Edit: Added initial test results.
After some tests with larger plates and Arduino sized PCBs it turned out that 100W USB-C power was not enough for my Hot Plate to get enough heat into the PCB to melt the solder reliably. It was more like a warming stage and I had to do the soldering with hot air, which went very well but was not desirable.
So I took all the MCHs and attached them to a good sizes plate (big enough to solder 2 Arduino shields at the same time) and wired them in series. The resulting resistance is 37 Ohm, which is resulting in 3.2A or a whooping 390W of heating power. AC is controlled by a solid state module and protected with an extra 15A circuit breaker.
Some pictures of the build and setup.
5 MCSs wired in series, the one with the highest resistance (2nd from the left) as much as possible in the center.
AC wiring with solid state module and 15A breaker.
And I powered the system up and got some interesting results. First, the thermal sensor is visible at the lower right corner as far away from the heating elements as possible.
Running a full profile without PCB on the plate got the sensor up to ~240degC, which seemed to be lower than expected. Another test run with a PCB and solder pieces (leaded and lead free) melted the leaded solder easily, but the lead free one barely got liquid and did not melt into a nice ball due to its surface tension.
To verify the performance I connected the AC business end through my trusted KILL-A-WATT power meter. And I got an unexpected surprise. The power started at ~300W as expected, but dropped with increasing temperature to only 120W at 200degC. I have to read up on thermal properties of those MCH elements.
The solution seems to be counter intuitive. I have to decrease the number of heating elements, to decrease the resistance and increase the current. Because of P=I*I*R this should give me more power. Let's find out.
To be continued....
And finally a little detail:
For the breadboard connections, I tried something new. I used 2-pin jumpers for a single connection to give them better retention force. The jumper pins are thinner than regular ones to not bend the breadbord inserts permanently. With 2 pins they hold better in place, they only have 2 minor drawbacks. First, you have plug them in with the correct orientation to not short the signal with a neigbouring one. And 2nd, they take up 2 spots of the row of 5 connected holes.
Jumper wires with 2-pin header:
Inserted jumper wire (Ground and one signal):