iPhone 6 touch function not working after replace screen

Trouble Phenomenon

American customer’s iPhone 6 has been dropped and the front cover glass has broken. The touch function is lost even after replacing a fine LCD screen.

Repair Process

First replace the old screen with a fine LCD screen and test. The touch function cannot be used, which suggests the problem existing on motherboard. Take out motherboard and check, as shown in picture 3-21.

Picture 3-21

As the touch function is lost, we check the touch circuit directly. First measure the diode data of communication coupled capacitor between touch process chip and touch control chip, finding no problem. The coupled capacitor’s location is shown in picture 3-22.

Picture 3-22

When measuring I2C chip bus, find the diode data of TOUCH_I2C_SCL is several tens. The normal one should be over 300 and the clock should be the same as data. Generally, the low diode data is caused by short circuit. The main chip of this signal is only the touch chip. The circuit diagram of I2C chip bus of touch chip is shown in picture 3-23 and its measuring points in bitmap is shown in picture 3-24.

Picture 3-23

Picture 3-24

According to the experience, the damage of touch process chip will cause the failed usage of touch function in addition to the short circuit. Thus, we decide to remove the touch process chip, as shown in picture 3-25.

Picture 3-25

After removing it, the diode data of the two bus circuits of I2C chip are the same, which becomes normal. We know the touch process chip is damaged now. Replace the touch process chip and the trouble is removed.