Hi everyone, I recently purchased a dead D620 because I like to fix broken computers or LCD monitors. I am not a professional by any means, but I get by on patience and weak knowledge of electronics.
So, since most computer shops will not try to find a fault in a mother board, I thought I would give it a shot. I have heard of a lot of no power on situations with the Latitude series so I hope this helps.
What I found were two SMD's coming directly from the power jack on the mother board with cracked solder joints. You will need a super magnifier for this one. I believe these SMD's are diodes, they probably get warm and cold so much the solder cracks. You will get sporadic power on and frequent low voltage glitches through out with the poor connection and a weak battery. I re-soldered one of them that was actually falling off the board. the computer still did not work. I went back in, and found a hair line crack on the second SMD. I pushed on it and got varying voltage on the other side of it. I re-soldered that one and know have a nice working Latitude D620 for my wife.
You will of course have to be handy enough to dismantle the entire computer, but give it a shot before replacing the whole motherboard. I wish I had this information before, but now we all do.
Good luck!

One to store for reference.

It would be nice if the solder whizzos here could also come in and share what they've found on repaired laptop mobos.