Chosen Solution
I currently own an iPhone 6S , which has a CLEAN logic board (no water damage and no visual physical damage) , and the iPhone does not power on at all. It might be a short in VCC_MAIN, but I have no idea how to identify the shorted component. How can I truly identify the shorted “line” and the shorted component on the logic board that prevents my iPhone from powering on? Any suggestions would be really appreciated. Thank you all for viewing my question! (PS; Basically, I am a novice at mobile phone repairing, and I would like to investigate and solve the problem by myself so that I would gain a bit of experience all in all) UPDATE: I now know that VCC_MAIN is shorted, and I now seriously need to find an effective and time efficient way to identify the cause of the short. Correct me if I am wrong. Now, I am powering on my DC power supply and adjusting the voltage to almost 4 volts. I then attach a wire to an anode side of a capacitor (which is the rail side of the cap) and attach the probes of the DC power supply to that wire. I then use a freeze spray to spray the whole logic board in order to find an area that heats up. It is usually a cap that is shorted, so I would check if there is any cap that is heating up. If no capacitor heats up, then this would mean that I have an issue with a circuit that gets VCC_MAIN passing through. I should then check which component causes that short, and then try to remove that component of that circuit (which is usually the one that is being fed by VCC_MAIN). That’s all what I have learned from my research. Am I doing anything wrong at this point? I need the advice of all of you people in this forum! Please help me here! Thanks a lot in advance.
Finding a short on VCC_MAIN is the hardest short to find because there are capacitors on that line all over the board. All it takes is one bad capacitor to make all the others look bad. So you can’t measure them in situ to find the bad one. Additionally, when you have a clean board, there’s not always a visual clue, like heavy corrosion form water damage, to point you in the right direction. The best bet is to question the owner of the device and ask lots of probing questions about how the phone failed (i.e. things got hot near the FCAM and suddenly the phone died), following what activities (I was hiking, water skiing…). Also ask what kind of chargers they use (could blow the charge circuit) and whether the phone looks scuffed up or not (drops, and physical abuse). If that doesn’t give you any leads, then a thorough visual inspection, preferably with heavy magnification, may give some visual clues. If you suspect a cap, then pull it out and test the VCC_MAIN again to see if it solves the short. If not, put it back and write it down. You can’t conceivably do this for every component though. So I would start checking the logical failure points…the Charging circuit (Tristar/TIGRIS), the PMIC and the PMU_RF. If you start seeing shorts in secondary power rails generated by the PMIC or PMU_RF, then you can try to either find a downstream problem or it may be the actual PM chips themselves. In the end, it’s a lot of trial and error and experience. Everyone has their own technique. Just ask questions here and you will get some advice that may help you pinpoint the problem. The YOU can contribute to the site for all those following in your footsteps.
For a strong short that has no resistance to ground you can find the shorted line by measuring around randomly to find caps that beep –have continuity–to ground on both sides with a multimeter. Once you find the shorted line, use a schematic to identify all the possible components that could be the CAUSE of that line being short to ground–something with a path to ground that could have failed and turned into a wire to ground. Use visual inspection, or heat, to try and find the offending component and remove it.
The method that has just worked for me: (I used to do this as a side a couple of yrs ago but real life got in the way so been away from it since then). Dug the tools out recently and went back to board repair. iPhone 6. History unknown as I bought a bunch of ‘dead’ phones hoping to fix a few up. Plugged power supply in and it took 2 amps no boot. Quick look showed a dead short on vcc_main. All components on that rail looked intact and I spent maybe 15 mins looking closely. Tried the thermal (FLIR ONE PRO on my phone. Not the best but it helped). Showed a small IC getting real hot. Took it off the board and set aside. Still short. Increased the amps to 2.5 and FLIR cam again. Found a cap on the other side getting hot. Quick squirt of alcohol round the board but this was the only cap boiling. Flicked it off and short gone. I just use a combination of thermal, alcohol squirts and increasing the current slowly until something shows itself. I’m sure alcohol alone would work if you don’t have a thermal cam. Anyway replaced the cap and the IC and phone boots as normal.
