I remove any relays, ICs, DIPs that could get water trapped in them. Wash with toothbrush in sink with soapy water. Wash again with toothbrush and naptha. Sit in front of a box fan. Naptha works great at removing flux and displacing water. It also dries with no residue (alcohol leaves behind white powdery stuff).

Wash and rinse with an old paint brush and dish soap in a wash tub type sink. Sometimes I spray down with Simple Green (or sometimes Mean Green). Let air dry standing on end in a tube for a few days (usually takes me that long before I am ready to mount back into the machine).

Have never removed anything other than the batteries (which make sure you do before getting the board(s) wet.

Doing the soap and water for over 25 years (the Simple/Mean Green part I started a few years ago and only on the really nasty boards). The 'dust' just with soap and water.

I will have to try the simple/mean green. I got some right now that are super, dirty.

If you don't remove flipper relays water gets trapped in them. Same with dip switch banks... if this causes a long term issue... not sure, but i avoid it.