Iphone 6 backlight

[golf r]

Hi everyone hoping somebody can help. I have had a few of these issue lately.

iphone 6 comes in with backlight can see display but faint, ive done full backlight line replacement heaps of time D1501,u1502,l1503 ect basic replacement.

So i have no backlight but i have display i can see. Genuinely with this issue L1503 has had a coil poop/and D1501 is burnt everything else is ok no short ect but ones i complete the line with new chips 1502 smokes and the coil gets very very hot..when i complete the line again with new chips there no display but coil is getting very hot.

is the vcc main line or the pp1v8 butting heads ones theres power to the board.im lost for something to smoke the fast it has something to do with backlight voltage and something else

[Jessa_the_Professa]

Let me guess--you are declaring "no short" because you do not have a full 0 ohm short in continuity mode. But you're not ruling out partial short. How can you detect partial short?

[Alber_the_Learner]

Since our collegue golf r did not answer Jessa's question and for learning purpose for others that will read the post, I am gonna answer it: How can you detect a partial short?

By measuring the resistence to ground of the line usinig your multimeter in Diode Mode and compare the results to a known good board. You should put the Red probe on ground and the black probe on the line. If you get a lower mV reading, there is a partial short. And If you get a higher mV measuring? It is an OL (Open Line).

[golf r]

Let me guess--you are declaring "no short" because you do not have a full 0 ohm short in continuity mode. But you're not ruling out partial short. How can you detect partial short?

Hi Jessa,1st of all its great you started this forum we need to all come together to fight the big guy Apple lol.

So in continuity mode i have nothing jumping out theres a short to ground reason why this issue has stumped me and Im a proud person to ask for help But im lost here on this one.I have checked the resistance on the backlight lines everything seem to very close the zxw resistance numbers.

but ones i complete the line with new backlight ic,coil,diode ones vcc main is plug in small smoke show then heat from the coil after that .

[crea2k]

Have you checked for long screw damage ?, I had one line this and it would blow again straight away and was long screw damage.

[Sly]

Hi golf_r,

I know the question is directed to Jessa, but here is my 2 cent worth, maybe it can help you, or someone else.
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Problem: Coil keeps smoking (a bad habit that is hard to stop)
Goal: Eliminate as much variables as possible to determine why.

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Eliminate Variables:
(Side note: The backlight circuit is a good place to practice this thinking habit, since it is a fairly small, and a well known circuit.)

1. Reference Voltages:Consider writing down in an excel documents measurements from a known working device if you can instead of ZXW. I've found it to be more accurate, different region of the world have slightly different versions of phones, and plus they could make a mistake as well .
2. The Screen: Lets make sure we are using a known good screen, some screens can have their own backlight lines shorted which would always kill your backlight circuit no matter of how awesome of a job you did.
3. Repairing the backlight circuit: You've replaced the IC,diode,coil, but you don't mentioned what caused to the problem originally.(Shorted cap? IC Failure? Make sure to confirm what caused the problem if possible)
4. Under fill: Has all the under fill been removed from the other capacitors for those lines. If not, a new short could of have been created while installing the Backlight circuit with hot air.
5. IC "Landing Pad": Clean up (properly) all the under fill where the IC goes. If there is some left, it will stop your IC from seating properly.(But don't pull pads )
6. Bad IC: This one will be hard to eliminate.I don't know any real none intrusive way to test this beside having faith in your supplier from previous purchases history and rate of component failure
7. Long Screw damage: (***Suggested by crea2k***)

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Assumption

• Your backlight ICs are mostly good
• Tested with a known good screen
• The original failure has been identified as a failed cap, and removed
• The under fill has been removed around the other caps in that region before soldering with hot air.

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So, now what...?

Lets try to reason this again now with everything above stated clearly

• Old problem identified and solved
• We've confirmed that no other shorts, or partial shorts were present as ***suggested by Jessa_the_Professa***
  (This is from the professa, don't skip this step!)
• Using a known working screen
• Then when we install the IC, take a few diode measurements from the FPC connector
• We test by booting the phone, magical smoke comes out of the coil again

Lets assume that the ICs are good for the moment.It bring us to the under fill around the IC "landing Pad"

Double/triple check that area to ensure that no under fill that would make your IC shift and bridge lines together.

If your coil is smoking then it consuming too much current. Have a look on a schematic. Which IC lines are the coil connected to, A3 and C3, and what is the name of those lines?
What is that line between them... B3? What is the name of that line?

How easy would it be for those lines to touch if there is some small amount of leftover under fill that pushes your IC when your trying to seat it in place?

That would mean (Insert A3,C3 line name) ,and (insert B3 line name) are touching


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New Tests:

Measure the line that the coil is connected to directly. Once again B3 is right between A3 and C3 and a little left over underfill could easily push those together which could short VCC through the coil, then through the IC, then to ground.

You can also visually inspect the balls of A3,C3, and B3 to make sure they are not connected (I think those are on the outside layer, but I don't remember).

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Sly, You've wasting my time, I've already checked all of those things.

Well if everything above is true, then:

• The screen is good
• The original fault has been identified, and repaired
• No new shorts or partial shorts has been created during the repair process
• All components are properly installed, visually inspected, and measured

And your coil is still smoking... Give it a nicotine patch... :/ I don't know lol

All that is left in my eyes are:

• The other lines for the IC which are data line. (Probability, Meh...)
  but a dead short on those could draw too much current. Although be wary about cpu line measurements, those lines are sometimes naturally of very low resistance and can be deceiving. Make sure you know what I mean by that before chasing a phantom short on a cpu data line.)
• Your backlight ICs are bad (Probability, reasonably high)
  Anyone can take ICs from water damage boards, or "dead boards" cleaned them in an ultrasonic cleaner and reball them without testing, and sell them.
  
  Think about how low these are sold for. Then think of the cost to the supplier to just take them off a board, clean them, and reball them.It not far-fetched to assume that some suppliers are getting their parts that way, and wont test them because it is too time consuming for the price they are selling them at.
  
  And what is one of the most common point of failure on a iPhone 6?
  Backlight
  
  What is one of the most likely damaged ICs of those water damage boards, or dead boards then?
  The backlight driver you've just installed on your customers board.
  
  Try ordering from a new reputable supplier to eliminate that potential problem.

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You're still here...Wow

I don't have the answer for you, but this is how I approach some of these problems when I encounter them.

Did you resolved the issue?

if not, do you know of any other variables that could cause this to happen?
Take some time to think about it, and let us know. That is what makes you a good tech vs a please bro tech.
And If you have some other ideas please share what you think. It's ok to be wrong. Better be corrected by someone who helping you than standing corrected by a messed up board.


Good luck!

[Jessa_the_Professa]

coils blow up from heat, and heat comes from shorts. You must have a short somewhere. What does "backlight lines measure pretty close to zxw" mean? What is your exact bl_anode line reading in diode mode, red probe on ground?