How to confirm if motherboard is broken
If it starts to boot (goes to the boot logo/animation) or Recovery/Download mode at least tries to load, then the motherboard is NOT damaged.

Your device is soft-bricked, and just needs to have a compatible ROM flashed, and your userdata erased for your device to be fully functional again.

The person is the service centre doesn't know what hey is talking about (I wouldn't trust those people - they just want your money).