Hi!
If you have an ultrasonic cleaner, you have to use it with your phone. If not, try to manually clean your phone with isopropanol alcohol (if you'll get it ;) ). First check any salt track on MCU and FLASH areas and , as hrco said, power elements( located on PCB SIM side above Battery connector...
You also can try to scan both PCB side on 300 dpi and send it to me
[email protected] (please jpeg only!). I'll try to help you...
Good luck!