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No cell service but wifi and bluetooth fine. How to diagnose?

I have a damaged iphone 12 mini I’m trying to repair. It had been dropped and both front screen and rear glass were cracked. I have swapped the chassis and a fitted a new aftermarket screen. All seems to work except I get no cell service (no signal bars, no identification of msisdn in settings, and nothing helpful in field service mode). Wifi, gps and bluetooth are fine and the phone detects when the sim is present or not. The sim works fine in another iphone. Any suggestions on diagnosing if this is an antenna issue, a main board fault, or something else?

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The easiest way to check if you have a parts problem or a board problem is to input *#06# into the phones keypad. If it displays the IMEI number it could be a parts problem but if it doesn’t display the IMEI number it will be a board issue most likely with the Baseband CPU.

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Although this could be an antenna problem, based on your description my inkling is that this is a board problem. The thing with nearly all iPhones X and newer (aside from the XR) is they have a dual board layout. The logic board is actually made of two separate boards sandwiched together. The bottom board contains all the components related to cellphone service, and other wireless functions. The top and bottom boards communicate via an interposer board and a heavy enough drop can break certain lines of connection between the two boards, causing a bevy a cellular issues.

Some additional info may help us narrow this down further. Where did the replacement housing come from? Did you transplant all the parts from the original housing? or were some already in the donor housing.

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Thanks. It looks more like a board problem as the *#06# is not showing the imei, although that does show in the settings which is a strange. The chassis was bought complete with antennas, speakers, and charging coil so I’ve transposed the main board, cameras and battery. I could try a bit of hot air on the main board to see if connections get remade.

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@si_p You could try applying some hot air with a bit of flux around the outside of the board but you really need to separate the layers and check the pads as after a drop they have a tendency to tear off and you will need to run a jumper or make your own pad.

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@imicrosoldering totally agree here. Separating the boards is probably going to be necessary. Especially if the housing was mostly complete, really in my experience antenna issues are pretty rare and usually happen because of a drop where glass got in and mucked stuff up. Or a cable was torn during a repair.

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Thanks both. I will see if I can get the two boards apart. Do either of you have a recommended temperature for the hot air station?

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@si_p Oof. That’s a good question. I don’t actually use a hot air station for that. I have a board heater. And recommending temps to others is difficult because what it’s reading on your station isn’t necessarily accurate. And I know my station isn’t calibrated, I adjust temps purely by feel.

That being said, the temp for melting that solder is going to be lower (in theory) than the other stuff on the board. That way soldering the 2 boards together doesn’t mess with all the other board components during manufacture.

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Hi there, it sounds like an Antenna fault so i suggest you try and replace the antenna and if that doesn't work it will be a motherboard issue .

Thanks

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