Hi everyone

Enough said - Dewey endorses the point I was trying to make in the first
place - and he is the customer, a very punny customer at that!

It's Friday and already gone 5.30pm and I have a full day of golf, followed
by rugby on the TV and I am gone..........

Have a great week-end everyone, enjoy

Graham Naisbitt

> To all,
> Since my sense of humor is not as well known or understood as "Mountain Dew"
> Doug's is, I want you to know that the "pun"ishment you endure is hopefully
> soothed by the humor,truth and wisdom(well one out of three qualifies me for
> the majors) that comes along with it.
> All of what everyone has said is pretty much correct over a given set of
> situations or applications.I do hope you believe that I have recognized,
> assessed, evaluated and analyzed all of that over the years so our designs,
> choice of materials, application techniques and all the"illities" have been
> developed to make the most robust and safe product possible for the use in
> our "known" Aerospace environment.I have a "cradle to grave" responsibility
> so the practices and procedures at our maintenance facilities are as an
> important  key, as our initial design and assembly is, to meeting our
> committment to long term reliability.
> Our efforts to uphold this "hi rel" approach are continually being
> compromised by global and strategic cost reduction initiatives so it is
> imperative that we disseminate this information so there is an awareness of
> the ramifications of one's design and material choices.This is why I support
> the IPC and this technet.The old addage is true"If you can't lickem,
> joinem(Doug and Graham would have been disapointed if I hadn't said
> this)".We need to make people aware that our requirements are real and
> justified.They don't have to design to meet them if their product and use
> doesn't dictate, but they do need to recognize them.
> So the next time you are at an IPC meeting go up to people like Dave Hillman
> of Rockwell Collins and thank him for all the hard work and dedication he
> has contributed to make the industry better as a whole(a little PTH humor).
> After all,if our job was that easy, Japan would be building Airplanes.Oops!
> They are.That's another thing I let slip out of the "dust bag".
> Dewey
> 
> 
>> -----Original Message-----
>> From: Tegehall Per-Erik [SMTP:[log in to unmask]]
>> Sent: Friday, March 26, 2004 1:59 AM
>> To:   [log in to unmask]
>> Subject:      Re: [TN] acyrlic coating deformed BGA balls
>> 
>> Graham and David,
>> 
>> I think you mix apples and pears regarding the "dustbag" issue. It is not
>> the low Tg per se that makes AR coatings susceptible to become "dustbags",
>> although it contributes. In contrast to epoxies, polyurethanes and
>> silicones, acrylics do not cure, they just dry. That is, no cross-linking
>> occurs between the acrylic molcular chains in the dryiong process. This is
>> the reason why acrylics are so easily dissolved in many solvents (and why
>> you can't have a solventless AR coating material). The absent of
>> cross-linking make it less rigid and allow it to "flow" and encapsulate
>> particles above Tg. A low Tg silicone coating will never do that even if
>> the Tg is much lower than for AR coatings. It is attractive to have a
>> coating that is easy to remove but the "dustbag" properties is one thing
>> you have to pay for it.
>> 
>> Per-Erik Tegehall
>> IVF
>> 
>> 
>> -----Ursprungligt meddelande-----
>> Från: David Douthit [mailto:[log in to unmask]]
>> Skickat: den 26 mars 2004 04:30
>> Till: [log in to unmask]
>> Ämne: Re: [TN] acyrlic coating deformed BGA balls
>> 
>> 
>>> 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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