Pete - Pete please,
Paralene is NOT impervious. NO coating is impervious.
I agree to your concerns about the mentioned stresses, our materials have
been in use in such conditions for over 50 years - yes 50! The first
conformal coatings were used on RAF night fighters for the early radar and
wireless sets in 1940 - as was pointed out to me by engineers at (then)
Marconi in Rochester, Kent.
The stresses you mention for avionics systems is one of the reasons why
ceramic circuits were, and in some cases remain, the preference of system
builders.
As to when failures might occur, is the reason why we have to meet extended
and accelerated life/aging tests.
Regards, Graham Naisbitt
[log in to unmask]
Concoat Limited www.concoat.co.uk
<http://www.concoat.co.uk>
Alasan House, Albany Park
Camberley GU16 7PH UK
Phone: +44 (0)1276 691100
Fax: +44 (0)1276 691227
Mobile: +44 (0)79 6858 2121
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of <Peter George Duncan>
Sent: 30 May 2001 01:27
To: [log in to unmask]
Subject: Re: [TN] Antw: [TN] MSD's
Guenter,
I already understood a fair amount about what you told me, but confirmation
of my understanding was very welcome. Components, boards and coatings have
to be able to "breathe", and the moisture content does reach an equilibrium
point. The equilibrium point, though, will vary depending on RH,
temperature and pressure. The point is, what damage does all this moisture
transport do?
In avionics boards, particularly military avionics where boards can be
subjected to extremes of all of three factors in a matter of seconds, I am
having concerns about the effects of moisture content on the performance
and reliability of equipment. For example, at -40 deg at high altitude,
water will be very frozen, and water expands considerably when it freezes.
What does this do to the internal workings of all the components that live,
say, in tropical countries when on the ground? Most equipment is not
hermetically sealed, so on the ground, it's 'feast time' for hygroscopic
materials, which may then get the equivalent of serious cramp when
exercised at altitudes where pressure, temperature and humidity are all
very much lower. The high moisture content would freeze and expand, forcing
apart the boundary layers between the materials within the components.
There are increasingly few MIL-Spec parts available these days, and
increasingly often, OEM's have to opt for Industrial grade components to
achieve functionality. These have lower ratings in a number of areas -
temperature and moisture sensitivity being two of them. How often does the
freezing and thawing, the forcing apart of component internal material
boundary layers and repeated dew points inside components have to occur
before the component suffers a drop-off in performance, or fails?
Short of thoroughly cleaning and baking all the boards immediately prior to
conformal coating, and then coating them with something impervious like
parylene, is there a solution to this scenario?
Pete Duncan
Guenter Grossmann
<Guenter.Grossman To: [log in to unmask]
[log in to unmask]> cc: (bcc: DUNCAN Peter/Asst
Prin Engr/ST Aero/ST Group)
Sent by: TechNet Subject: [TN] Antw: [TN]
MSD's
<[log in to unmask]>
05/29/01 05:13 PM
Please respond to
"TechNet E-Mail
Forum."; Please
respond to
Guenter Grossmann
Pete
I couldn't lead you to a single publication about this item but I try to
give some answers from my experience.
I believe the most important thought that has to be kept in mind is the
fact that any polymer is permeable to humidity. There are variations in the
amount of water a polymer can absorb until it is saturated and there are
variations how fast the water diffuses through the polymer until it reaches
an interface but this is a natural process in polymersthat can not be
avoided. On this thought I'd like to look at your questions:
- Yes, component encapsulations and PCB materials will absorb humidity
until an equilibrium of the water content of the polymer ant the
surrounding atmosphere is reached. This is not a stationery condition but a
dynamic process that depends on the changing environment. How fast the
water contents of a polymer changes depends on the material properties, on
the difference in the water contents of the polymer and the surrounding
media and on the temperature. A water splash will not significantly enhance
the contents of moisture due to the fairly low diffusion coefficients of
the polymers used.
- Yes there are studies about the influence of moisture on the performance
of components in the field on IC's. Humidity combined with the ionic
impurities that are present in IC packages are an important root cause for
corrosion on the surface of chips.
- If moisture is trapped under a conformal coating it will be
1) absorbed by the coating
2) absorbed by the components encapsulation
3) absorbet by the PCB
4) diffuse through the coating
These effects will lead to a varying contents of humidity in the
encapsulation of the component depending on the climatic conditions around
an assembly.
Problems might arise if a coated assembly is standing in a warm humid
environment for a while and then undergo a fairly rapid change to low
temperature. Then the relative humidity in air pockets below the coating
will rise, eventually above the dew point. This can cause all sorts of
things like shorts and corrosion.
- Additionally, hygroscopic residues below the coating will , like
polymers, absorb moisture. The source of this moisture is the surrounding
medium in this case the coating. This means, the coating is getting drier
thus absorbing moisture from the environment with the effect that a
transport of water through the coating takes place. The unfortunate thing
is that the equilibrium of moisture absorption depends on the water
pressure in the materials. And this pressure can be quite high it can lead
to blisters below coatings when the volume of the contaminant is increasing
dramatically during absorption ( As it is the case when salts are absorbing
so much water that a saturated water/salt solution forms).
Did that help you? I hope I didn't tell you any old cheese as we say in
Switzerland.
Have a great day
Guenter
Guenter Grossmann
Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland
Phone: xx41 1 823 4279
Fax : xx41 1823 4054
mail: [log in to unmask]
----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----
----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
