Short on PP_CPU, PP_GPU, and PP_SOC common failures?
Long story short: water damaged iPhone 6s (daughter absolutely *had* to wait in line, in the rain, to ride Tower of Terror). After installing a new screen/digitizer assembly and sending it to a "microsoldering shop" (they took my money and did a half-assed job not fixing the phone), the phone powered on but drained battery in about an hour. Finally it got to the point where the phone would boot, but as soon as you entered the passcode it would hang. A forced-restart worked, but it would just hang again on the lock screen.
*sigh* Resisting the rant at this point... ;-)
So I bought an AmScope (thanks for the recommendation Jessa, I've been wanting an excuse to get a microscope for a long time) and took the phone apart. At least the repair shop claimed to have done an ultrasonic cleaning and the board was indeed very clean. The repair shop had identified about 8 caps around the PMIC (U2000) that were badly corroded and they said they "fixed them", and by "fixed" they apparently meant removed the caps but did not replace them.
I realize you can get away with pulling a few caps here and there, but 6 to 8 caps on the output of PP_GPU (or any LDO for that matter) is not a good idea. The camera LDO (U3200) was also badly corroded, and the input cap (U3210) to the LDO needed replacing as well. So I ordered the parts and started poking around while waiting for them to arrive.
So far I have found that PP_CPU, PP_GPU, and PP_SOC (BUCK0..2) are shorted. PP_VCC_MAIN is fine, as well as all the other outputs of the PMIC (BUCK3..8).
I was wondering if anyone has seen something like this and could say "oh yeah, we have seen that before and x% of the time it is the PMIC...", or similar?
I know removing the output inductors (L2000..2003, L2010..2013, and L2020..2021) would be the way to isolate and test the PMIC from the output filter caps, but I'm reluctant to remove them because they are right up next to the CPU and they appear rather fragile.
Any tips, suggestions, or insight would be greatly appreciated.
TL;DR - see "TL;DR start here" below:
Well, the *why* of it all is a slippery slope. I'm a geek by nature, and spent my childhood taking everything apart to see how it worked. I've been into programming and electronics since I was around 12, but being technically and mechanically inclined can get you into trouble sometimes. I have spent the greater part of my life always "doing/fixing it myself", which for me means doing tons or research about the task, seeing what others did, checking out procedures and tools, etc. This all takes a *lot* of time. Sometimes things go well, other times the results were not worth the effort. I basically invest a lot of time and money learning to do something *once*.
Always the biggest problem when I start a project I have never done before is that I don't have the experience of having done the job many times before. When you pay someone to do a job, you are not just paying for the time or materials (those aspects are usually the smallest part of the cost), you are paying for their expertise and experience of having done the job before and that they have the skill and knowledge to do the job right. I'm learning that sometimes paying someone is faster and easier than all the trouble of doing it yourself, especially if you are not set up to do the job.
Fixing iPhones is one of those things that I really did not want to get into. Becoming a repair shop is not on my todo list, and phones are expensive enough that I really don't like messing around inside them. Getting schematics and phone information is an exercise in visiting websites that make me nervous, the technical info is hard to get, and you never know if the information is accurate or reliable (or legal).
However, I have two daughters and thus have had my share of replacing phone screens and batteries; and I'm really thankful that iFixit exists to provide those parts at reasonable prices. I never had to deal with a water-damaged phone though, until now. So this time I decided I would pay the experts, but sadly it did not pay off. I did get an ultrasonic cleaning out of it (I think, I can't prove that though). I'm not willing to spend $500 for the entry-level cleaner that really does the job (according to Jessa and Louis). I already spent more than that on the microscope, but I wanted one of those for a long time, and the rest of the family likes to look at the mirco-world too. :-) So the microscope was worth the cost to add to my bench.
Wow, look at all that text... The older I get, the more I ramble. Finally getting to the topic at hand, i.e. the phone:
TL;DR - start here:
I did not replace any of the caps yet. I started testing for shorts first since the phone went from seeming to work, albeit draining the battery faster than normal, to locking up after booting. The repair shop also failed to replace the input cap on the U3200 LDO, and before the phone started locking-up my daughter pointed out that the camera and flash-LED were not working. After checking the schematics, oh look, the missing cap was on the input to the camera LDO...
Q: "What do you call an LDO with no input or output caps?"
A: "An oscillator."
Ok, that was bad. ;-) But it is true. Every LDO I have ever worked with states in the datasheet that the input and output caps are required, X5R or X7R, or the LDO will oscillate. It really added to my frustration to see the repair shop did not replace the U3200 LDO input cap.
I understand what the diode (beep) test is actually doing, so I also checked the actual resistance of the "shorted" paths, both ways (negative on the cap, then negative on ground):
PP_CPU: 33ohms, 88ohms
PP_GPU: 9.6ohms, 11.2ohms
PP_SOC: 9.6ohms, 11.2ohms
All the other outputs of the PMIC were 2.5K or more. Actually, none of those three above are complete shorts to ground, since in that case I would expect to read about 0.52ohms (which is what I get when I check the ground-side of the caps).
It is interesting if you consider the voltage and current rating for the output of something like PP_CPU, which is 1.03V @ 12.5A!! I realize that is a max rating, but let's just say the load is 10ohms (the resistance I'm seeing) @ 1.03V, the current would only be 103mA. So maybe I don't have any shorts after all? To draw even 2A @1.03V the load resistance during operation would have to drop to 0.515ohms...
This is the biggest problem I'm having with this repair, I don't have another known good iPhone 6s to open and compare to, or lots of experience fixing phones where I would know these values and be able to use experience and insight to guide me. So far I have not been able to find anyone who has made a list of "normal" values (voltages, resistance measurements, etc.) for the various phones. Does anyone know the normal resistance of the PP_CPU, PP_GPU, and PP_SOC nets?
Is this a good question?