Thank you, Guy.
Yes I was, but as long as we are on the subject, it typically is sufficient for most MSD's as well.
I have done a lot of bake studies to determine optimum bake times/temps for both bare PWBs and populated assemblies, as well as individual components.
The J-STD-033 is intended to be used as a guideline and it states that several times, and in my opinion you will do much more harm by baking many of these components (and circuit boards as well) at temperatures of 125-150 C for the lengths of times listed.
On the other hand, I understand very, very well that certain PWBs and certain components do require much longer times above 105 C in order to ensure enough moisture is removed to preclude damage due to steam pressures during reflow, especially with higher Pb-free reflow processes, and that there are some circuit board types and components that do not do well unless they are baked longer. And for this reason the standard was written to ensure that if followed to the letter a minimum amount of damage will occur to the components. But again there is a caveat, and that is the fact that reliability and solderability for both components and circuit boards can be compromised if bake times/temps are too high or too long.
Many will not stand up to the bake times/temps listed in the Tables within the standard. If there is a questionable part or board, then the manufacturing engineer should know how to qualify the bake time and temperature to optimize the moisture removal without risking the hardware.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Wednesday, November 14, 2007 8:59 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Breakdown
I think Richard was talking about the board.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gumpert, Ben
Sent: Wednesday, November 14, 2007 9:36 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Breakdown
Richard,
Is that adequate for MS parts? That doesn't comply with J-STD-033.
I realize that I can discard any part removed and replace it with a fresh part, but cost sometimes prevents that. I'm also concerned about adjacent parts that are not the target of the rework.
Ben
-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]]
Sent: Wednesday, November 14, 2007 8:25 AM
To: TechNet E-Mail Forum; Gumpert, Ben
Subject: RE: [TN] Conformal Coating Breakdown
Ben,
You do not need to bake at 125C. You simply need to get the board above 100C, perhaps for a little longer time.
I have yet to see an assembly that did not lose 80% of its saturated moisture content when baked at 105C for 12 hours. If you get below 20% moisture content, you should not have any trouble.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gumpert, Ben
Sent: Wednesday, November 14, 2007 6:01 AM
To: [log in to unmask]
Subject: [TN] Conformal Coating Breakdown
Anyone have some information on what happens to conformal coatings as you heat them up, what temperature do they start to deteriorate? (Specifically acrylics and urethanes)
What I'm really getting at is that I have some boards that need hot air rework, but they have been coated. There are moisture sensitive components on the board, so I would like to bake the boards prior to rework. I would prefer to make the bake as short as possible, but that puts the temperature
(125°C) at the top of the temperature range for these coating materials. (In fact, some boards that I have baked have gotten a brown tint to the coating).
Will I do any damage to the conformal coating by baking at this temperature?
If not, what temperature would cause damage?
Thanks,
Ben Gumpert
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