Many DIY reflow hotplates use a PCB heater, where long, thin traces in the board are used as resistive heating elements. This seems like a great idea, but as [Toby] explains in the project video ...

[Wq] has created a beautiful coaster made out of PCBs that can keep your drink warm with an internal heater ... On board PCB traces, in the shape of a Hilbert curve, create the heating element ...