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iPhone 11 PP1V8 trace DC biasing expectation

Dear all,

We have a measurement project on a working iPhone11. There is no short or open as far as we know (not a return). Store-bought for the specific project, was working perfectly before the PCB removal.

I would like to understand if I use PCB-level pads to DC bias the PP1V8 trace, do I expect the SDRAM to heat up within the A13 package? Or is the DC blocked by the Caps on the way? Can we think of this as a RC circuit and expect transient behaviour and expect a full open when t >> Tau?

I used different pad configurations, and can measure µA to mA currents running on the trace (from different pads) through the SMU, but nevertheless, the SDRAM (or any part of the package for that matter), shows no signs of heating up under the thermal camera.

Does this match up to the expectations of the seasoned iPhone fixers?

Thank you in advance!

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I have no idea what you're trying to do, but assuming that the trace you're powering actually feeds the LPDDR4X SDRAM on the A13 package, then I wouldn't be surprised if you saw current draw on that rail and the SDRAM warm up due to static power dissipation. LPDDR4X generally requires VDD1=1.8V generally and I would expect that to be the case on the A13 package.

Keep in mind that there are probably multiple 1.8V rails on the device feeding different things, but I haven't looked to much into it. There will definitely not be DC blocking capacitors on a 1.8V supply trace to the SDRAM, but again, I don't have the device schematic in front of me so I can't tell you what that rail even touches.

I wouldn't expect significant current flow on this supply if the SDRAM isn't active, which in turn means limited heating. These parts have very low static power and you probably aren't supplying the other rails critical to operation (VDD2/VDDQ).

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@chrisgreen thank you for the answer! Below I attached the PCB schematic.

Block Image

I have been told by credible sources that, if DC voltage is applied directly to the red pin (small pin below package site, upper left) and a any neighbouring GND (greys) after detaching the package, the SDRAM should indeed heat up in a short time and show up on thermal imaging. This is the PP1V8 trace. So, I know that the small pin below indeed connects to a trace at SDRAM.

What I am trying to understand is that, the small pin below seems to be connected (shorted) to other sites on the face of the PCB, including plates of some capacitors (red on the left of the package site), this is why I was asking if DC is being blocked if I apply current, for example from the large red pin on the left side of the PCB using any GND. Here I assume, all grounds are common???

So the question is, can I apply and get DC 1.8V DC into this pin from big pin on the PCB, or will I be blocked by the red capacitor plates that are on this trace??? Would applying 4V from this single outer pin worth a try? Should I connect a power source to the battery socket and try if I see something?

Thanks in advance for any follow-ups!

So far:

  • The voltage applied was 1.8V, I believe it is regulated to 1.1V before entering the LPDDR (not sure about this).
  • Since it's not on read/write, I also expect this only to supply a current of ~5 mA to be getting into the SDRAM (assuming a idle power dissipation of 0.8W). But this should be sufficient to show up on thermal camera.
  • When applying voltage to the outer red pin, we measure 100s of µA, so current is indeed running somewhere, but the SDRAM is not heating up.
  • When applying voltage to the capacitor plate (that is shorted), we get about 2 mA current with no transient behaviour (not sure how long it takes to fully charge the capacitor, but nothing was burnt)

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Some thoughts:

1. The capacitor isn't blocking DC, only one pin is attached to the net and therefor is acting as bypass/filter capacitor to clean up the voltage (some Googling can explain this better)

2. LPDDR4X requires 3 voltage rails typically (can't speak for any weird Apple specifics): 1.8V (you're supplying this), 1.1V, and 0.6V. Presumably these are all fed to the SoC on separate pins. The 1.1V supply dissipates the most power under normal operating circumstances.

3. The power figures I can find for a random Micron LPDDR4X module are a <=6mA on the 1.8V rail under operation, and 0.36mA for a self-refresh. This equates to worst-case heating power of P=IV -> 0.006*1.8~=10mW. This isn't a lot and may not be observable with thermal imaging.

4. For a high-speed design like DDR4 memory, there may be ground isolation (a ferrule, resistor, both, or something else) for the different voltage rails, so your assumption that you can use any ground could be incorrect, though unlikely.

par

Hi Chris, thanks!

1. That is very useful to know thanks!

2. I have found these 3 rails, however the question still remains whether to use a common GND

3. It is similar for this Samsung one, but this should indeed show up on a thermal camera within 5-10 min

4. This is also very interesting. How would one check for this? Check if all GNDs are shorted?

Thanks in advance!

par

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