The Touch function of iphone 5 cannot be used.
1. Turn on the iPhone and the find the problem is the same as the customer’s description. After teardown the iPhone, first test with a fine screen and figure out whether the damaged screen caused the failed usage of touch function. But the problem is still the same even when the damaged screen is replaced, which suggests the problem is not caused by screen but motherboard.
3. Continue to measure the each power supply circuit of touch control chip U12 (white touch) and touch processing chip U14 (black touch). The voltage of PP1V8_GRAPE and PP5V1_GRAPE_VDDH is normal.
4. Then measure the power supply circuit of PP5V7_SAGE_AVDDH and PN5V7_SAGE_AVDDN, which is also normal.
5. The material object of touch control chip and touch processing chip is shown in below picture.
6. Each circuit of touch chip becomes normal, so touch process chip is very likely to be damaged. Replace the touch process directly, as shown in below picture.
7. After replacing a new touch process chip, assemble iPhone and test. Touch function recovers. We can be sure it is the touch process chip that causes the problems of touch screen.