iPhone 6 Completely Dead and Unresponsive

I am a Smart Phone repairer and I recently had an iPhone 6 dropped off to me for repair. The problem description given by the customer was simply that the phone was unresponsive.

I proceeded to examine the phone and tried all the suggested fixes, Changing the LCD/Digitizer, Battery, and Charging Port. I also tried a hard reset, cleaning the connectors with 98% isopropyl alcohol.

It isn't being recognized by iTunes at all and there is no display showing (Even with no ambient light) I've tried everything I can think of short of replacing the logic board and it is completely unresponsive.

The owner did say that the phone had been repaired and looked at by multiple other phone repairers before bringing it to me.

What are the possible causes and solutions to this problem.

Thanks in advance

Stelios Sgoutas

Easy Tech Repairs Adelaide.