What is the black spot on electronic circuits?

Question

On some circuits like music toys, remote controls, ... etc

whats the black subctance on the circuit and what's under it?

Answer 1

I am not sure what you are seeing, but I think you are talking about the potting compound used to fix certain components to the board so they cannot be moved.  They also use it to protect you from high voltages in high-voltage power supplies.  Plus, you can "fix" frequency-determinng circuits so they cannot be moved and the frequency altered.

Potting compound can be removed with heat from a shrink-tube heat gun or smd rework station.

Answer 2

It is a protective covering for the main integrated circuit in the device. In some low cost devices the "chip" is mounted directly to the printed circuit board with out it's own protective package.. While I suppose it is a special case of a potting compound, it is harder than many potting compounds, and generally can not be removed without destroying the device.  Chip on board devices would typically be considered disposable, as you mention, they are common in toys.