|Ask the Experts|
December 12, 2019
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?
|Expert Panel Responses|
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.
Engineering and Operations Management
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:
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 1 atm bake, the ramp rate should be controlled, this prevent PCB and component damage by reducing the chance of violent out-gassing of moisture vapor.
CALCE, University of Maryland
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.
Senior Applications Engineer
Flexible Circuit Technologies
The concept seems like a good idea, but as Finstad suggested, the real issue is heating the boards. A vacuum oven does not lend itself to convection heating or radiation heating (minimal) which leaves heating by conduction.Michael J Gay, Isola Laminate Systems, USA
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.
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