LCD Screen Backlighting
The backlight circuits in all iDevices with an LCD (excluding the classic iPod or Nano) share a common architecture:
- A power chip generates the backlight signal.
- An inductor, commonly called the “backlight coil," amplifies the signal.
- A diode prevents reverse voltage.
- A ferrite bead filters the signal.
- A ribbon cable carries the signal to the backlight LED strip.
- Some devices have an additional backlight driver chip.
Troubleshooting the Cause of a Backlight Failure
Most iPad/iPhone hardware operates at voltages between 1.8 V - 5.2 V. However, the backlight circuit operates at about 15 - 20 V. At this higher voltage, the backlight components are more prone to damage when a short circuit occurs. The high voltage backlight circuit is also prone to corrosion from water damage.
The Most Common Causes of Display Failures
- The LCD screen - LCD failure can occur from drop damage, water damage, or it can simply be the result of a defective part.
- The backlight filter - When a short in the backlight circuit occurs, the thin wire inside the filter breaks, severing power from the backlight LEDs.
- The backlight diode - Like the backlight filters, the backlight diode is a fragile component. In cases where a backlight filter is particularly burned up, you’ll often find that the diode has failed as well. Diode failure in the absence of filter damage is rare, but it can happen.
The inductor coil is rarely the point of failure in modern iDevices.
Backlight Short Circuits
The most common cause of a self-induced short occurs from working on the device with the battery still connected. Even when the screen is dark, there is voltage in the backlight circuit. A slipped pair of tweezers or misalignment of the LCD connector can short the backlight circuit to ground. (The iPad mini is particularly prone to this fault, as simply removing or inserting the flex cable into its connector at a slight angle is enough to bridge the backlight’s high voltage pin to the adjacent ground pin.) Avoid self-induced shorts by always disconnecting the battery before working on a device.
Another cause of backlight shorts is faulty assembly procedures. iPhone screens have a soldered joint on the LCD flex cable connecting the thinner backlight flex cable that powers the LED light strip. During device fabrication, these solder joints are protected by piece of black tape—however, during the screen refurbishing process some manufacturers neglect to replace the tape, apply it misaligned, or fail to apply it securely. As a result, the screen initially works during testing—but once the metal LCD shield is installed, the exposed solder joints touch the grounded frame, shorting the backlight circuit.
Backlight shorts can occur when the latch for the ZIF connector securing the LCD flex is missing. The LCD flex slides out an angle and the high voltage backlight pin contacts the ground pin, causing a short circuit.
Other Common Causes of Backlight Failure
Water damage is a frequent source of backlight problems. Water will corrode the LCD connector pin/pad junction, which breaks the electrical path to the connector and can damage the filter.
Backlight circuit failure can also occur from damage to the electrical traces on the circuit board. If the electrical traces buried in the board are inadvertently severed—for example, from trying to fasten the board with too large a screw—the backlight circuit will not conduct power to the backlight LEDs.
Diagnosing a Backlight Failure
To diagnose whether your device is “dead” or just has a malfunctioning screen, try connecting it to your computer. If the computer recognizes the device, then the problem probably resides with the LCD screen or backlight circuit. Additionally, iPhones will notify the user of a backlight problem by repeatedly playing a chime sound and vibrating.
The good news is that nearly all backlight failures are repairable. Once the damaged component is identified it can simply be replaced. If this isn’t something that you can do yourself, call a knowledgeable microsolder shop and send it out for a quick repair.