December 10, 2013 in REFLOW SOLDERING
When I made the post about building a reflow oven and the one about temperature uniformity I explained that my oven was heated using five 200Watts heating lamps. This was not enough when using a fan to recirculate air inside the oven to improve temperature uniformity as the heat loses higher than expected.
We have made some modifications on the oven to avoid these problems:
Firmware changes. These involved changes in the PID controller programming and the way heating ramps starts. We will give you more information in another post.
- Add 3 more 200Watts heating lamps. So the oven is now equipped with a total of 1600Watts of heating elements.
- There is a turbine below the oven that injects fresh air when the process is finished to cool down the PCB faster. This turbine has been changed to a small high static pressure turbine instead of the low power, low pressure old turbine. This has significantly improve cooling speed.
- When the soldering is finished a solenoid linear actuator opens a hole at the top of the oven to help the hot air leave the oven. This solenoid has been changed to a new one with much larger linear movement.
To see if the modifications improved the oven we did some tests:
We repeated the test we made with the old oven. We measured the temperature sensors in 4 places distributed inside the oven to see if the temperature was uniform.
As you can see in the graph the results were quite good, the temperature is really uniform during the soldering process. The only point a bit different is the “T4″ during cooling phase, this is because this sensor was just over the fresh air turbine we use for cooling down but even this point has a decent uniformity. You can see our previous post about temperature uniformity here.
MAX SPEED TESTS:
As we have added more power to our oven so we expected being able to heat the oven much faster. To test that we ran two soldering processes with 100% ramps. Running two tests also allowed us to test repetability.
So a whole soldering process in less than 200 seconds, that’s fast. Anyway, this was just to test the oven don’t try to solder components like this or they will be fried.
CONFIGURE THE OVEN
Now that we were pretty sure the oven was working we decided to search for the correct soldering parameters for Lead free solder.
There are many places were we can find good guidelines for reflow soldering. I would recommend:
After reading this my targets were:
- Flux activation phase, between 150-200°C for 100 seconds, commencing at 150°C and reaching slowly 200°C at the end of the phase.
- Wetting time, time over 217°C, not exceding 60 seconds.
- Max temperature 245-255°, about 15 seconds.
- Max heating ramps of 3°C/second.
- Max cooling ramps of 6°C/second.
Now it is a matter of trial and error, I placed a dummy PCB board on the oven and the controller temperature sensor on the board and started to make trials until I was satisfied with the result. Don’t make your tests inmediately one after another, you have to let the oven cool down completely or your results will not be repeatable next day when the oven is at room temperature.
So here you can see the results for the soldering with 100% ramps (blue) and the one with the final settings I chose (green):
As you can see my green profile matches all my targets. The controller parameters were:
Ramp 1: 55%.
Temperature 1: 195°C.
Ramp 2: 70%.
Temperature 2: 255°C.
Time 1: 50 seconds.
Time 2: 15 seconds.
Time 3: 120 seconds.
Remenber that in our controller these parameters mean:
The use of a dummy board is necessary as the performance of the temperature sensor is not same over a PCB board or holding loose in the air. In a perfect world oven suppliers recommend to use the actual board you want to solder to fine tune your oven, but as this can be difficult for small quantities just use the most similar board you don’t mind to destroy during testing.
To visit our first post about building a reflow oven, go here.