Graham,
Good to hear from you. (Why do I feel like a student facing an exam?) Anyway here goes.
Tg (Glass Transition Temperature) is a temperature when an organic polymer under goes rather
distinctive changes in its' physical characteristics. For instance from a rigid almost
brittle state to a softer more pliable state (like ice cream as it warms coming out of the
freezer but before it melts)
To quote IPC-HDBK-830 Section 10.1.1
"The Glass Transition Temperature (Tg) is the temperature at which an amorphous polymer
transitions between a super-cooled liquid and its glassy solid. As a result, amorphous
polymers exhibit substantially different physical properties above and below this
characteristic temperature. The Tg is strongly dependent on the rate at which the temperature
changes through this phase change."
There are about 13 different methods used to measure Tg. It depends on what characteristics
you are most concerned with as to the method chosen. There will small differences in the Tg
depending on the method chosen. Also the transition does
not occur at a singular temperature but is spread over several degrees. Another variable is
the amount of moisture in the polymer. When the polymer is saturated with water it can lower
the Tg 5 to 10 Degrees C.
Some of the physical characteristics of most interest for coating materials are barrier
properties (permeability), CTE (Coefficient of Thermal Expansion), dielectric properties,
bulk resistance, and Young's Modulas (hardness).
The permeability of coating materials is not well understood because there is so little
information readily available on the subject. All polymers are permeable, at various rates,
to moisture plus certain reactive gases, fumes, free air radicals, and other liquids. The
problem is the phrase "various rates". The information that is available is based on per mil
(.001 inch) thickness per a square area per a time period. A gentleman at Lucent Technology
informed me that the rate of permeability varies exponentially (Yes Brian I spelled it
correctly this time) with the thickness. It will also vary with temperature.
Because polymers expand and soften when above Tg the permeability increases greatly (the
space between molecules increases as they become mobile). I liked Brians' analogy when he
wrote "Below Tg a polymer is like a block wall but above Tg the blocks are still there but
now there is a 2 or 3 cm space between them". A "block wall" does permit moisture and other
materials to seep into and through but when these spaces open up "all bets are off".
The Tg of certain types of coatings (ones which have cross linked molecular chains) can be
varied by adding "hardeners". Increasing the Tg of these types of coatings will make them
more "brittle" and CTE becomes a serious issue. Unfortunately there are numerous types and
"flavors" of coatings now available. Choosing a coating can become quite confusing.
New designs and expanding uses of electronic circuitry has made choosing a coating a very
serious issue. The concepts of "good enough" and/or "worked before" no longer apply. The
industry is changing to fast. There simply is not enough information available to make a
really good "informed decision" (although the lack of info is a salesman's paradise).
Adding to the dilemma is the lack of testing capabilities which can provide the needed info
(I just learned there is a Bellcore Spec, written in 1995, to test assembled circuits in a
combination of MFG, ionic particulates, plus airborne suspended acids in a chamber).
To sum up the permeability issue there are very few solid numbers to use as a reference
(starting point). There is virtually no info available about variations due to temperature
(particularly above Tg). I am also rather amused when I see one temperature given as the Tg
for a polymer when in reality it is a sliding window (moisture content). There are enough
problems that already exist with integrity (coverage) of coatings plus thickness. My
conservative position is that above Tg the barrier properties of coatings are virtually nil
and reactive contaminants can reach the circuitry. When the temperature lowers and humidity
rises moisture forms and will permeate to the contaminated surface. ( At > 60% RH and at
temps from 0 to about 40 Degrees C in the localized surface area of a contaminated surface, a
layer of water will form that will be greater than 5 molecules thick. At a thickness >5
molecules this moisture film will begin to chemically act as bulk water, generally with a Ph
factor between 2 and 3.)
This is why I made the statement that above Tg coatings are no barrier (better to be safe
then sorry).
As you are aware I have written and presented (May 9th in Washington D.C.) a paper titled;
"Avionics Systems, Reliability, and Harsh Environments", which contains more info on this
subject. Jorges issues are becoming far too common with very little in the way of "hard
numbers" to help.
The primary concern of OEMs is to get past the warranty period. Once there only the end user
is truly concerned with FEMA, if they can afford it. This causes a fair amount of finger
pointing at best. Without readily available test equipment or protocols it is difficult to
verify failures related to these issues, thus I am not impressed by "there are no reported
failures" statements. There are, however, a lot of no-fault-found results. My personal
experience has been that up to half of the no-fault founds were related to these issues.
Whew! Time to get off the "soap box".
David A. Douthit
Manager
LoCan LLC
Graham Naisbitt wrote:
> David
>
> Could you please explain what your understanding is of Tg (Glass transition
> temperature) and what you think happens to the coating?
>
> Regards, Graham Naisbitt
>
> [log in to unmask]
> www.concoat.co.uk <http://www.concoat.co.uk>
>
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> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]]On Behalf Of David Douthit
> > Sent: Wednesday, June 13, 2001 06:16
> > To: [log in to unmask]
> > Subject: Re: [TN] Ok conformal coating, but how ?
> >
> >
> > Jorge,
> >
> > Be careful. The 76C that you mentioned is well above the Tg
> > (Glass Transition Temperature )
> > of many types of coatings. While above Tg coatings are no longer barriers.
> >
> > David A. Douthit
> > Manager
> > LoCan LLC
> >
> > Jon Moore wrote:
> >
> > > My first direction to approach this would be acrylic coating
> > applied with a
> > > selective spray system so that you don't have to mask.
> > >
> > > Jon Moore
> > >
> > >
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