Boots only up to OS log in screen, then shuts down

I got this with moisture damage and managed to clean up about 3 green corroded spots below the logic board with flux and heat and then it seemed fine. But after a few days it had trouble booting again. When I make it boot by disconnecting battery and just plug in mag, it sometimes goes as far as log-in screen, but then shuts down. I will of course revisit my former corroded spots to lock for disconnets, but I was wondering if the pattern of coming up to log in, but not beyond may give me a clue where to focus my attention looking for disconnects.