Where to buy WD-40?
Your circuit board will be fine. But WD-40 is a cleaner and lubricant, and leaves an oily residue which does not dry quickly. If you do not want this on the board, you have to wipe it off. If your circuit board (motherboard, etc) has stickers and similar stuff, WD-40 can dissolve some of the adhesive and cause the stickers to come off.