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From:
Graham Naisbitt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 26 Mar 2004 18:05:50 +0000
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Hi David

No, but I have been flying with them for more than 30 years now. You would
think I would qualify for some sort of prize? Second thoughts — don’t
answer that!

Re your response — well said, although you don’t include many other factors
that we have to consider when designing coatings, and our customers have to
consider when evaluating them.

It is my experience that you David, consistantly highlight Tg as the prime
issue in the coating performance; and as has been so amply demonstrated by
everyone involved in this debate, there are so many other issues that come
into the equation. Barrier is the wrong word and water will go through
everything, even glass given enough time. Better to think of a conformal
coating as a protective film, which is a breathable membrane that helps to
protect the circuit......

.....that sounds soooo much better than dust bag!

But they do work!

Graham Naisbitt

>> 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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>> 
>>   
> 
> 



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