I just changed changed a screen in a 5s and was getting the Apple logo with lines going across then to a red screen boot loop. After replacing the proximity cable, I now get the blue screen boot loop. I got the phone after someone else tried replacing the screen so I can't say whether or not the screws were put in the wrong place as Jim's link above suggests is the cause. I can say I have examined all of the screw holes and can not find any damage to the pcb or any tracks. I will report back when I do find the solution but the link above is not a sure diagnosis.