The bottom heater is very important to support the soldering.
You should always distinguish between a single sided assembled or double sided assembled PCB.
For a single sided assembled PCB you can always set the bottom heater to „follow 0°C“. This makes the soldering very efficient. The oven can reach a maximum heat climb of 80°C per minute. This varies according to the design and construction of the board itself including the amount of copper on the board, the size and number of components on the board, the presence of heavy copper inner layers, the use of thermal reliefs on the vias, even the colour of the solder mask.
Setting a fixed temperature of e.g. 175 ° C is more suitable for double-sided assembly, as the bottom of the board is less heated.
14. Set the bottom temperature. For double sided soldering we suggest that the bottom heater is set to a maximum temperature of 175°C for lead free soldering and 150°C for lead soldering. This to keep the temperature on the bottom side as low as possible and yet high enough to support the solder process on the top side.
15. Set the offset with which the bottom needs to follow the top temperature. Generally we define the bottom heater to follow the top IR-lamps curve on a distance of 40°C. This means that the bottom heater parameter is set to -40°C. This way the bottom heater supports the top curve in the best possible way. If more power is needed the -40°C can be reduced to -30°C or less or even follow (which is 0°C).