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March 15, 2017

Rechecking Thermal Profiles

Is it necessary to replicate the thermal profiling process during long term mass production? Is it necessary to recheck accuracy of reflow oven profiles periodically, if so how frequent?

H.A.

Experts Comments

Thermal profiles should be checked frequently to avoid costly product mishaps. If the reflow oven isn't working properly and doesn't alarm for one reason or another, then routine profiling would catch this.

It is a check of the process and the equipment. It costs little to do this but may save lots of boards from being under-reflowed or over-"cooked." Even if all the heaters are responding properly, if the conveyor speed has increased or decreased for some reason, it can spell disaster for the product.

It's good practice to run a profile at the start of each shift along with a check that the proper recipe is being used and nitrogen is flowing appropriately if being used.

Frequent profile monitoring will dry out profile boards, so keep an eye on the board to ensure that thermocouples are properly attached and that the board is not separating. Layer separation will change the thermal properties of the profile board.  Proper profiling and frequent checking of the profile is the best insurance in the reflow and wave areas.
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Gary Freedman
President
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
If you have requirements for thermal process traceability, or if you want to integrate MES into your production line, you will need to not only check the PCB profiles periodically, but to profile each and every PCB. For companies that want to avoid rework or have clients that do not accept reworked boards, it is also necessary to use a continuous and real-time profiling system in order to stop production when an out of spec situation occurs, and to verify that the loaded oven recipe is the correct for the PBCs being produced.  

For applications outside of high reliability products, when the oven is stable, and when the process window is not very tight, many factories are content with periodic profiles once a week or after preventive maintenance. Because the process is "hidden" inside the oven, the engineers  do not have access to process information throughout the week. It becomes a matter of risk analysis for them (or their clients) whether to do periodic profiling and its frequency.  

Modern reflow ovens, wave solder and selective soldering machines are far more stable these days compared to several years ago. They also have their own alarm systems for excessive drift that some factories rely upon, and therefore reduce the need for periodic profiling for them. Such alarms are designed to keep the machines stable and working correctly. Studies have shown, however, that process (profile) drift can still occur undetected.  Examples are during changes in the factory exhaust system, fluctuations in oven load, changes introduced during preventive maintenance, human error and other.  

For the sake of disclosure, I work for KIC which manufacturers profilers, oven setup software, and automatic profiling systems.
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Bjorn Dahle
President
KIC
Bjorn Dahle is the President of KIC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.
It is very important to know you have the right oven recipe to correctly solder your customer's products. This can only be confirmed with an initial thermal profile of the actual product through your process.

This only needs to be done during the original process design for that specific product. Once the recipe for your oven is established for each product, you no longer need to run a thermal profile of the actual product. Save that product used to set up the recipe for when you need to establish a new recipe for the same product on a different line or in a new oven. Now all you need show, on a regular basis, is that your oven is consistent and in control using statistical methods, charts, graphs, etc.

This can be done using one of the many tools specifically designed to measure reflow ovens using process control methods (SPC). These tools can withstand repeated runs at reflow temperatures and not be damaged. This is best repeated as often as possible, however it should be done at least after any oven maintenance, and after a recipe change, just before you let your customer's product through the oven, to show your oven it ready and in spec. Then, if your oven runs the same recipe for days or weeks, at least weekly if not more.

This will allow you to prove your oven process is in control and consistent day to day. Without this record of performance, you cannot know if you have a consistent process or not.
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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
Yes, you should periodically run a thermal profile. With the controls in place on most modern convection-based ovens, the risk of a suddenly occurring out-of-tolerance condition is small, but not zero. Furthermore, slow "drift" in performance over time can be hard to detect before it causes problems. Heat transfer in a convection-based reflow system depends on both the temperature difference between the heated air and the product (linear relationship) and the amount of air movement (linear within a limited range).

If a zone cannot maintain temperature, the oven will be able to detect this and throw an alarm, but if the air flow changes, the oven may not be able to detect this. Some ovens monitor/control the "static pressure" in the air distribution plenum, but even this type of monitoring can be fooled by clogging of the plenum. Finally, even if both air flow and temperature are correct, a conveyor speed error can throw off the profile.

