SAMSUNG Z Flip no signal after drop damaged for a few days.
Model: SAMSUNG Z Flip
Announced: 2020, February 11
Defect Phenomenon
The phone screen was broken by drop damage, after replacing the screen and using it for a few weeks, no service issue happened. Tried to reset the factory setting, then the phone cannot be activated.
Troubleshooting
Insert a SIM card to test, find the SIM card cannot be recognized.
Read card failure might be the reason to cause the phone cannot be activated. We need to fix the SIM card issue.
Repair Process
Disassemble the phone and take out the motherboard
The motherboard is very tiny with a compact double-layer design, also, as we can see the motherboard is deformed.
For the SIM card cannot be recognized issue, we can focus on checking the SIM card slot. 
When measuring the 6th pin of the SIM card slot find the diode value is abnormal
(the measured value is OL, normally it should have a diode value)
Check the SIM card slot under the microscope we find the joint pad with the motherboard is open. Is that possible this is the reason cause the SIM card cannot be recognized?
Remove the shielding cover and measure the diode value again still shows ‘OL’.
This model has no bitmap and schematic yet, so we can only find a Xiaomi model with the same platform for reference first.
After checking we find the 6th pin is the power supply pin, and the voltage is provided by the NFC chip, between the SIM slot pin 6 and the NFC chip, there is a resistor.
For the SAMSUNG Z Flip, the resistor is not printed on the motherboard
NFC chip and the SIM card slot are located on the different layers of the motherboard. Besides, the SIM slot pin6 is connected with the unprinted resistor.
So the issue might be the poor connection between the two layers of the motherboard.
Normally, we can recombine the two layers of the motherboard to fix the issue. But as we can see the board is deformed, in this situation, recombining the motherboard might cause a very serious problem.
What to do?!
After carefully checking, we find the NFC chip which provides power for the SIM card slot with the same spec as the SAMSUNG S8.
Bring a SAMSUNG S8 motherboard, check the pads under the NFC chip, and find the 4th pin of the NFC chip is connected with the SIM power supply pin. So we can remove the NFC chip, then jump wire to fix the problem.
Jump wire
- one side from the unprinted resistor location which is connected with the SIM card pin 6
- another side jump wire to the 4th pin of the NFC chip
After jumping wire, cure it with UV mask and UV lamp. Then solder back the NFC chip to have a check
Measure the diode value again when the motherboard is cooling down, find the diode value back to normal
Assemble the phone to test, and the SIM card can be recognized.
Then we can fix and tidy the jumper wire with the solder mask. After that, the phone can be activated.
Also, test all the functions are OK.
Summary
The issue is caused by the SIM card cannot being recognized.
Since the motherboard gets deformed, we can only jump the wire from the NFC chip to the SIM connector to fix the power supply for the SIM slot.
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