Hello,
this is Louis from the Electronics department of the Formula Student Team ELBFLORACE.
We are using similar PCB specifications (FR4) for our own designs and also operate a three‑stage reflow oven with one reflow zone and two preheat zones. Our temperature setup is 140 °C for the first preheat stage, 190 °C for the second, and 260 °C in the reflow zone. The actual reflow occurs after roughly one minute in the reflow area.
FR4 can generally withstand temperatures higher than its nominal rating once or twice during manufacturing, so you do not need to worry about the material being damaged by short exposure.
In your case, the issue is most likely not the PCB material but the components used. Many electrolytic capacitors and connectors, for example, have a maximum temperature rating of around 240 °C, so this must be considered and adjusted accordingly.
Additionally, make sure to adapt the time spent in the reflow zone to the thermal mass of each pad. Large copper areas—such as GND polygons with increased copper thickness—require noticeably more time to reach melting temperature.
I would recommend experimenting with several similar temperature profiles on different PDB boards to find the most suitable setup.
I hope these tips help you in selecting the right temperature profile.