slight impact damage, now dropping signal

I under take a lot of iphone repairs some board level stuff as a side buisness.

I have an iphone 4 that has been dropped on the floor

not enough to break the screen but enough to make the signal sporadic sometimes fine other times goes to searching mode and also drops calls.

I have transfered logic board into a know working assembly and the problem is still there.

any tips as to what might be the problem thanks