motherboard not booting up

You'll have to give us more than that...it could literally be anything. Start by telling us the history of the device before it stopped booting up. Did the phone come in contact with water or was it dropped or damaged.

I can see that the CPU shield was removed and that the battery is poked and washed out. Was this for a repair you or a repair shop attempted?

Try these things (if you haven't already) to see if anything helps:

• Try a hard reset (Home & Power button for 10 secs)
• Use a genuine Apple Lightning connector
• Use a genuine Apple charging brick and let it charge overnight
• Try to connect it to different computers (mac/pc if possible) with an updated iTunes
• Put the phone in recovery mode (search DFU Mode). Careful, this can reset your data
• Use 3uTools for putting it in and out of Recovery Mode
• Use 3uTools to flash the firmware (you can't downgrade)
• Check the condition of all the flexes (with magnification)
• Disconnect the Front Camera Ribbon from the logic board and reboot your phone
• Disconnect the screen assembly completely and see if iTunes will recognize the phone.

Otherwise, you may have a logic board issue. If you are uncomfortable doing these steps, consider finding a repair shop to do the troubleshooting. A shop that also does micro-soldering can go deeper if the problem is not modular in nature