iPhone 6S no audio devices found turns into no boot

[GuruFix]

Hi got a new case where im stuck.

Got a iPhone 6S that looks OK and this customer had trouble with calling all of a sudden. When we test the dictaphone app we get the famous message (for iPhone 7's):
There are no audio devices found. We rebooted the phone but the message was still there.

Exam: found that the logic board is slightly bend from bottom to top but not in an extreme way. Diode mode around the audio IC was all good and found no numbers way off.

Work done: iPhone 7 and iPhone 6S audio IC are the same i found out. Since the audio IC is easy to replace i decided to replace the audio IC with a brand new one.
Diode mode some pads that have visible a trace to a nearby component, just like the iPhone 7 but found not out of the ordinary. After replacing the audio IC the phone didn't boot anymore. This lead me to undo my previous work and left the audio IC off the logic board to see if the phone boots again. But still no succes! Im not sure if the phone 6S boots without an audio IC so this lead me to undo my whole work and that is putting the old/native audio IC back on the board. But the phone still doesn't boot.

The phone is able to enter recovery mode btw but when we enter that state i don't have image nor backlight. Measured the backlight line's in diode mode to be sure but there is nothing wrong there.

I don't know what my next step must be? Is it possible that the audio IC wasn't placed correctly and a dataline touched a power line and therefore the NAND is corrupt? Im afraid that im guessing here.

Attached are the diagrams of the iPhone 6s trying to on DCPS with the original/native audio IC back (previous work undone).

Some help would be great! The data is also important btw.

[Jessa_the_Professa]

6s can boot without audio ic.

That DC power supply looks like the phone is boot looping, or starting to boot. In general, if a phone can get past 100mA on DCPS during boot, then it will either boot or boot loop.
Split a screen to rule out backlight and look for image with a light from behind--does it make apple logo when it starts to boot? i.e. when DCPS gets to 200-300mA do you see Apple logo?

[DocVolt]

I had something pretty similar happen to me and it was the smaller audio ic beside tristar

[GuruFix]

Good to know that the iPhone 6S will boot without audio IC.

Today i checked for image (with a split screen) but there was no image / nor backlight.
I removed the CPU shield and saw 3 solder balls in total! One was on U4020 Backlight driver and one was near U3300 strobe/flash driver. Im going to remove the underfill
and place some new ICs. Hopefully the CPU didn't get to hot!

Im never using a pre-heater on a iPhone 6s from now on.

[GuruFix]

Update: i've removed the underfilled IC's U4020 and U3300. Cleaned the pads very well and diode mode under U4020 compared to ZXW it looks fine.
Replaced only U4020 with a brand new one and tried to boot the phone. Im assuming the iPhone 6S will boot without U3300?

But still no image! Measured the J4200 LCD connector but found nothing wrong! What to check/to do?

[Jessa_the_Professa]

Okay--solve image before backlight. We will worry about backlight later. First--does DCPS seem like the phone is indeed booting? i.e. it goes above 100mA and dances up and down? If so, let's solve image. If there is no detectable short or open at connector, then we switch to second test---voltage testing. Check for image voltages which are the chestnut outputs. Find chestnut on the schematic--find his outputs. do voltage test with screen connected and device under power and seems to be booting on dcps. What are the results?

[GuruFix]

Thanks Jessa for the update. Phone is dancing on DCPS up and down, not exactly as a normal booting iPhone.
All chestnuts outputs are present and correct.

Measured J4200 again (completely) and found a fault. Overlooked this previously!
J4200 line 9 MIPI_AP_TO_LCM_CLK_CONN_N is O.L. and needs to be around .300 To be sure i've measured @L4200 pin 3 called MIPI_AP_TO_LCM_CLK_N going directly from L4200 to RAM still O.L. The CPU and/or RAM did get too hot! When i check the pad on CPU its exactly around the spot that did get to hot (top left).


Next step reballing RAM and CPU? I'm still not experienced enough to complete this repair. A training/cursus would be nice in this case.

[Jessa_the_Professa]

Is this data recovery? I have solved that problem before for data by "borrowing" a clock from another device booted at the same time with a long jumper. If not, then yes--reball CPU is next step. Our MasterClass spent 10 days all day every day doing just CPU rework, so it isn't something that just needs a few tips. Get some iCloud locked boards and some 138 paste and see what you can do. Start with iPhone 7 where you don't have to split the SDRAM and CPU like you do for 6 and 6s.