iPhone Backlight Failure – What Causes it?

This post is currently being drafted, and should be considered incomplete. It was posted early in this state at the request of a reader.

In Mobile Phones and other electronic devices, there is a common circuit known as a “Boost Converter“.

The Boost converters task is to take a given voltage, and increase it to a higher voltage. This is vital for certain operations, such as driving an LED array, used to provide backlight for an LCD panel.

The circuit is typically made up of the following components:

  • An Inductor – Used to accumulate a Magnetic Flux.
  • A Driver IC/Mosfet – Which causes electrons to rapidly flow through the inductor, then releases them in a repeating cycle.
  • A Diode – Which passes the electrons on, only allowing them to flow in one direction.
  • Capacitors – That take the electrons from the diode, and store the now higher voltage temporarily.
  • A Fusible Link (Fuse/Filter) – Which protects the aforementioned against a shorted output.
  • A Load – That uses this higher voltage for a given purpose, such as driving an LED Array.

You can read all about Boost Converters here.

 
 
 

Backlight Circuit Failure

There are 2 types of observable failure we see in iPhones. These can be separated into an event caused during use/damage, and an event caused during repair.

Spontaneous Backlight Failure

Complete Backlight failure occurs when a capacitors dielectric suddenly fails, resulting in a short between conductors. In ceramic capacitors, this usually happens from shock/flexing, resulting in stress fractures in the capacitor. These stress fractures result in the capacitors electrodes making a direct connection.

TDK have an excellent article explaining this occurrence, as well as countermeasures engineers/manufacturers can use to avoid it.

Starting with the iPhone 6, due to the flexibility of the device (dubbed “BendGate”), this type of failure became a common occurrence.

The result of a shorted capacitor on the Boost Converters output, is typically Catastrophic, Complete Backlight Failure.

The industry refers to this failure as such due to the chain reaction caused by this type of event. A breakdown of the chain of events is as follows:

  • A Capacitor develops a short – resulting in excessive current draw.
  • The Diode fails – Usually explosively, resulting in the Anode and Cathode shorting together. At this point, the diode becomes a wire.
  • The Inductor overheats – Due to the excessive current, resulting in its windings shorting together, and it losing inductance.

The same failure can occur as a result of Dielectric Breakdown. This is generally the result of an engineering oversight, where the rated voltage of the capacitor is too close to the circuits operating voltage.

This can also be observed in the iPhone backlight circuit, where 25V capacitors are used in a 22V circuit, rather than 35V capacitors. It’s objectively speculated that this design may have been some form of intentional Planned Obsolescence.

Determining which of these two events resulted in the Catastrophic Failure, is usually a matter of measuring the resistance of the capacitor(s). A short caused by fracture is typically very close to 0 ohms, where a short caused by dielectric breakdown generally has some resistance.

Visual inspection can also provide some insight on the cause of failure, as cracks in the ceramic substrate can often be observed. It’s important to note that this isn’t always the case though, especially when the fracture occurs under an end cap.

Accidental Damage

The second type of failure is generally caused during repair, or from a poor quality part that lacks insulation.

This occurs when the output is shorted externally, causing the fusible link to fail as intended.

The most notable event that results in this type of failure is technician oversight, where the battery is left connected during repair.

It important to note that this can occur with the device turned off, as in many cases the Boost Converter obtains its power from an always live rail, such as PP_VCC_MAIN.

For this reason it is imperative to disconnect the battery during repair. However, failure to do so tends to result in fairly isolated damage to the fusible link/filter, and not a catastrophic chain reaction failure.

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