Backlight 101. It's Never the Coil.

Not even the rice myth is as rampant as the little lie that is posted on repair forums around the world on every single day.

"I can't see anything on the screen, but I know the phone is on, help!"

--"It's probably the coil"  

--"Yeah, bro.  Replace the coil."  

A bunch of likes, upvoting, and general head nodding then occur from people that have never actually seen a backlight coil.   This happens every day, around the entire world, in every repair forum.  You know you have done it yourself.  Everyone has, but it is now time to step into the backlight.

What is a backlight anyway?

The LCD screen on your phone, with all of its rich colors and detail is just a dim slab of nothing without the backlight.   Looking at a replacement screen, you'll see the wide digitizer cable, and the smaller LCD cable carrying the data to color the pixels of the screen.  Looking even closer you can see a thin little third flex soldered to the assembly. This the backlight flex.   Through this flex flow the electrons that light a tiny strip of LED lights---the backlight.  The light is diffused and spread around nicely by several backlight sheets.  The whole thing is married to the back of the LCD.    As long as the electrons march through the flex, the strip of LEDs light, and the backlight comes on.

So how do we push electrons through the backlight flex?

Every backlight circuit on a mobile device is more or less the same.  Like you, I am out of my comfort zone when I move from thinking about plugging in connectors to trying to analyze a circuit.  But let's put that aside and forge ahead.

The brains of the display is the Power Management ic chip.  This guy generates a voltage that is passed to the coil. Remember a voltage is a hill for the river of current to flow down.

 An inductor coil is a simple little square component.  It is what it sounds like.  A coil.  A coil of wire.  A tiny slinky.

The magical power of the coil is to boost power. The coil, or inductor, can transform the relatively small incoming voltage from the Power chip into the big 15-20 V required to light the energy sucking display.  The flow of current through the coil does some magic called electromagnetic induction that can be at once incredible and overwhelming.  But really, it is just mechanics.  When electrons go for a swirly ride through a coil, they generate a big voltage.  A big voltage that can push electrons all the way to through to the flex and light the LEDs.   All we need to know is "The coil.  Big Slinky.  Sturdy. Boost voltage"   In order for the coil to stop working, you'd have to come by and snip off the wire as it connects the coiled spiral of wire inside to the pad below the component.  Practically, this can only happen in very specific cases of water damage, where water essentially eats the base wire of the coil.   Since the days of the iPhone 3Gs, modern coils are surrounded by waterproofing.  Hence--they never fail.

Next we have the backlight diode. 

A diode is a pretty simple component.  Diode = one way current flow.  

The diode is a cop, a bouncer, an organizer--it will only let electrons file across its sensitive middle if they move along single file from one side to the other.  It is constructed from delicate materials--chocolate on one side, vanilla on the other.  Red light. Green light.   And that kind of makes it prissy.   It is not a simple semiconductor--not like a wire or a rod.  No. Inside it has two different materials seamed together.  This is what makes it directional, it stands on two simple pads, and  you must  install it only one way.   Guess what happens to the poor little guy if an excessive unregulated voltage were to hit it?  Like, say, someone pokes a screwdriver snagging a component to the frame, shorting the circuit?  The diode will wilt like the delicate flower it is, and let out its magic black smoke.  We all know that once any component lets out it magic black smoke, it is curtains.   

 Wait, backlight diode?  What?  You may be skeptical.  You may not have heard about the backlight diode before. However,  I've bet you're heard of "the backlight ic." News flash. In modern phones and iPads,  they are the same.  Identical.   An ic, or integrated circuit, is an overstretched promotion for the simple diode.  This misuse of terminology was intentional.   When folks started hiring other people to fix their iphone backlight circuits, the implication that what was being done was the  installation of some sort of ic chip perpetuated a sense of difficulty of the operation.   While some  phones do have a small ic providing some logic for the backlight, nearly all iPhones  and iPads today just have a simple diode.   The terminology "backlight ic" remains because--- "Install an ic, means hire a professional"  "Install a diode, means give it a shot myself"    

 A quick search of eBay for the phrase:

  • "iPad backlight ic" = 25 different listings.  
  • "backlight diode" = 1 listing.  
  • All 26 are the same diode.

