iPhone 6S Touch Backlight Repair Guide

The backlight power is supplied by the boost IC-U4020, and the output power signal is sent through inductors FL4211 FL4212 FL4213 to the display screen interface J4200. Unlike the iPhone 6, there are two sets of boost inductors and rectifier diodes. If one set is damaged, it does not affect the light display. However, when taking a photo with the front camera using the flash, it can damage the light control IC. The front camera’s flash is achieved by enhancing the brightness of the backlight, so it has one more set of boost inductor and rectifier diode than the iPhone 6. In a normal device, the voltage measured at capacitor C4023 when the light is on is around 17V. This voltage is related to brightness, and when the brightness is set to the highest, it is about 20V. The lowest is around 16V.

If there is no display and no light, while the screen is installed and powered on, measure whether there is about 17V voltage at capacitor C4023. If not, continue to measure whether the display power IC output is 5.7V. If the 5.7V voltage is normal, it indicates that the CPU has issued commands to activate the display and backlight circuits, and the problem lies in the backlight power IC-U4020, which can be replaced with U4020.

If there is around 17V, measure the resistance to ground of pins 1, 2, and 3 of the interface to check for short circuits in FL4211 FL4212 FL4213. If either FL4212 (PP_LCM_BL_CAT1) or FL4213 (PL_LCM_BL_CAT2) is damaged, it can cause a display problem where one side is bright and the other is not. If one side is bright and the other is not, check for short circuits in FL4212 and FL4213. If shorting FL4212 and FL4213 still results in one side being bright and the other not, then it is a problem with the boost IC, which can be replaced since replacing the boost IC-U4020 is troublesome. Alternatively, it can be resolved by shorting the green line shown in the figure below.

If the inductors on the channel have been ruled out and the backlight power U4020 has been replaced but the fault still persists, remove U4020 and measure the resistance of each pin on the pad to ground to see if there is an open circuit or short circuit. If there is, check whether the connected inductors or capacitors are damaged, and troubleshoot one by one.

The touch IC of the iPhone 6S is on the display screen, and there is no touch IC on the motherboard. Of course, the power supply part must be on the motherboard! The power supply for the display screen and touch screen is provided by power IC–U2000 and U4000, with U4000 providing positive and negative 5.7V and 5.1V, while the power IC provides 1.8V power supply.

When there is no display and no light, first measure the resistance of pins 1, 2, and 3 of the display screen interface to rule out the problem of inductors FL4211 FL4212 FL4213. After ruling out the inductors, install the screen and power on, measuring whether the voltages +5.7V and -5.7V in the figure below are normal. If not, check the short circuits of FL4220 (LCM_TO_CHESTNUT_PWR_EN_CONN),

check the short circuits of FL4221 (AP_TO_LCM_RESET_CONN_L)

replace the display power IC–U4000

If all the above 5.7V voltages are present, then check whether the inductors FL4200 FL4207 FL4208 are corroded or damaged, causing the three power supplies to fail to reach the interface. You can short these three inductors! After ruling out the 5.7V voltage, check whether the 1.8V voltage is normal (check the short circuit of FL4205)

If there is display but touch is unresponsive, check the short circuits of FL4209 FL4210