Home made reflow plate toasting PCboards. What is the trick?

[jorgemef]

Hello,

I built myself a reflow hot plate base on eletronoobs design -> https://electronoobs.com/eng_arduino_tut161.php.
I added my own code and implemented 3 profiles.

//{"leaded",{"Preh",RAMP,60,150},{"Soak",RAMP,120,165},{"Refl",HOLD,20,225},{"Cool",RAMP,120,50}},
//{"lead-free",{"Preh",RAMP,60,150},{"Soak",RAMP,120,180},{"Refl",HOLD,15,245},{"Cool",RAMP,120,55}},
//{"dessolder",{"Preh",RAMP,60,150},{"Soak",RAMP,120,180},{"Refl",HOLD,15,245},{"Cool",RAMP,120,55}},

Had to adjust somehow how it works as the plate is not fast enough to heat on the reflow phase even with PID maximum. At maximum it heats at 1-3ºC/s depending on the ambient temperature, and has a lag of 15-20s, so for the reflow step the code is relying on the time more that on the inpu/output temperature.

{"leaded",{"Preh",RAMP,60,150},{"Soak",RAMP,120,165},{"Refl",HOLD,50,225},{"Cool",RAMP,120,50}},
{"lead-free",{"Preh",RAMP,60,150},{"Soak",RAMP,120,180},{"Refl",HOLD,15,235},{"Cool",RAMP,120,55}},
{"dessolder",{"Preh",RAMP,60,150},{"Soak",RAMP,120,180},{"Refl",HOLD,15,235},{"Cool",RAMP,120,55}},

Anyhow the profile looks good but my issue is that the lead free profile toasts the PCB.
Even the leaded profile which I add the plot in attachement burns somehow the PCB.
In the image, from left to right, original PCB, then pcb with leaded profile attached as well then lead-free profile results.

I tuned the temperature reading on the board surface with BM869s so the mentioned temperature is really the temperature to which board was exposed.

So what is the limitation with this kind of process? I noticed that 63/37 starts melting/flowing when plate reaches around 210-215 but is flowing well with 225 but then fr4 is toasting.

BR,
Jorge