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September 18, 2018

Pre-bake in a Vacuum

Can the time for pre-baking of components and bare PCBs to eliminate moisture be reduced by using a vacuum oven? What percent of bake time reduction could we expect?


E.B.

Experts Comments

From experience I can tell you that the time reduction is going to happen for sure. The vacuum will help getting the moisture out faster. Regarding the time reduction, that will depend on the surface, thickness of the component as well as the material to be processed.
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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.
Yes, in theory this will work. The vacuum will lower the boiling point of water vapor, and also decrease the partial pressure of water vapor present in the oven. Both will work to speed up drying of the components. As to how much, you would have to calculate that, or determine it empirically.

The drying process can be monitored by measuring weight loss. An analytical balance of at 0.1mg or finer resolution is required, and good weighing practices must be observed. Also be aware that by pulling a vacuum and heating the parts will have other effects:
  • Parts will heat almost entirely by radiation, which is a different mechanism than convection. Heating rate will be very dependent on how well the heaters in the chamber are "visible" to parts.
  • Internal vapor pressure  within boars/components will be increased, since the temperature of the parts will be further above the boiling point.
  • Oxidation of finishes will be reduced, which is a good thing. Oxidation rates can also be reduced by nitrogen-purging the oven.
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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.
Vacuum ovens will certainly reduce baking temperature to below 105 C. As prescribed in IPC-1601, a weight loss v/s time experiment needs to be conducted in order to establish the bake time duration. As with 1atm bake, the ramp rate should be controlled, this prevent PCB and component damage by reducing the chance of violent out-gassing of moisture vapor.
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Bhanu Sood
Laboratory Director
CALCE, University of Maryland
Bhanu Sood is the Laboratory Director at the Center for Advanced Life Cycle Engineering (CALCE) and actively assists companies and organizations in all aspects of electronics reliability. Sood's key focus area is in design reviews, custom tests, and failure analysis services. He has authored several articles on board and component level reliability and unique failure mechanisms in electronics.
Using a vacuum oven to prebake bare flex and rigid flex circuits prior to soldering operations can be beneficial due to the fact that you can drive moisture from the circuits at a lower temperature.  You will also get a more thorough moisture extraction in a vacuum.

In my opinion, the time savings are negligible since heat transfer to the circuits is not as efficient in a vacuum as it is in a standard convection oven where heated air is circulated around the circuits.
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Mark Finstad
Senior Applications Engineer
Flexible Circuit Technologies
Mark Finstad has over 30 years in the flex circuit industry in both design and manufacturing. He is a regular speaker at IPC APEX (Professional development courses) and PCB West (flex circuit design courses). He is also vice chair of IPC-2223 and active member of IPC-6013. Finstad has extensive experience with both domestic and off-shore manufacturing.
Reader Comment
The concept seems like a good idea, but as Finstad suggested, the real issue is heating the boards. A vacuum oven does not lend it self to convection heating or radiation heating (minimal) which leaves heating by conduction. Conduction only happens using direct contact with the board on a heated surface in the vacuum oven shelf. This requires a single shelf for each board and by experience, can take hours. The process becomes more complex if the board is partially loaded.  

Remember the distance the water vapor must travel is long and arduous.  This requires energy to get the molecules moving - ie body at rest etc. From experience, one needs to get the boards at a temperature of about 50-60 C to make anything happen at a reasonable speed.    

One could preheat the boards in a standard oven then transfer them to the vacuum oven, but this defeats the purpose of using a vacuum and increases the risk of handling damage.  

The better choice for speed and protection of the components and surface finish is a nitrogen oven or drying cabinet.
Michael J Gay, Isola Laminate Systems, USA
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