Reflow ovens and wave solder machines come in a huge variety of flavors and price points. The spec sheets for these machines tell the story of impressive features and capabilities such as flux management, gas flow management, static pressure control, active cooling zones, dual-lane independent speed control, low nitrogen usage, etc. Truth be told, reflow ovens today are far more stable and capable than they were years ago. They also can provide an enormous amount of data that can be fed to the manufacturing execution system (MES). It seems that the only data these machines cannot provide is the single most important data, the very objective of having an oven in the first place.
Like with many things in life, if you want to go beyond scratching the surface and get to the core of an issue, you need to keep asking questions to each answer given. Here is how that discussion tends to go when talking with process engineers about their reflow ovens:
Q: What is the purpose of a reflow ovens?
A: Melt solder.
Q: That’s it?
A: And cool it.
Q: That’s all?
A: Solder components onto a substrate.
Q: How do they need to be soldered?
A: Create strong solder joints.
Q: That’s all?
Q: What about the components?
A: What about them?
Q: Any consideration for them?
A: Sure. You cannot fry them in your oven.
Q: How are you supposed to heat and cool them then?
A: You have to stay within the tolerance specs.
Q: Which tolerance specs?
A: The ones given by the component vendors. And the solder paste company.
Q: How do you know whether you have soldered everything inside these tolerances?
A: We measure the time vs. temperature profile.
Q: And then what?
A: Then we compare the measured profile to the specs given to ensure the entire assembly is in spec.
Q: So essentially the job of the reflow oven is to create the correct profile in the assembly?
You ask anybody enough of these questions and you always arrive at the same answer: Creating the correct profile. Therefore, we can think of reflow ovens and wave solder machines as Profile Making Machines.
Therein lies the irony. With all the sophisticated features and capabilities and with the hundreds of data points a second that an oven can crank out, the single most important data is nowhere to be found. Smart ovens record, display and share this essential information. Otherwise, third-party equipment such as manual profilers or automatic profilers are needed to provide information about the profile on the assembly.
This is also a good time to visit the discussion about machine vs. process monitoring. There is no inherent conflict between these two technologies. They are simply different. Machine monitoring does exactly that: it monitors the machine, including performance, deviations, set points, actual temperature and conveyor speed, static pressure, airflow, O2, and much more. This can be useful information. They are not, however, sufficient in that they do not address the very purpose of a reflow oven. What is the profile on this assembly going through the oven now? Is that profile in spec?
There are numerous variables beyond the control of the reflow oven that will affect the profile on the assembly. Examples include changes in the factory exhaust system, temperature of incoming PCBs, etc. The most fundamental example is operator error where the wrong oven recipe was loaded. Monitoring the stability of a machine running the wrong setpoints is not only obviously an exercise in futility, but it demonstrates the mechanics of monitoring something removed from the real purpose of the machine: creating the correct PCB profile.
A profile making machine needs to tell you about the profile it is making.
By Bjorn Dahle, President, KIC