-conductive (or conductive) particles or residue on contacts or traces, or left wet for long enough could cause corrosion.
Rinse it with distilled water as soon as possible then dry it quickly. I use a hair blower then leave a fan on it for a couple of hours.
As far as i know, white vinegar and distilled water (1:3) was my best option so far (but only if You see that something corroded, if not, as zuldan said before me, only distilled water would be sufficient).
Leave it over night, and dry it well. I use compressor (2 bar max) to dry almost fully, and then i put the motherboard in oven for 45 minutes at 60 degrees (place it on something non conductive as wood or card box parts, don't leave it on metal parts in the oven).
One personal experience although not mobo. My old 15" Acer monitor from 1994 was sitting outdoors in the rain for several years. Summers and winters and we have had -35 ... +30 centigrade temperatures here up in the north. The monitor is still alive and kicking after drying and cleaning. Later I recapped it and resoldered flyback transformer, horizontal output stage + other relevant components. Biggest problem is the corroded VGA D15 connector. The cable is fixed and I should change or extend it to avoid damage to display card VGA connector.
Here's a pic of the vga connector 😀
Fibreglass soaks up water like a sponge and it can take a long time for it to dry out completely.
