|Faulty power supply (either burning out board or making it appear it is gone).|
Missing posts in your case or metal out of place or bent causing a short on the back of the motherboard.
Trying too hard to hide the wiring and running some of them past sharp corners inside the case (common on cheap cases).
Bad memory causing problems that appear to be a burnt out board.
"earthing" usually called 'grounding' -- static electricity that may be small enough you do not notice, but enough to fry components if you touch them before grounding yourself (touching screw on switch plate or water faucet) or using a carpet or other material on work surface that holds or makes static charge.
Using steel wool to clean ANYTHING that is going into computer build.
Try bench testing out of case with just minimum plugged in.
You have to be a little bit crazy to keep you from going insane.