Display does not work / will not boot after repair

Hello,

I started to repair my iPhones (actually my one: 5s and the one from my wife: 5c) and now I have the same problem on both. I changed the displays for both ones and they worked out for some time. But now after repiar action they won't work anymore.

Detailed description:

The 5s: I changed the display once and threw it down exactly one hour later. It worked for a while, but after 3 weeks it started to make some spurious moves without touching the display. So I wanted to change the display again, but now, right after the diplay change, the display stays black and nothing works. I followed the instruction from ifixit carefully.

The 5c: I changed the display, it worked out well for one week, then it suddenly showed a white screen and switched off. I tried to change the display again, but the ame problem here..Does not start.

It is not the battery, I charged both of them (at least the 5s was fully charged),..

Does someone know the issue or can help me out?

Another question: Do I have to reassemble the whole mobile or can I test and boot it before?

THANKS!