boot up iPhone 6 with front panel components unconnected

Hi Chris, thanks for answering. I plugged in another older phone I have which is a 5C, and it showed up as a USB device on the computer (by looking at the macOS "System Information" dialog) before I unlocked with the passcode. I don't think you can get access to data while it is locked, as you might expect, however it does perform the USB enumeration step, meaning that the phone has performed a handshake with the computer indicating that it is, indeed, a USB device. Since the broken phone doesn't do this, I am led to believe that it is not booting and that it would not be worth my time to fix the screen since there are likely other issues. However, I am open to the idea that this behavior is different between the 5C and the 6. I may try to get a junk phone to use the screen and test this theory.

@James Anderson iOS restrictions came in from iOS 11.4. 1 so the 5s might be on an earlier version. But it could be a board problem. You would have to rule out faulty parts first if you wanted to continue with the repair.

right you are Chris, the older phone is on iOS 10. I am probably going to have a shop replace the screen. I also tried using a USB adapted to plug in a keyboard, to try to enter the passcode with that, but I guess the USB restrictions you are talking about may also apply to hosting USB devices. Either that or there is a board problem. Thanks again.