Black Screen but phone works after water damge treatment

Ok so my girlfriend was pushed into a pool with her iphone 4, when she got out she noticed it was at the bottom of the pool, the back glass had been cracked just a bit and she tried turning it on, mistake i know, i sent the phone to be treated for water damage but was told it didnt work. When i got home i heard siri talk when i held down the home botton, the phone works perfectly fine and itunes reads it too, i even backed it up. Everything works except the display, i had a whole new LCD tested on it but nothing, so im guessing the corrosion got the one part of the mother board where it displays everything, is there anyway of fixing that besides buying a new motherboard?