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Liquid Damage - No Power - Possible BB/BB_PMU Problem?

Hey guys. I have an iPhone XR that I am working on that suffers from liquid damage and currently does not power on or charge properly. I have ruled out a parts problem by swapping out the lightning dock charging flex cable and a new battery to no effect. The logic board has been ultrasonic cleaned, as there was visible corrosion on the board.I've taken some measurements and I am leaning on something wrong with BB_PMU or BB itself, but wanted some other opinions. I'll outline everything I've done thus far

Problem:

Liquid Damage. Visible corrosion in the area around C4191 (PP_VDD_MAIN).

Phone does not power on

Phone attempts to charge at varying ~0.10A

Diagnosis & Repair Attempts So Far:

Ultrasonic cleaned the board due to the corrosion.

Tried new lightning dock flex cable - No effect

Tried new battery - No effect

Found short on PP_VDD_MAIN. Replaced C4191 and short cleared - Still no power or proper charging.

TRISTAR/HYDRA Tester says Hydra is OK.

I have checked a whole bunch of power rails both for voltage and for shorts. I don't THINK I found any shorts (the ones that appear low seem to be alright according to ZXW Tools) , but I did find some rails missing that are tied to BB and BB_PMU. I will post my measurements below.

The first measurement reading on each line is diode mode measurement to ground (red probe on ground, black on rail) and the second measurement is the voltage. Testing was done with the board connected to a DCPS set to 3.8V no screen, battery, or lightning dock connected. A few of them at the end I just got their diode reading and not voltage, but will gladly get those if someone needs to see it.

Various Power Rails:

pp3v0_nand - 0.449 | 2.61V

pp1v8_s2 - 03.15 | 1.8v

pp1v8_always - 0.632 | 1.79V

pp3v3_usb - 0.324 | 3.3

pp_vdd_main - 0.325 | 3.82v

pp_gpu_sram - 0.481 | 0v no screen connected

pp_vdd_boost - 0.393 | 3.76

pp1v1_s2- (short?) 0.095 | 1.069

pp1v2_SOC - 0.538 | 0v

pp_cpu_sram - 0.335 | 0v

pmu_vbatt_vsense- 0.786 | no batt connected

pp1v26_s2- 0.446 | 1.27v

pp0v9_nand - 0.474 | 0v (normal?)

pp_batt_vcc - 0.442 | 3.82v no batt connected

pp_cpu_ecore - 0.443v

pp_cpu_pcore - 0.259PP_SOC_S1 - 0.267

pp3v0_s2 - 0.417pp_gpu - 0.018

And then here are the measurements strictly involving BB/BB_PMU. These are all voltage measurements. The ones that are zero volts, I also took diode measurements and none of the zero volt rails are short:

pp_0v9_SMPS1_BB_SRAM - 0.9V

PP_0V8_SMPS2_BB_DIG - 0V

PP_1V3_SMPS3_XCVR_ANA - 1.24v

PP_0V77_SMPS4_BB_CORE - 0.87v

PP_1V0_SMPS5_XCVR_CORE - 1V

PP_1V85_SMPS6_IO - 1.846v

PP_3V15_LDO1_VFE_AUX - 0V

PP_1V8_LDO2_VFE_LNA - 0V

PP_3V3_LDO3_USB_HS - 3.3

PP_1V8_LDO4_BB_MEM - 0v

PP_1V24_LDO5_USB_SS - 1.25V

PP_1V1_LDO6_BB_IO 1.1V

PP_1V2_LDO8_HBPAD 0V

PP_1V8_LDO9_GNSS 0v

PP_1V8_LDO10_XCVR_HI 1.74

So it seems to me that my main problem is likely the BB_PMU IC itself which is U_PMIC_K in ZXWTools or the BB itself which is labeled U_BB_K, but that is just somewhat of an educated guess.I was hoping someone could take a look at what I've measured and help me in the right direction. I don't have either ICs in stock for the iPhone XR so before I ordered some chips needlessly, I wanted some help in making sure I'm on the right track in my thinking. Thanks guys!

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The Baseband circuit is a bit more mysterious, compared to the main PMIC. I wouldn’t put too much emphasis on the voltages as it isn’t always clear which rails power on under which circumstances. Check them all in diode mode first to see if there are any shorts.

I do find some of your readings to be a little off. For example, PP_3V0_NAND should be bang on and so should your PP_0V9_NAND. Your VDD_BOOST seems low at <4V (the whole point of this line is to boost the circuit to >4V when the battery is weaker or there is high current draw).

The thing is water damaged phones can be a real pandora’s box and most tech don’t “fix” them unless it’s for data recovery because they tend to be unreliable, at least the ones that had severe failures.

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Minho is right. Forget about the baseband and get your PP3V0 NAND to where it should be. Set your DCPS to 4.2V Are you sure it is not in DFU mode?

von

Thanks for the reply. I guess here is where my knowledge gets rusty because I am not quite sure what to check from here. So for example, I check PP3V0_NAND for short to ground using diode mode measurements and it's almost spot on with what ZXW says it should be measuring 0.449V at C2616.

PP_0V9_NAND measured at C2602 I get 0.474V.

VDD_BOOST measured at C3110 I get 0.392V.

So 3V and 0.9V power for NAND is created at PMIC. Since these lines are lower than where they should be and there are no shorts on these lines, should I be looking at replacing PMIC or could one of the various capacitors/resistors on the lines cause them to be low?

Edit and no, it is not in DFU mode

von

It will not be the PMIC these chips are under filled and pulling the PMIC would be the last resort as the CPU is on the other side. The lines you are diagnosing are not next to each other on the PMIC. If it is producing the correct voltage for the other lines it is functioning correctly. One way of checking would be to inject 3V into that line and take a reading. If it measures 3V than I would start looking at the PMIC, if it doesn’t produce the 3V I would be looking at the NAND itself.

von

Alright. Thanks for the tip. I'll give the 3v injection a try and report back. If it starts looking like the NAND itself, I'll be extremely limited in my current capacity to do much at that point. I don't have a programmer, although I've been putting off buying one for a long time now. Maybe I should have one...got any recommendations?

Anyway, I'll inject 3v tomorrow when I get back to the shop and report how it behaves.

Thanks again.

von

@techwizards I would just re-ball it to start with and do some measurements while it is off. As for NAND programmers there’s the WL B110 or the JC which have a few models. Best to do some research and read reviews. I haven’t used mine in months, if I can get the phone into DFU mode I use the iBox mini.

von

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TechWizard wird auf ewig dankbar sein.
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