Hi Robert,
Unfortunately we don't have a dry box, but I have wondered about something.
We did buy a McDry cabinet when I was working back in Tulsa, and it was a
self regenerating dry desiccant cabinet. Periodically, it would turn a
heater on where the desiccant was stored and dry out and reset the
desiccant, which had me wondering, why wouldn't you do be doing the same
thing as a dry desiccant cabinet if you took a few packs of fresh desiccant
and sealed them up inside a MBB, won't that pull out moisture as well?
Steve
On Mon, Apr 16, 2018 at 3:24 PM, Robert Kondner <[log in to unmask]>
wrote:
> Why bother with heating, a dry box will suck out moisture. Right?
>
> Bob K.
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of Douglas Pauls
> Sent: Monday, April 16, 2018 2:26 PM
> To: [log in to unmask]
> Subject: Re: [TN] Vacuum Baking
>
> 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
> >
> > --
> >
> >
> >
> > This email and any attachments are only for use by the intended
> > recipient(s) and may contain legally privileged, confidential,
> > proprietary or otherwise private information. Any unauthorized use,
> > reproduction, dissemination, distribution or other disclosure of the
> > contents of this e-mail or its attachments is strictly prohibited. If
> > you have received this email in error, please notify the sender
> > immediately and delete the original.
> >
>
--
Steve Gregory
Kimco Design and Manufacturing
Process Engineer
(208) 322-0500 Ext. -3133
--
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