To help bring an end to this question, I did a small experiment.
The following were used in this experiment:
Sampo KM-B12 1200 Watt portable induction hob
Flat-plate steel griddle, approximately 12" square
A sensor consisting of 6 4" diameter loops of wire, connected to a 3 mm run-of-the-mill LED through a 301 ohm resistor
With the sensor on top of the steel griddle plate, and the hob on high power, the LED produced no visible light and the coil of wire didn’t heat up above the temperature of the griddle plate. Based on this test, I would say that any currents induced in the senor assembly to be insignificant.
An interesting result was observed with the griddle plate removed and the sensor assembly directly on top of the hob. Note that most hobs sense if sufficient mass is present and shut down iif there is not, so this test may not even be possible on most hobs. With the hob at full power, the LED was observed to be dimly flashing. Enough energy was coupled into the coil of wire to illuminate the LED, but the LED was far dimmer than it would have been with a typical 10 mA current. In fact, the flashing could only be observed when the room was totally dark.
Based on these results, I would have to say that using a hob with a “normal” thickness of material between the hob surface and the PCB is unlikely to cause damage. Putting the PCB directly on the surface of the hob could result in damage but even in this situation, it’s unlikely that traces will be vaporized - and this situation of PCB directly on hob surface is not how a PCB would be heated.
So to the question of why a hob might be used, most of them can be set to reach and maintain a given temperature and they do it quite accurately. The rate of temperature increase can be quite rapid, and is controllable by the power setting. Thus, the temperature and the ramp rate can be controlled accurately.
As to making the hob unsafe to use for food, a smooth steel plate can be used to heat the boards. Don’t use your normal cookware!
Jon
This photo shows the sensor and griddle plate on the induction hob. The hob is a Sampo KM-B12 1200 watt table top unit.
This photo shows the 3mm red diffused LED and 301 ohm 1/4 watt resistor, along with a crappy soldering job.
This photo shows the result of the sensor directly on the induction hob with no intermediary layer of metal.
The LED is flashing but it’s extremely dim. The multiple image is caused by my movement in a dark room;
It was a 1 second exposure at an ISO equivalent of 1600, with an F-stop of 13. The LED was so dim it
could not be seen with room lights on.