The effects of thermal inertia in a chamber reflow oven like eC-reflow-mate-V4:
- Inertia in heating up the infrared lamps
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. To ignore this effect we set temperature points and temperature hold times. The time to reach these points is also determined by the thermal inertia of the infrared lamps and is not due to poor temperature regulation.
Infrared lamps in eC-reflow-mate
- Thermal inertia of the oven
To be able to solder properly, oven and PCB must go through the same heat up and cool down phase.
eC-reflow-mate-V4 is much beter insulated than the first version we built. Therefore the effects of thermal inertia are much stronger in V4 than they were before. Building more aggressiveness into the PID values of the thermal controller does not make the oven react faster while heating up as inertia has its strong effects. The same effect plays in the cooling down phase.
An ideal solder profile as customers expect, is only possible by using a continuous oven. Instead of the 3000W of our eC-reflow-mate, it uses at least 18,000W available over different heating zones. Each zone is set to control 1 fixed temperature value and that with a continuous flow of sufficient power so that there is an excess of calories available to heat up the PCB. The possible cooling effect of the surrounding oven is compensated by the preheating time of the oven needed before you can use it. In a continuous oven, the soldering curve is manipulated by the speed of the transport belt rather than by controlling the temperature.
In short, you cannot simulate a soldering curve achieved by a continuous reflow in a chamber oven. However, you can achieve a decent soldering point and a dwell time and that over a soldering area of 250mm x 350mm with a delta T of max 25°C.
In the latter, we score well with our V4 and that because of the good insulation.
- 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.
These 3 effects are normal and should be taken into account while choosing the best soldering profile for a particular board. Also the where and how to place the external sensor on the board, has an effect. See external sensor.