iPhone 4 not booting (Motherboard is ok, tested with new battery and.

Hello

I have an iPhone 4 in for repair.

The phone does not boot.

I tried:

• holding the power button
• connecting the charging cable
• hard reboot by holding home button

There is no life in the iPhone.

The computer does not recognize it.

I tested:

• An other (charged) battery.
• A new dock connector

..does not seem to work.

• I placed the motherbord of that iPhone 4 into my iPhone 4 and that works.

When I place my own motherboard into the iPhone it does not boot.

So the problem must be something else that is:

• not motherboard related

• not the battery

• not the connector

Any ideas??

Thx.