On to the filter.  The real bad guy.  

A backlight filter is a miniature ferrite bead encased in ceramic with two conductive surfaces.  Like a coffee filter prevents the passage of chunks of Starbucks from sticking to your teeth, a ferrite bead can prevent the passage of unwanted high frequency signals in a circuit.   Since a DC current circuit doesn't have to worry too much about frequency modulation, I think the backlight filter is there to suppress electromagnetic  inteference (EMI) noise.  Which probably means, in the context of a backlight,-- suppress flicker.    

All iDevices have at least one backlight filter.

The intact filter is required to complete the path from the diode to the LCD connector.   The problem is that they tend to be very susceptible to 1.) water damage and 2.) current spikes.  In practice, the filter is the first component to go when the circuit is in trouble.  As such, they act as a fuse.   Intentional or not, I don't know.  However, when a sudden drop in voltage from pulling the LCD connector in the iPad mini while the battery is still connected causes a transient spike in amperage, like a dutiful Romeo, the backlight filter will sacrifice itself, breaking continuity.   Some backlight circuits have one filter, some have two or three.  All of them are required for the intact delivery of the voltage necessary to light the LCD.
When the filter is destroyed, it will usually have a very characteristic appearance.  (See pic above)  The ceramic splits and a pin prick of solder is exposed.  That filter must be replaced.

LCD connector in an iPhone 5 with water damage

iPad 2 LCD connector Water damage---pads and even the pins of the connector are completely corroded away

The last guy in the line is the LCD connector.  Most of the pins of the connector are receiving data to build the image on the LCD.  Only a few pins are required to deliver the voltage to the backlight LEDs.    Can you guess which ones they are?  Take a look at the LCD connector of any water damaged phone. Look for a pin that is the blackest with the most chewed up pad--that's the one meant to carry the big voltage for the LCD display.

Normally, a failure in any component of the backlight circuit will cause a dark display when the device is on.    However, in later model full sized iPads--the iPad 3, 4 and Air things are a wee bit more complex.  The backlight circuit is split into two.  There are two coils, two diodes, two filters that come together to feed one LCD connector.   This strange situation can result in this common problem:

Half Dim screen = only one side of the backlight circuit failed.  Easily repaired by replacing the failed filter.

One filter is clearly dead.  The other looks like it has one foot in the grave.

Why do we care about all of this?  Let's apply:

If you're trying to fix a phone with black on the LCD connector and you wonder why the screen won't light up, what is MOST likely to be the problem?

A.) The coil.

B.) Anything but the coil.

Give yourself a star if you chose B.  No backlight is a CLASSIC and COMMON problem in all mobile devices. It is almost always repairable, you just have to know where to look.

How to Fix any Backlight problem:

  1. First check the LCD connector.  If you see black, that's the problem.  You'll have to reflow those joints, or replace that connector.
  2. Once done, hunt for the backlight filters.  They may be near or far from the LCD connector.  You are looking for ceramic components with a black center.  Think ferrite bead = iron= black and find them.  (I can't believe that this little relationship just occurred to me for the very first time right now.)  If they are damaged, they will not have continuity from one side to the other.  Get out your multimeter.  Set it to continuity testing.  If you don't hear a beep on the backlight filter, that's the problem. Many times, if the backlight filter is severely burned, it has become welded inseparably from the underlying pad.  In these cases, it is not possible to replace the component.  But it is still possible to save the device---build a custom microjumper to restore continuity in the line. If you do find a backlight filter that looks severely burned, then
  3. Check out the diode.  If you find the filter you can find the diode using your multimeter even if you've never seen the device before.   Put one probe on the end of the filter.  Stab around with the other probe until you find a diode-looking component that has continuity with the filter.  That's the backlight diode! Test the diode.  Set your multimeter for diode testing.  Measure black probe to one side red to the other.  Reverse probes.  You should get a "1" = infinite resistance in one direction, but not the other.  If you get a "1" in both directions, replace the diode.  Remember that the new diode should go in the same direction as the original diode! After you have done this, and your device will still not show a backlight with a known good display, THEN and ONLY THEN should you consider the coil.  But honestly,
  4. Just quit right there.


It is Never the Coil.

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