iPhone 7 starts up with a black screen or no display, yet the boot current is normal. Screen replacing also cannot solve the problem. We can draw the conclusion that the logic board backlight circuit broke down. REWA LAB offers a logic board solution regarding the iPhone 7 black screen issue(iPhone 7 home button not working).
iPhone 7 Black Screen Repair – Logic Board Solution
Power key power up, the phone shows no response.
Take apart the phone and disconnect the battery. Connect the phone with the DC Power Supply. Booting current displays normally, yet the phone goes with no display.
Disconnect the power supply and detach the display assembly. Replace with a new display assembly. Connect the phone with the DC Power Supply and power up. The phone still goes with no display
Preliminary judgment – backlight circuit of the logic board broke down
Take out the logic board. The test grounding resistance value of J4502 – normal value. Which shows that backlight circuit is also normal. Yet backlight signal is connected directly with backlight IC U3701.
Preliminary judgment – backlight IC U3701 broke down
Apply some BGA Paste Flux, take down the backlight IC with Hot Air Gun at 320 degrees.
Clean the tins with Soldering Iron and then clean again with PCB Cleaner
Apply some BGA Paste Flux. Align the backlight IC to the right place and solder it with Hot Air Gun at 320 degrees.
Once the logic board is cooling enough, clean it with PCB Cleaner.
Assemble the phone and test. The phone boots normally with screen display—fault cleared