I have actually had very good success reflowing boards by placing the thermocouple directly on the PCBs. I have consistantly been reflowing both lead and lead-free board profiles without burning boards. I initially invested in the oven to be able to build boards with BGA parts on my designs. I was skeptical that I would be able to, but it has been surprisingly easy to do. I use a 100-pin 0.8 mm pitch part on one of my designs and a 256-pin 1.0mm pitch BGA on another design. Even more amazingly, I was able place and reflow 624-pin 0.8mm iMX.6 processors on some once disfunctional boards from my workplace. I clean the BGA pads well, apply solder flux to the pads, and then carefully align the corners of the part on the board. The flux holds the part in place and will self-align during reflow process provided that you are not off by a row of pins.
To derive my profiles, I empirically tested an average sized bare 4"x6" PCB over many cycles adjusting the ovens profile and comparing that with an independent thermocouple connected to a multimeter and then to the industry standard reflow profiles. One thing that I noticed, was that the fan should kick on soon after the peak temperature has been reached. The board's cooling rate is still within the recommended profile spec, but this helps prevent the board from burning. I use "Mechanics" Solder Paste (AliExpress) for my lead solder, but have to use the lead-free solder (Sparkfun) and profile for boards with BGAs.
For my QS-5100 w/thermocouple attached to PCB, I have the following profiles:
The QS-5100 has a MAX6675 thermocouple amplifier on its control board that uses a K-Type thermocouple. It attaches to the control board via a Phoenix-style connector that allows wires to be removed/inserted via screw terminals. I bought a cheaper set of thermocouples from Amazon for short term testing and a metal-shielded thermocouple for long-term installation. Here are those:
If you are looking for something cheap as external measurement and do not have a multimeter that has temperature, then you might try this dual temperature meter:
I received the cheap thermocouples from Amazon a few day’s ago and temporarily replaced the oven’s built-in one and attached it directly to the PCB that I was reflowing. As expected, I could dial in the board’s reflow profile much more closely to the manufacturer’s specification and the results were accurate and repeatable. There was some overshoot by the controller (expected), but that is easily mitigated by adjusting the temperature and time settings slightly.
I need to perform more testing, but I am pretty convinced that this will be a more accurate and repeatable method than the controller using the ambient temperature sensor. I plan to permanently install the metal-shielded sensor into the oven once it arrives. I’ll post more as I get a little more time and can work out the profiles some.
You might try aiming a low-output external fan (4-6 inches away) at the built-in fan opening on the back of the unit. It seems to circulate the air in my small oven without interfering with the oven’s built-in fan.
I'd recommend placing a thermocouple directly on the board and paying particular attention to the Wetting stage (Reflow and Keep stages). This is the most likely period when the boards are browning and needs to be tightly controlled. The leaded solder paste profile indicates that the total time from 183C to 215C then back down to 183C is 60 seconds maximum. You might try reducing the Reflow hold time (maybe to 15-20 seconds) and having Keep kick in at a higher temp (maybe 205C) to actively cool the temperature back down. At least on my oven, the fan actively cooling is still within the 2-4 degrees C rate that is recommended for the cool down stages.
I received my QS-5100 oven last week. As Ian mentioned in his YouTube video, these ovens appear to be better quality than the T962 models. I removed the top cover on my unit to inspect it and also ran some temperature tests to test its accuracy. For their low-cost, it appears to be well built and insulated (with no masking tape inside!), and have nice control boards, fans, and sensors.
It does have one flaw that I observed when trying to baseline the temperature profiles. The oven's thermocouple is placed right between the two IR heating elements. The fact that the oven's controller is basing it's profile on the ambient temperature and that reading is is further influenced by the proximity to the IR heating elements make the desired profile and actual board profile very different. You can check this by taping a thermocouple (from a multimeter) to a spare PCB and comparing the temperatures between the meter and the oven during a reflow process. They're probably significantly off from each other.
Since reflow profiles are specified for the solder junction temperature and not ambient temperature, then the profile really needs to follow the board's temperature and not the ambient. I ordered another thermocouple and plan to replace the oven's current one with a thermocouple that I can Kapton tape to my boards during reflow. Hopefully this should allow for a more accurate and repeatable reflow process.
So if you're having trouble with dialing in the profiles, then I recommend getting a multimeter and thermocouple and compare the temperatures and work from what the board temperature is and not what the oven is perceiving from its ambient sensor.
Thank you for the info and YouTube video on QS-5100 reflow oven. I was wondering if you could comment on your experience with using the QS-5100 with any BGA parts? Any helpful tips / techniques for reflowing these parts?
You also mentioned that the reflow profiles that you use now are different than what is recommended in the manual. Could you you share what profiles that you use (and the specific solder paste). Thanks for all your help!
