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August 4, 2016

Reflow Oven Calibration Schedule

Can you offer some guidelines regarding how often we should re-calibrate our reflow ovens?

M.D.

Experts Comments

Most manufacturing best practices say 1 year as a general guideline, but ultimately this needs to be determined by your own liability. If your oven is used for curing (or some other low precision requirement) for example, your liability may be quite low, and only require calibration every 18 months. Now if you are making pacemakers, your liability goes up (and the level of precision as well), so you may want to calibrate more often (say every 6 months). You also need to take into account of variability of the oven your calibrating.

If you are consistently finding large offsets at calibration, the interval should be shortened. If your oven hasn't changed in 10 years, you may want to lengthen the interval to every 2 years. The real question to answer is "How long can I afford to make bad product, and how likely is that to happen?"
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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.
A good basic recommendation is to run a weekly profile, and to do control charting on the temperatures and timings to detect drift. This type of profile should not be done with a board, it should be done with a more thermally robust piece of tooling. The objective is to track change, and any breakdown in the tooling, like delamination in the PWB used, will affect the results. Re-calibration should be done only when drift is detected that exceeds process limits defined by the process engineer.

The above will let you react to long-term drift, but if something changes suddenly within the week, you certainly need to know, or you potentially have a week worth of product at risk. The main failures that you need to account for are:
  • Convection failures, e.g. fans and motors or changes in plenum flow due to contamination
  • Heater failures (or cooling system failures)
  • Conveyor speed error
Many (not all) ovens can reliably detect the first two classes of failure. The third is harder to detect, and can be just as detrimental  to product quality. The old tried-and-true method is a daily check using a ruler next to the conveyor and a stopwatch. This will confirm that the speed has not changed dramatically, and more precise measurement can be done in conjunction with the weekly profile.
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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.
What does the reflow oven manufacturer recommend?
Lee Wilmot
Director, EHS
TTM Technologies
Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.
The oven manual or the oven manufacture is the best source for a recommendation on the calibration frequency. However, many choose to calibrate their ovens annually, which is practical for most, and gives a reasonable record of historical calibration performance.
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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.
First I would check the manufacturer's recommendation and have that on the internal PM schedule. Using a KIC or MOLE once a week or even once a day is a good practice to ensure that the oven works within the needed parameters. I have detected variations in the temperature before due to motors malfunctions even though the reflow oven would not exhibit an alarm mode.
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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.
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