I have design a PCB size about 100x60mm it reflow very nice on the hot plate but If I convert my pcb to a panelization 2x3 quantities size became 200x180,the edges began to lift up due to expansion during soldering. I think it is called as "PCB Warping" maybe this expansion was happening when the PCB was in single form, but it was not noticed because it was not that much. But now it's starting to cause problems. how can i solve this problem.

At the moment, after I put the pcb on the hot plate, I press both sides with the help of pliers. I'm waiting for 10 seconds, when the expansion is corrected, I leave it with normal solder. But I think I can solve this problem by adding miling in some places on PCB design and adding features like V-cut to the panel.

result is:

My hot plate is this:

I have design a PCB size about 100x60mm it reflow very nice on the hot plate but If I convert my PCB to a panelization 2x3 quantities size became 200x180, the edges began to lift up due to expansion during soldering. I think it is called as "PCB warping" maybe this expansion was happening when the PCB was in single form, but it was not noticed because it was not that much. But now it's starting to cause problems. How can I solve this problem?

At the moment, after I put the PCB on the hot plate, I press both sides with the help of pliers. I'm waiting for 10 seconds, when the expansion is corrected, I leave it with normal solder. But I think I can solve this problem by adding miling in some places on PCB design and adding features like V-cut to the panel.

Result is:

My hot plate is this: