2 GSM iPhone 4's not powering after screen replacement.

Hello,

I've been repairing CDMA iPhone 4's for a while now. Probably done 2 dozen screen repairs without any problems. I recently picked up a few GSM iPhone 4's with cracked screens but no other issues. I replaced the screens on the first one and it's bricked. No power at all. Figured it was some kind of fluke so I went to the second one. Replaced the screen. Bricked. No power at all. I am at a complete loss as to what may have happened here.

I've tried everything I can think of. Switching batteries with batteries that i know that work. I've checked the power flex cable for any kind of breakage. I've dissembled both, switched motherboards, checked all the connectors.

If there is anything anyone else on here can think of to recommend trying let me know. I got them both on the cheap but I"m still out $300 that I could have sold them for. All I can think of is that I somehow shorted the logic boards. Kind of amazing to me that I'd short them both though given that I've done dozens of cdma replacements and never shorted a single board.

Any kind of helpful suggestions that don't equate to "you're an idiot" would be appreciated.

Thanks