Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

dry oven

Larry Smith


dry oven | 6 August, 2002

I have two basic questions: 1. We have a number of small convection ovens and want to modify some of these for baking out parts when needed. We have tried pumping nitrogen into these but I still run between 35 and 50 RH. Has anyone tried converting a regular oven and how did you get the RH low enough to bake out components.

2. For as long as I can remember all assemblies here go thru a four hour bake before conformal coating. What are the benefits of this.

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dry oven | 7 August, 2002

N2 PPM vs %RH? With a firm grasp of the obvious, reflow oven manufacturers rig N2 ovens differently than non-N2 ovens. The N2 displaces O2. Usually N2 oven monitor something or other to give an indication of N2 ppm. %RH really isn't part of the equation.

WHY BAKE? If boards have excessive moisture, conformal coating can 'blush' [get a 'milky' look]. Baking boards prior to CC eliminates that problem. We do not measure %RH in the bake. We control temperature and %RH in the plant. We bake. We spray. We inspect. We ship. The fine SMTnet Archives contain suggestions on bake recipes.

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dry oven | 12 August, 2002

Been there, ... done that, the T-shirt.

When with a major IC manufacturer, we tried this. IT DIDN'T WORK! The problem was that most convection ovens leak. We could not achieve a significant N2 atmosphere.Eventually went to vacuum-assisted bake-out to keep our lines running with dry parts. This is similar to the TVP process. Contact me if you want further information.

Stu Leech

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Kevin Facinelli


dry oven | 14 August, 2002

One thing we did for our oven is hook it up to the compressed air system in the factory and bled in a small volume of air to create a positive pressure in the oven. The air is clean (non-oiled) and comes off the dryer. We can attain 5% RH. Remember you are just trying to create a positive pressure and small exchange of air. I think we calculated that the air was turning over once every couple of minutes.

Just an ideal that works for us,


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dry oven | 14 August, 2002

Larry, You can try one of two approaches, a bake above 100C will drive off moisture or use a vacuum bake. The vacuum bake process is slow since there is less/no air to conduct the heat. In the hybrid industry the standard vacuum bake was 16 hours at 125C. For PCB's we used an air bake at 125 C for 2 to 4 hours prior to coat. We also did a pre-assembly bake at 110C for 2 hours to drive off moisture. Den

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dry oven | 14 August, 2002


Until I got deep into development of a vacuum assisted moisture removal process, I always thought that the bake process would be saddled by the lack of atmosphere in the chamber. I later learned that the conduction of temperature at reduced atmosphere is flat until you reach 1/1000 atmosphere. It dives steply at lower atmoshpheric level. This due to the law of "least mean of the squares." There are fewer molecules to carry the energy but there are fewer collisions between molecules compensating for the reduced population.

Stu Leech

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Plasma prior Conformal Coating