Intermittent Cell Signal Issues Persisting After Troubleshooting
To introduce myself I am a professional repair technician who has gone through ISO 9001:2008 certification in mobile device repair with about 1 full year worth of repair experience and 100s of iPhone 6 devices fixed, and this still stumps me.
I know a lot of people have asked about this, but no one seems to be able to give a definitive answer, and I was hoping over the past few months maybe someone has stumbled upon a feasible solution.
We have three iPhone 6 phones in our shop, 2 we own, 1 customer's device all with the exact same symptomolgy: Intermittent service Searching/No Service. At random the device will connect to the towers with good service (3 to 4 bars) and again at random they will switch back to "searching/no service". This issue occurs on all the network types the phone is capable of operating on (AT&T's GSM and LTE and Verizon's CDMA and LTE)
We have confirmed that it is not the following:
-Date and Time are correct
-Toggled Airplane Mode
-Different SIM Card
-Toggle LTE on/off
-Different Network's SIM Card LTE on/off
-Reset Network Settings
-No liquid damage
-No apparent frame bending
-Multiple new primary cellular antennas (completely installed new charge ports, ensuring all grounding rings and sticker surfaces were cleaned before installation)
-Multiple new cellular interconnect coaxial cables (the one that runs the length of the motherboard on the device)
-Multiple new upper antenna interconnect flexes (the L shaped one at the top of the motherboard)
-No damage with a continuity reading to all coaxial connectors on the board to their respective pads
I've done what I feel like is a lot of research but keep turning up empty handed. It appears like this issue is caused by a chip on the motherboard that was not underfilled that has cracked or damaged solder joints, I just don't know which one. Logically it must be after the baseband but before the antenna amplifiers, so a switch of some kind? To my knowledge each frequency type/band has its own amplifier (GSM/CDMA/LTE etc.) but they all share the same modem (the baseband). Maybe I'm wrong though, just going on very unclear googling.
I am experienced with board level repair and troubleshooting, and I know it can be like opening a can of worms but I am confident and have nothing to lose in attempting a repair of this kind.
An indepth explanation of both the physical, electrical, and firmware structure of the iPhone's cellular antennas would go a *long* way for me if anyone can.
A little bit goes a long way, if anyone has any thoughts let me know!