April 28, 2014 in REFLOW SOLDERING
This post is about how to assure that your oven temperature sensors are always on contact with the pcb we want to solder.
One of the most important things about soldering everytime right is the hability to maintain a consistent contact with the pcb your want to solder and your temperature sensor during the whole process. It happens sometimes that the sensor is not contact with the pcb, if this is the case the profile will not be always the same. See the following graphs.
Every graph correspond to a real soldering process, as you can see they are pretty uniform even if they were done in different days. The yellow one is different from the others, the soldering process is longer and quality of the soldering was not the same. The difference is that the sensor detached 1mm from the surface of the PCB 20 seconds after the soldering process started. This made the sensor take more time to heat as it was not in contact with the pcb.
It is not easy to maintain the contact of the temperature sensor to the pcb during soldering, we can not just put a sticky tape over the thermocouple to glue it to pcb as the pcb surface is full of solder paste and not soldered components so this is not possible. We decided them to build a clamp that maintained the contact as a resort, but this is not easy in an oven reaching 250ºC, also the clamp must be narrow to avoid shadowing parts of the circuit to solder. You can see our clamp here.
It is quite easy to build one, you will need a rigid ground cable, heat resistant Kapton tape, aluminum tape and a K-thermocouple.
First you will need to take some rigid ground cable and strip the isolation from it leaving only the rigid copper wire.
Then you can attach the thermocouple with kapton tape in one end of the copper wire and them on the other extreme at the length you would like to have for the sensor:
Now you can build the “feet” of the sensor by turning the copper wire to give it the form of a feet. This avoid the sensor to turn once installed in the oven.
The last step is to cover the sensor and the cable with aluminum tape, this will protect most of the sensor and reflect quite a lot of the infrared emissions.
And now as you can see we can assure a perfect contact during soldering using the kapton tape to maintain a little tension on the arc of the wire.