I need to design a circuit to warm my PCB. There are many ways to build up such circuit. But I learned from a post "Warming PCB in a low temperature environment" that maybe I can use traces as heater.

My first idea is to use one of the internal layer as a heat bed and place copper traces there. I have searched the Internet for a while but I can not find any application note or any discussion on this topic.

So my question is: is it good or proper to use the internal layer as a heat bed? If not, any disadvantage?

(I am not familiar with the fabrication process of PCB boards. So I am not so sure whether could I place traces in the internal layer)