iPhone vibrating when connected to charger, but no image

Hello everyone!

I bought the iPhone 6 in hopes of fixing it, the owner told me that it only shut down and it was not responsive with no back story of the phone, at first the phone didn’t showed any signs of life neither on the charger, I tried the button combos but nothing happens, when I got it open, there where a complete mess, lots of dust, mising screws, but nothing so much critical, I replaced the battery and tried turning it on, nothing happened, I inplugged display connectors and battery connector, plugged it again with attention to make it sit well, still nothing.

I connected it on the charger and pressed home button and power button combo and it had 2 very strong and weird vibration waves, but still nothing happened, I tested it with another lcd, but the same occured, also I plugged it to the PC, in hopes it will recognize the phone, this time I had same 2 Vibrations, and smaller one in short time, all that with the old one and other lcd, there were no indications of water damage but I cleaned all the connectors carefully, the last thing I noticed was that on the last socket connector for lcd there was something black on one of the contact lines, I looked it under the microscope and it looked like it’s missing one of the contact lines, if anyone can reply and recommend me something it would be high appreciated!

Thanks in advance!