unresponsive iPhone 4 touch screen

Hello I'm trying to fix a friends iphone 4 GSM model on 5.1 it was dropped and has no water damage, the touch screen does not work at all i have taken the logic board out on placed it in a working phone and it still does not work, i have restored to stock 5.1 firmware so its not a software issue. i have used Isopropyl Alcohol 100% pure to clean all contacts and have checked the connections several times and still a unresponsive touch screen. and yes i have tried holding home and power buttons to reboot many times still no go.

any suggestions would be of help.