> Graham,
>
> Welcome back.  You must have enough frequent flyer miles to be part
> owner of British Airways.
>
> Here goes round two!
>
> Tg can be measured many different ways (up to 13 I believe).
> Each gives a slightly different reading. This generates a 3 to 5
> degree C  "window" for Tg.
> Tg will also vary with moisture content (by as much as 5 degrees C).
> Also if the coating materials are a "mixture" it is impossible to
> attain a completely homogeneous mixture
> which will also create variations in Tg.
>
> For my use there is no single Tg but a "window"
> that can be as much as 10 degrees C wide.
>
> "So what"?  you ask.
>  Here is a material that has two different sets of physical properties
> depending on the temperature.  The first problem is there is usually
> only one set of spec numbers issued with the product.  This can be
> somewhat misleading.
>
> These coating materials are suppose to provide some form of a
> "barrier"  that prevents
> malfunctions or disruption of the circuitry. Determining the
> "barrier'" effectiveness is very difficult.
>
> First, all polymers (coatings) are permeable to gases. Water vapor is
> one of many
> The rate of permeability varies with coating thickness and
> temperature. There is a
> dramatic increase in permeability when the Tg window is exceeded.
> There should be a requirement for coating vendors to provide
> permeability charts for there products.
> Some of these gases, such as Hydrogen Sulfide, are quite reactive.
>
> Second, there is Young's Modulus. This is the relative "stiffness" of
> the material.
> When the Tg window is exceeded there is a dramatic drop in The Young's
> Modulus.
> This is the "softening" of the material. In addition there is a
> dramatic rise in the CTE of the material.

    At some point above the Tg window the combination of reduced Young's
Modulus

> and increasing expansion of the product allows solid particles
> deposited on the surface
> to "sink" into the coating.  This results in a  coagulated goo with
> virtually no barrier properties left at any temperature.  I  have seen
> this with the low Tg AR coatings.
> As near as I could figure this process began at about 40 degrees C.
>
>
> What makes this whole issue seriously difficult is that the
> "effective" life of the coating
> is primarily dependent on the type of atmospheric conditions it is
> exposed to.
> How fast the air is being exchanged at the surface determines the
> overall levels
> of exposure. I as mentioned in my response to Dewey changes in the
> industry are causing increased density, speed, and lower signal
> levels. This results in thermal problems with the hardware. Internal
> forced air cooling increases atmosphere exposure by up to 200 time
> over passive thermal exchanges.  David Hillman mentioned some "ugly"
> situations I was involved in. These were a combination of internal
> forced air cooling and low Tg type AR coatings.
> I have determined that this combination is extremely risky.
> Understanding the critical role
> played by localized atmosphere exchanges is necessary when selecting
> coatings.
>
> High humidity > 60% is critical due to surface deposition of submicron
> particles and reactive gases. At this point an aqueous film > 5
> molecules thick can form. This film can dissolve
> salts and transport ions (leakage currents). There is also the fact
> that it is very acidic
> The National Association of Corrosion Engineers, NACE, has determined
> the Ph of these films to be between  2 and 3.  The amount of time
> required to create this acidic film has
> been measured by Bell Labs at about 4 weeks in an office environment
> with the boards
> opened to the air.
>
> The combination of reactive gases/liquids, submicron particles,
> temperatures,
> humidity, and the and rate of atmospheric exchange will determine the
> life cycle of the coating barrier. Modern circuitry is rapidly
> becoming more sensitive to this combination.

>
> Neither the mil standard nor the IPC guidelines address this
> combination. They are material qualification documents that insure
> certain minimum requirements are met. They are needed because there
> are more coating materials being sold then grains af sand on the beach
> that do not even meet these minimums (my favorites are the diluted
> "Super Glue" types).
>
> All this assumes that the board surface was "clean" at the time the
> coating was applied.
> If not, all bets are off!!
>
> The Mil specs and the IPC documents will not insure the coating will
> actually work  for your
> end use environment.  DO NOT BLINDLY EXCEPT  THEM AS A GUARANTEE.


David A. Douthit
Manager
LoCan LLC


Graham Naisbitt wrote:

>Hi everyone,
>
>I have been in France for the past couple of days, so did not respond to
>everyone earlier.....but here goes the duel! (You can't get rid of me that
>easily!)
>
>The subject in question is David's oft repeated reference to low Tg....and
>for the record the Tg of 1B31 is 14DegC not 15.
>
>So what?
>
>As Dewey said, and is no doubt backed-up in the Collins report submitted by
>Dave Hillman, this is not necessarily a bad thing.
>
>Circuits exposed to wide temperature variations, coupled with high levels of
>humidity are quite commonly conformally coated. The coating MUST therefore
>be capable of surviving temperatures commonly down to -65DegC and up as high
>as +125DegC or more.
>
>I have the distinct impression - and David please correct me if I am wrong -
>that David thinks that an acrylic coating such as 1B31, permits external
>"stuff" to descend through the coating and hence onto the circuit surface.
>Whilst this might be the case, there are most certainly many other forces at
>work or conditions that must be considered and it is inappropriate to
>zone-in on one issue in isolation.
>
>Definition of Tg: The temperature at which an amorphous polymer changes from
>a hard and relatively brittle condition to a viscous or rubbery condition.
>
>A simplistic view of a material's glass transition temperature (Tg) is the
>temperature below which molecules have very little mobility. On a larger
>scale, polymers are rigid and brittle below their glass transition
>temperature and elastic above it.
>
>The Space Shuttle Challenger disaster was caused by a rubber O-ring that was
>below its glass transition temperature and thus could not flex adequately to
>form a proper seal around one of the two solid rocket boosters.
>
>So how about a coating that has a high Tg, Would this be better David? In a
>purely high temperature environment the answer would be a definite maybe!
>However, below its Tg it is brittle, therefore the lower the temperature the
>more brittle it becomes and hey presto - the coating shatters allowing all
>that nasty stuff to get in through the cracks!
>
>So, as stated by Dewey: The key is to understand the end use environment and
>choose the coating that has the widest processing window, tolerates rework
>and functionally does the job.
>
>You will now have to excuse me for saying so, but this is the very reason
>why we have so many different coatings in our range - "horses for courses"
>or, as Dewey so poetically referred to it: We have a huge selection of
>quality Dust Bags whatever your needs - we even do latex but I don't want to
>go there right now....
>
>TTFN everyone
>
>Graham Naisbitt - with a little (lot!) of help from Phil Kinner.
>
>
>
>>To all,
>>I thought for a moment we were going to have an old fashioned duel between
>>David and Graham.It would have been a shame to loose all that intellect and
>>humor in one fell swoop.We would have missed Graham, also( just kidding
>>Graham).
>>
>>
>>
>>
>
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