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iPhone 6 multiple partial shorts

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  • iPhone 6 multiple partial shorts

    I have an iPhone 6 that came to me with a searching for service problem. It had been to a shop which said they could not fix. I powered it on and no signs of life, put it on the charger and it booted up. I then did a check on the imei number which was not showing, removed and reballed baseband while the battery was charging, checked diode readings around baseband. I then powered the phone up and signal was good, shut it down and put it back together. The next morning I tried booting it up but nothing. The battery was dead again I put dcps on it and it was drawing 0.022 mA before prompt to boot, I thought it must have something to do with the way I soldered the shields on so I removed the shields and tested with multimeter which was about .299. I contacted the customer and she said that it had originally been to the shop as the display stopped working, they fixed that but a few days later it started getting no service. I should have checked before doing the repair I will definitely be doing that in the future. My conclusion is the shop put it in ultrasonic cleaner without removing the shields to clean the flux where they where working on the backlight filters and caps. I was told the phone has never seen water, the housing sticker is still white but there is no sticker on the logic board also when I peeled the shield above pmic I could see it was stained. I thought the smartest thing to do first would be to give it a hot bath for a couple of minutes with out the shields on. I then checked diode mode on vcc main and it was down to .22 I started a close inspection of the board and noticed there was signs of heat around the backlight driver which they did not change. I checked that circuit on diode and it was reading .37 and the sdram is showing .295. I don't have a shop or advertise, if I receive a repair it is through word of mouth. I know how to solder and reball chips, I don't know very much about the different lines of an iPhone. I gave the customer another phone as I am not very confident on this being a phone again. I hope these questions will help me out in this field.

    So a few questions.
    1- If there is a short on vcc main will you always have a short on the backlight circuit.?
    2- How does a component become partially shorted and can a cap be partially shorted?
    3- Are there smart and stupid electrons or are there strong and weak electrons. what determines which electrons take the shortest path out in a partially shorted circuit?

  • #2
    1- there will be a short on part of the backlight circuit, e.g the part fed by vcc main. The output side should be ok though as it has to go through the ic first (I think, need to check schematic to be 100% sure).

    To confirm this yourself just open up the schematic, look for vcc_main coming into the backlight driver. As long as that isn't connected to the output any way the output will not be shorted unless the backlight ic is the cause of the short.

    2 - Think of it like this, add flux to a piece of wire and solder it to the board, that'll produce a good 0hm short. Now take a piece of wire that's nicely oxidised and place that on the board but don't solder it. It's touching the board but the oxidation coating on the wire acts as a barrier for electron flow so it's not going to be 0ohms, but some higher value.

    Now inside ics you have tiny circuits, ic meaning 'intergrated circuit' . So in the same way you can get a short on your motherboard circuit you can also get a short inside a chip which is just a circuit, but on a much tinier scale.

    lets say you pull too much current through the backlight driver and inside the backlight driver are two nano traces that run next to each other. If you pull loads of current through it one of these traces may melt a bit and let's say a whisker of metal touches the trace next to it from this melted material. All this is on a microscopic scale and if you've ever seen a de capped chip you'll see how close everything is so it doesn't take a lot for one trace to touch another and cause a short.

    this can happen from dropping too as if you know how close the traces inside the chips are, a drop from say 1m is huge in comparison and the force inside the chip may well damage a circuit inside the chips.

    as for capacitors they are just layers of conductive and non conductive material. If the layers touch that's how you get a short. Also force too much current through one and the angry electrons don't want to go nice and slowly through the material, they just ram raid their way through with force and short out the layers.

    think of a capacitor like say ten garden sieves sat on top of each other. Pour sand in the top eratically it filters through the layers and comes out fairly smooth. Stop pouring sand and it will keep flowing for a few seconds out the bottom. That's pretty much how a capacitor works, as a smoothing / buffer for electrons.

    a capacitor is basically just a big multi layered sandwich.

    3 - an electron is just an electron, it's resistance in the short that means whether you get a full or partial short.
    Last edited by crea2k; 01-25-2018, 11:46 AM.

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    • #3
      Thanks for taking all that time to answer that. I really appreciate it. There is some information there that I didn't know and no doubt will save me from a lot of heart ache.The reason I was asking about the backlight circuit, mainly the three backlight caps is that they don't show up on the vcc main line. I tried relieving the short by removing the backlight ic followed by the three backlight caps and the filter which had a wire soldered along the top. diode was still reading 0.22 I then removed the diode which made the line open. Next I got a donor board with no shorts, shorted vcc main and tested the backlight caps and they read 0.16 I then took the wire and shorted out the backlight caps and tested vcc main which was all good. But I think if I bridged the diode, vcc main will have a short. So even though vcc main stops at the coil on the schematics it actually continues on at a lower voltage through the diode and into the back light circuit? So if like this board if checking and found some sort of short on vcc main and the backlight circuit I should concentrate on the vcc main short first and then backlight or am I missing something in my testing logic?

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      • #4
        Generally if that backlight diode goes you can see as it gets burn or has a hole in it. You can soon test it in diode mode to rule it out though. If the backlight caps are short you would have some kind of backlight problem as they are supppsed to be on the other side of the diode, so the electrons only flow one way.

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        • #5
          Look at the schematic—think of the cool as a wire, think of the diode as a one way valve. Vccmain goes through the coil and diode and appears on the other side as backlight anode line —this is why working on a device with battery connected is a bad idea—main voltage (which is battery voltage) will always be present at the lcd connector

          so Yes—a main short will appear as a backlight anode short. Solve main short first.

          figure out where main is leaky and bring it back up to normal diode mode reading.

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