Chosen Solution
i took apart this a1278 and now it wont turn on. it seems to get power and is fully charged, but i cant hear fans spinning nor the boot up chime. i have checked every ribbon cable for issues and all seems fine, also the battery seems fine.
With any dead system you really need to trace out the power path from the battery to the logic board. I would go back following the iFixit guide here MacBook Unibody Model A1278 Logic Board Replacement I would walk it back to Step 23, Inspect each cable connector of damage. You either missed something or something got broken in the process
The SATA cable is not pluged properly or try to disconect and reconnect fan cable or check if battery cable is properly connected.