The best method of monitoring is to use a purpose-made tool that will perform without degrading significantly over hundreds of profiles. Run this tool at least monthly, or more often depending on criticality. Plot the parameters of interest and use control charts to monitor for trends and out-of-control conditions. In our facility we use a tool that allows for monitoring the following variables:
  • Mass density (two levels)
  • Across-conveyor location (three positions)
  • Along-conveyor location (three positions)
  • Conveyor speed (derived from temperature/time offsets)
We plot the following parameters on control charts:
  • Peak temperature (five channels)
  • Liquidus time (five channels)
  • Conveyor speed deviation
The tool that we use was built by us based on my own design, but pre-packaged tools are available. If you choose a pre-packaged tool, look for data that ensures the thermal masses do replicate the behavior of a PWB adequately. I have seen folks use aluminum plates, for instance, and these are not a good analog of a PWB (too low a specific heat, far too high a thermal transfer rate).
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
The short answer is Yes. After refining the oven recipe to produce the ideal profile, the only way to verify the oven is continuing to produce that profile is to either continue to profile the product or profile the oven. When we say "profile the oven", it means to prove that oven is behaving in the same way over time by running a test pallet at a fixed recipe and recording the same results.  This proves your ovens capability to reproduce the correct profile.   

Frequency can be determined by customer requirements (prove the oven is good before each run), by product liability (pace makers vs. calculators), by production needs (how much time do I have to profile), or by your company's tolerance for scrap and rework (how long can I make bad boards).  Ultimately you will have find the right balance that fits your product and company goals.
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Mark Waterman
Engineer / Trainer
Electronic Controls Design, Inc. (ECD)
Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.
Checking profiles on an ongoing basis is a must due to the deterioration of the equipment and the variations in the settings. If the product volume is high and the mix is low, then I would recommend checking  the thermal profile before every shift. Whereas if the product volume is low and the mix is high, which indicates the thermal requirements of the oven has to be changed then I would recommend checking the thermal profile prior to running the boards which would determine whether or not the oven has stabilized at the correct temperature to run the boards.  

This may sound like overkill, but the variation in component sizes and the heat absorption rate of each component and board is different and the reflow process is a general heat application system, meaning it heats everything in the oven based upon the oven temperature and conveyor speed. Hence to verify the board is getting the correct amount of heat to prepare and dry out the solder paste and reflow the solder paste for a long enough dwell time to solder all the components, the thermal profile is your due diligence in verifying the process compatibility.
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Other than profiling each assembly/board when running it the first time, a periodic check is recommended to make sure that the machines are running properly. We are running weekly profile checks and analyze the data - we are looking for "drifts" in temperature readings and belt speed as well as consistency in between different machines on the floor. They are the same model and type so we want to make sure that they are working exactly the same. 
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Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
Periodic thermal profiling for each and every product built is very important. If any of the heating zones is not working as expected the result may be active flux residues or improperly formed solder joints which can and most likely will cause issues with reliability. As important as continuous monitoring of the thermal profile knowing how and where to place your thermocouples. Thermal profiling must be done on full assemblies to replicate production and the T/C must be placed at locations with the highest thermal mass.
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Eric Camden
Lead Investigator
Foresite, Inc
Eric has been in the electronics industry for over 14 years and manages the C3 technical user group, Failure Analysis project management, Rescue Cleaning Division and is one of three Lead Investigators at Foresite.
All good inputs from the experts, but I'll add one more. When you create a profile for a given circuit card assembly, you need to replicate the conditions of actual production. Very few companies run a CCA all the way through the oven before entering the next one. Having two, three, four or more CCAs in the oven completely changes the temperature profile that is seen by a single CCA in the tunnel.

So consider this when creating the profile using a scrap CCA, and run a few dummy PWBs in front of and behind the profile board when it goes through the oven and make sure you compare that to the profile seen by the single test board. It is quite common for engineers to create a profile that is just above the temperature required to achieve liquidus for perhaps a minute or two, but when production starts, the loading of the oven with multiple CCAs causes none of them to get completely hot enough to achieve the desired peak temperature and time above solidus.

Ditto when using a process control fixture. Be aware that if the initial benchmark profile was established with just the tool in the tunnel, likewise you need to run the process verification with just the tool in the oven.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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