I have 3 questions about reflow. Hopefully someone can help me.
1. I am having a problem. I set the peak at 230 and my mole thermocouples are telling me the temp in that zone gets to 237. How is this possible even if the thermocouple came loose from the board? Even then the temp shouldn't get above 230. I am running without rail heaters and Speedline tells me that the accuracy tolerance for the Omniflow 7 without rail heaters is plus/minus 4 degress C. So, theoretically I shouldn't see temps over 234. Bleed over is not the problem since this is the hottest zone. What could be making my zone hotter than setpoint? Also, my PID is fast ramp, but I've waited until stabilization, so the PID is not the problem. I've also checked the offsets and they are not the problem either. Any help?
2. What happens to solder paste (any no-clean solder paste) when the suggested max peak temperature has been exceeded? I'm only talking about 7 degrees. I know that excessive heat can cause leaching, but what else? Chemically, what's happening when it goes over temp?
3. How does everyone out there verify accuracy of your ovens? What verification process/technique do you use to prove the accuracy of the heat zones and internal thermocouples?
what could be wrong: The mole can be wrong (try measuring the 100 deg celcius of boiling water to check it, and are the batteries ok?) or your oven is wrong. Are the thermocouples of the oven clean? Try exchange the thermocouples from the reflowzone with a preheatzone to check. What about temp-overshoot if there is a high heat resistance between thermocouple and ambient?
About the exessive temp: I do not think it can cause too much damage, as long as it is still about 235 deg, and not too long 20-60 sec >180deg More important are the preheat and soakzones and temp ramps.
How to check: I used to do it as follows: use a TEST-PCB with fixed thermocouples and connect it to the mole. Print the profile on transparant paper. do it again e.g. each week and put the transparant sheet on the new printout. Calibrate the mole each (quarter? year?) at a qualified testhouse depending on your volume. Your customers will be satisfied with this doublecheck. By the way I have a ECD mole for sale(see marketplace)