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Boot loop after back case replacement.

I did follow the guide to replace my iPhone 7 back case. Everything went smooth, I’m 100% sure I didn't mess with screws (everything was tagged and I also made photos after every step), but before final reassembly I decided to check if my phone works only to find out it doesn’t boot. Apple logo appears for 15 seconds, then phone switches off and on. 15 seconds later everything repeats, and phone never finish the boot.

Things I tried one at a time (and they didn’t help):

  1. Disconnected front camera
  2. Disconnected whole display assembly (it didn’t appear in iTunes).
  3. Disconnected back camera
  4. Disconnected back assembly (lightening port and audio speaker). This way it looped faster (in 7 seconds instead of 15)
  5. Disconnected power button assembly
  6. Connected new front camera assembly

I also tried to put iPhone in DFU mode in restore firmware. It succeeded both times, firmware check also succeeded, but iPhone still doesn’t boot. The only thing different is that it loops in 7 seconds instead of 15 like before.

I’m really stuck and don’t understand what I can check at this point. It doesn’t look like Audio IC or modem issue other users are having here, also it shouldn’t be front camera assembly, as I tried to boot without it (and boot with a new assembly) and it doesn’t work too.

UPDATE:

Could someone confirm that iPhone 7 with latest iOS 12 should boot WITHOUT front facing cam ribbon and Touch ID? I know that was an issue on iOS 11 and I’m trying to understand if I have a logic board or display ribbon cables issue.

Answered! View the answer I have this problem too

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Have you checked that all the correct screws are in the right spot? Some of the screws around the logic board are different lengths and can cause the logic board to reboot, just like a motherboard on a PC if the standoffs aren't installed correctly. If I recall correctly the difference in screw sizes are 1.4mm and 2.2mm, so even if you documented everything there is a chance you could have switched one or two up on accident. The good news is: You definitely did not break it since it turns on which is good.

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Well, from what I can see there are just 5 screws which keep the logic board: 4 2.2m standoff screws and one smaller screw which goes under the camera ribbon. I'm not sure what can be mixed here. I'm not putting any ribbon protectors yet, so that's all I used.

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What makes you think it doesn't seem an audio IC issue ? The iPhone housing replacement was done as the old one was damaged/warped ? If that’s the case it’s when such issue happens most often, as a consequence of pressure put on the logic board causing flexing. Bootlooping on iPhone 7 unfortunately points in that direction..

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No, the phone was working fine, although I dropped it few times in past. I bought the phone 2 years ago and never used any case: now there is literary no paint around the corners. So I decided to change the case to give it a fresh look.

I suppose I could break Audio IC during my repair, but the only thing I did to the motherboard was taking it off the case and putting it on a table. The other thing which make me think that it's not Audio is the fact that the home button doesn't respond, as the phone reboots just 5-7 seconds after Apple logo appears.

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Some phones exhibit Audio IC issues only after a full reboot. So any repair or update can make the problem "suddenly" appear.

However, with a short bootloop, like 7-15 secs, you are probably facing a baseband issue or some other repair induced damage. What I would try is to have just the board, a dock and a battery. Connect it to iTunes to see if it boots normally and is recognized. If it doesn't then you probably have a board issue (unless it's the dock or battery).

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Okay, I tried to boot logic board with the lighting connector alone (is it even supposed to boot like that?) and it didn't work. iPhone never appears in iTunes. I also tried to power it up with screen and it boot loops in 5 seconds like before.

The other thing I tried was disconnecting everything except power button ribbon cable (with antennas and everything else removed) and display, and it also boot loops at 5 seconds.

Can anyone confirm that iPhone 7 with iOS 12 should boot without Front Camera, without Touch id and all without all other ribbon cables except lightning port? If it should, then I suppose I somehow broke my main board.

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@lohmatij It won't boot just with the dock, you'll need the battery too. As for the other parts, they're not needed for booting phase, if the logic board works.

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Oh yes, the battery was connected, I forgot to mention it.

No matter what I tried it bootloops in 5 seconds (I mean that apple logo appears for 5 seconds before shutting off). It was definitely around 15 seconds before I updated iPhone firmware through DFU mode, so "clean" firmware takes less time. I wonder how it can be connected to baseband or Audio IC.

I also tried to boot it in original backcase (thinking that probably I get a short or something with a new case), but it doesn't help.

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I ended up applying for professional service, they told me that memory got corrupted. Not sure how it happened, but they fixed it

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