This is not why your device is rebooting. You first need to figure out the actual cause. And to do that, we first need to understand what you mean by "reboot"[br]
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Style 1: The phone shows an apple logo and then dark screen---but it never actually boots. If so, then this is what a phone will do when it is not connected to a charged battery. Troubleshoot on a DC power supply or a known good charged battery. [br]
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Style 2: The phone will boot, but only stays on for 3 minutes before rebooting. This is the symptoms of a sensor error--and in your case the missing sensor is likely in the charge port or in the connection of the new battery to the battery connector data pins.[br]
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The flex that you've damaged is not important for the boot up sequence or staying on for longer than three seconds.[br]
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Learning to microsolder is rewarding and fun---we teach a course every month here at iPad Rehab, however flex repair would not be a successful place to start learning microsoldering. Work on working devices when you are starting out. Learn to reball and replace easy bga chips and charge ports first.[br]
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Jessa