How to fix iPhone 6 black screen/WIFI/volume button failures?

The iPhone 6 has black screen, WIFI cannot be recognized, volume button not working issues after replace the iPhone display screen, how to fix?

Disassemble the iPhone by phone opening tools, and then remove the iPhone motherboard.

Meassure the diode data for display socket J2019 with Fluke 15B+ Digital Multimeter, we find the diode data for 14 pin AP_TO_LCM_RESET_CONN_L is infinitely large, which is disconnected.

Put iPhone motherboard under the Stereo Zoom Microscope to check , we can see that there is the trace of a breakdown for left screw column BS2502,  scrap it and re-connect the disconnection. Measure the resistance value to grounding for display socket J2019 14 pin AP_TO_LCM_RESET_CONN_L with multimeter, the data is normal.

Install the iPhone and test it, we find the iPhone volume button not working, and the WIFI cannot be recognized.

Measure the resistance value to grounding for iPhone volume button socket J0802, it is infinitely large, and perhaps it is disconnection. This circuit line is on the third floor of the left screw column that near the edge position, carefully scrape it and then re-connect it.

There are 2 reasons causes the WIFI cannot be recognized, one is the iPhone motherboard disconnection, and the other one is iPhone WIFI module IC problem. Remove the iPhone WIFI module IC and measure the resistance value to grounding for solder pad, no abnormal was found. Replace the new iPhone WIFI module IC, and then measure it agian, seems all data are normal now.

Finally, install the iPhone and turn on iPhone 6 to test, the iPhone display screen works fine,  test the iPhone volume button, it works properly, test iPhone WIFI, also works good. All failures has been fixed, the iPhone 6 back to normal work.