In the oven thermal inertia has two effects:
- Inertia in heating up the infrared lamps
- Inertia in heating up the PCB
- The infrared lamps that we use are specially made for us out of a high quality spiral wire encased in a quartz tube. To heat up the lamps we use a 10 amp current. It takes 20 – 30 seconds for the wire to heat up fully and radiate to its full capacity. This means that there will be a delay of 20 – 30 seconds between the oven receiving the command to increase the heat based on the soldering profile set and the temperature sensors showing the new temperature. There will be a time lag between the temperature profile set and the actual temperature profile displayed on the oven or on the PC. This is not due to poor temperature regulation but to the thermal inertia of the infrared lamps.
- Each PCB also has a thermal inertia. This varies according to the design and construction of the board itself including the amount of copper 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.
- The thermal inertia of the PCB has two effects:
- During the heat-up period the actual PCB temperature displayed by the external sensor will lag behind the temperature shown by the oven sensor.
- During soldering the board temperature shown by the external sensor may actually be higher than the oven temperature as the board continues to absorb energy even when the lamps are modulated downwards.
- Both of these effects are normal and should be taken into account while choosing the best soldering profile for a particular board. Also on the where and how to place the external sensor on the board it has an effect. See external sensor.