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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 26 Mar 2004 10:40:34 +0200
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Sorry, Graham, I must back up what David says. The phase change of the
cross-linked polymer over the Tg does not really make it into a viscous
liquid, so much as a rigid elastomer (Tg is not synonymous with MP). The
temperature of this change depends on the MW, which varies, hence
David's "window". It also varies according to the amount of absorbed
water vapour (or other gases).

However, for me, the key sentence in David's message is "In addition
there is a dramatic rise in the CTE of the material.". If the low Tg
coating underfills a component (e.g., a µBGA), then running a PCB
through temperature cycles will put undue strain on the solder joints.
There may just be sufficient ductility in SnPb joints to allow you to
get away with it, but have you considered what may happen with Pb-free?

I'll introduce another aspect: have you considered what may happen with
rework? Any chemicals used in this operation, either as liquids or
vapours, would be absorbed like gangbusters over the Tg. Now, consider
if a micellar substance (e.g., a surfactant) were used in a flux or
cleaning solution. The hydrophobic end of the molecule would hydrogen
bond within the loose intramolecular spaces of the coating, while the
hydrophilic end would stick out the top. With van der Waal's packing of
these molecules, your SIR would drop dramatically (cf., the drop of SIR
of PCB substrates taken over their Tg while soldering with WS fluxes
containing micellar PEGs - same mechanism).

Brian

I would consider a protection operating or stored at over its Tg as hairy.

David Douthit wrote:

>> 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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