Troubleshooting way for iPhone 6 touch function insensitive

Today, we need to fix an iPhone 6, which problem is that the iPhone 6 touch function can't used normally. This iPhone 6 has been repaired several times, but all failed. Now, VIP Pro Shop Team will use professional phone repair tools to troubleshooting it.

After we received the iPhone, we tested it and found that the fault was a little different. The specific failure performance is that the iPhone shows normal after entering the IOS, touch it with your fingertips has no effect; Touch it with your finger on the iPhone display screen, it works but not sensitive; If a little bit of force is used to press the screen to slide, it can be used normally. However, it's obvious that the owner of iPhone can't use the phone properly.

Testing it by disassemble the iPhone, we found the white touch IC and black touch IC has been replaced. Because the phone has been repaired many times, although not repaired, but these two chips have been replaced, which means that they are normal. And then we according to the fingers with a little force can slide to judge the line that from black touch IC to touch socket and touch socket itself are good. Because if they are abnormal, either the display screen will appear with a vertical bar or the touch is completely unavailable. Either lack of transverse or lack of vertical, but in the actual test did not find a lack of transverse or lack of vertical phenomenon, so the above judgments is correct.

And then according the above phenomenon, we judged the circuit line that from iPhone CPU to white touch IC also normal, so we decided to repair it starting from these two chips peripheral circuits. Use Fluke 15B+ Digital Multimeter to measure the black touch IC +-5.7V voltage and 1.8V voltage, all is normal. Measuring the white touch IC 5.1V and 1.8V, also is normal. Then we think it would be more precise to use the Hantek DSO1060 oscilloscope to measure, but the result of the oscilloscope measurement is normal. There is no idea now, we open the WUXINJI circuit diagram and analyze it, see if we can find out the reason and solution.

After checking the WUXINJI circuit diagram, seems find nothing. But we see a bias capacitor C2439 on the black touch circuit, and then think of the iPhone that was repaired, which the audio have sound, but the sound is small or very light fault, all of these failures are repaired by replacing the audio bias capacitor next to the iPhone audio IC. Will this touch faulty be caused by this bias capacitor?

Measure the diode data of other ungrounded ends first, we found that it's about the same as the normal board, but we still decided replace it with new one. Then we remove a same capacitor from damaged motherboard, and install it with QUICK 861DW soldering station. 

Install the iPhone and test it, now the touch function has regained sensitivity. The iPhone owner can use it normally!