After water damage no function but board seems working.

Hi guys,

I am repairing an iPhone of a friend and I now faced a problem I'm not familiar with.

Short version:

Phone remained in a jacket outside during rain. Since then no function. I opened up the phone but there was no corrosion visible. I put the board into my iPhone 6 and run it. Since it turned on the board should be okay.

I tried to find defect parts by exchanging them. I put a new battery, power button cable, lightning dock cable and volume buttons cable in. But still no reaction. Neither by pressing the power button nor by plugging in the USB cable. Even iTunes keeps silent.

Does someone of you know any component that be causing the defect I not exchanged, yet? Or could the housing cause the problem? I'm like 404...I'm thankful for every further idea :)

Chris.