Steve,
I would not be an advocate of this approach.  If you get the tape and reel
hot enough to make moisture diffuse out, you may also be causing the
plastic to outgas/offgass materials that would be detrimental to the
component.


Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins

On Mon, Apr 16, 2018 at 9:19 AM, Steve Gregory <[log in to unmask]> wrote:

> Hi All,
>
> I know this thread is about vacuum baking, but I would like to ask about
> baking components on tape and reel, I have not actually done this. I
> remember seeing discussion about the topic on Technet, but I can't remember
> all the details.
> I know when I ask what temperature is safe to bake tape and reel components
> "it depends" will come up, it probably depends on what kind of carrier tape
> on top film cover tape is used, is it pressure sensitive adhesive or heat
> seal cover tape? I'm seeing a need that may require more baking of reeled
> components now for some reason.
>
> Is it just me, or does it seem that more and more components are being
> designated as MSL 2A and higher lately?
>
> Steve
>
> On Mon, Apr 16, 2018 at 8:09 AM, Yuan-chia Joyce Koo <[log in to unmask]>
> wrote:
>
> > you can make board vertical stand in the PWA carrier tray with slot.  you
> > need surface area for moisture to escape...assume you use low temperature
> > in vacuum baking, little warpage would happen  unless it is very thin
> large
> > size board.
> > On Apr 16, 2018, at 9:23 AM, Robert Kondner wrote:
> >
> > Hi,
> >>
> >>  If you have a stack of PCBs, lets say 25 or 50 boards which is a stack
> 2
> >> or 3 inches high, can water vapor diffuse out the space between the
> boards?
> >> Does laying a thick tissue between the boards help or is it best to
> place
> >> each board in edge supported carriers?  I worry about warp if a board is
> >> edge supported while at elevated temps.
> >>
> >> Does anyone has knowledge here or even an educated guess?
> >>
> >> Thanks,
> >> Bob K.
> >>
> >> -----Original Message-----
> >> From: TechNet <[log in to unmask]> On Behalf Of David Hillman
> >> Sent: Monday, April 16, 2018 8:11 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] Vacuum Baking
> >>
> >> Hi folks - we did some vacuum baking experimentation a few years back
> and
> >> yes, vacuum baking can be effective but it is not as straight forward as
> >> you would imagine. You need to carefully select the vacuum bake
> parameters
> >> or you will actually impede water diffusion. You can cause ice formation
> >> rather than water evaporation depending on your parameters (put a
> shallow
> >> pan of water in a vacuum chamber and try it, its kinda fun). Wayne's
> >> approach is much simpler and far more practical/effective in terms of
> >> moisture removal.
> >>
> >> Dave Hillman
> >> Rockwell Collins
> >> [log in to unmask]
> >>
> >>
> >> On Sun, Apr 15, 2018 at 11:43 AM, Wayne Showers <
> >> [log in to unmask]> wrote:
> >>
> >> The problem with vacuum drying is not so much the boiling point of
> >>> water as in an unimpeded environment, those values are: At 25" Hg :
> 56C,
> >>> 27" Hg :
> >>> 45C, 28" Hg :40C, and so on.  The problem is degree of impediment
> >>> caused by the actual plastic.  To build off what Yuan-chia Joyce Koo
> >>> has stated but with a different vector in mind, the plastic material
> >>> of moisture bags for instance is highly resistant to diffusion.
> >>> Different polymers will follow accordingly.
> >>>
> >>> For information, many plastics must be dried immediately prior to
> >>> molding otherwise they suck up moisture like a sponge. Common
> >>> hygroscopic polymers are Nylon, ABS (Acetyl Butyl Styrene), PC
> >>> (Polycarbonate), and PMMA (Poly (methyl methacrylate)).  Nylon and
> >>> PMMA are very common materials in component shells.  ABS and PC index*
> >>> at temperatures not much higher than boiling water and are not
> >>> typically used.  PP (Polypropylene) and PE
> >>> (Polyethylene) are not so bad, hence their use in capacitors, but
> >>> these index in the 200C range making them a bear for SMT placement.
> >>> My general rule is the higher the MSL number, i.e., the more likely a
> >>> part is to absorb moisture, the easier it is to dry.
> >>>
> >>> For normal bake (NO vacuum), here is the formula I derived several
> >>> years
> >>> ago:
> >>> Thickness (mm) X Thickness (mm) (Divided by) Bake Temp Factor D125oC=
> >>> 0.4427 // D90oC= 0.1270 // D40oC= 0.0131, Result is time in hours.
> >>>
> >>> I have never experimented with Vacuum Acceleration, but I will query
> >>> the Penn State Plastics Engineering group with what we are trying to
> >>> resolve and post those results to the J-STD-033 committee as well as
> >>> here.
> >>> * Index is the temperature at which a plastic flows well enough to be
> >>> molded.  It is neither solid nor liquid, but soliquid.
> >>>
> >>> This all sounds like a job for Clumpy and Kloumpios when they are next
> >>> available.
> >>>
> >>>
>
>
> --
> Steve Gregory
> Kimco Design and Manufacturing
> Process Engineer
> (208) 322-0500 Ext. -3133
>
> --
>
>
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