Hello Techies,
Following Brian's posting, I saved another he made a little time ago that
you might find useful.
However, before I do that, may I only point out that there is no such thing
as the perfect or ideal conformal coating - it is all a question of
trade-off's.
For the original question of high humidity AND high levels of salt spray (or
should that be salt fog or salt mist? It does make a difference in test
conditions and results.) then a single part urethane will work well and we
can prove it. For those who want this proof, ask off-line please.
Selecting the right material is critical and making sure that it is:
a Applied correctly in accordance with the anticipated production
method
b Cured properly BEFORE testing commences
2 Part Urethanes are excellent but, a major pain and expense in production.
There are single part versions but these tend to be very moisture sensitive
thus have a short shelf-life and "use-by" time when opened (hours not days!)
However, there are even more urethanes (alkyds) that are far more forgiving
in every sense - and consequently highly popular.
So finally here is part of another Sermon according to "St Brian of Cyprus"
:-)
The Problem with Silicone Conformal Coatings...
Silicone coatings are a wee bittie of a con, IMHO (sorry, Dow Corning et
al.!). Their behaviour under humid conditions is roughly 10-20 times worse,
as a rule than other types. Let me expand on this sweeping statement. Single
component silicones usually require the presence of free hydroxyl radicals
to crosslink. That means that they will not cure in a perfectly dry
atmosphere. In this sense, before curing, they are hygroscopic. After
curing, the hygroscopicity is reduced, but they are porous (otherwise, the
trapped humidity would not have escaped). Two component products use other
mechanisms of curing, so are less hygroscopic prior to curing. However, they
are almost equally porous.
Other polymers, such as acrylics, epoxies and polyurethanes use other
mechanisms for curing and are considerably less porous (but they are all
porous).
Many years ago, I cast discs of typical products (the results are reported
in my book), which I used as a diaphragm between a humid and a dry
compartment. The comparison was striking.
However, in the popular imagination, silicones are the nec plus ultra
because the are not wet by water in the liquid phase, without consideration
of its behaviour in the gaseous phase.
So what can happen in worst case conditions? Let's imagine you have a tiny
salt crystal (or any other hygroscopic contaminant) stuck between two
conductors. Without a coating, the scenario is obvious, as soon as the
circuit is put into service in a humid environment. Aha!, you say, that is
why we coat, so that it would stop such a catastrophe. Not so! The zone of
polymer close to the crystal will haves it absorbed humidity sucked into our
salt crystal, drying it out. But, as nature abhors a vacuum, so do polymers
like to strike a humidity equilibrium with the surrounding air, so more
humidity is absorbed and the process continues. But, you may say that a tiny
speck of humidity not bridging conductors is relatively harmless. Again, not
so. As more humidity is absorbed by the crystal, osmotic pressure starts to
rise and will start to lift the coating off the substrate and this process
may continue to form a humid pool between conductors and, WHAM! This process
is called vesication and is more common than you think. It is illustrated
graphically and by photographs in my book. The result is sometimes also
called mealing, not to be confused with measling.
So your coating has only delayed a catastrophe and the delay time with a
silicone coating is only a fraction than with other coating types. One hopes
that cycling temperatures and humidities are such that the catastrophe will
never happen but the best way of assessing a minimum risk is by ensuring
that the minute crystal is never there, in the first place, by a thorough
and effective cleaning before coating. This was the philosophy behind the
long-defunct MIL-P-28809, but is still very valid. There may be some **very
rare** exceptions to the application of this rule, which are too complex to
discuss just now but you are safe in assuming that a proven cleaning quality
is a sine qua non to conformal coating.
Acrylic is better than silicones in this respect and electrically, but not
so good in terms of chemical resistance.
Another little known feature about silicones is that they transmit shock
better than most other materials. If you underfill an IC with a silicone and
the board is subjected to a shock test, the ICs will suffer more than with,
say, an epoxy underfill. This is because silicones are virtually
incompressible at high rates of stress (hence your super-bouncing ball).
Hope this helps (and raises the cat among the pigeons!)
And another:
A 0.2 cents worth: the TCE of silicones are ginormous (e.g. 300-350 ppm/°C)
compared with solder (c. 16 ppm/°C) and I suggest you may be right about
worrying about it. Furthermore, silicones have a unique property that,
however squausshy (thanks, Graham, for the term) they feel under the thumb,
they are rock-hard when subjected to a shock (hence super-bouncer balls and
bouncing putty). The combination could result in a squished silicone at
slightly elevated temperatures which would place a tension on the
BGA-ball-pad combination, which could increase by orders of magnitude should
it be subjected to a small shock at the same time. Any gap in the underfill,
especially if accompanied by flux residues or other hydrophilic contaminant,
could even fill with water under humid conditions of use. I feel vacuum
impregantion would be a must.
A few pages are devoted to silicones in Bartholomew's classic book on
underfilling in the chapter on properties of encapsulants, emphasising the
moisture permeability. He suggests that the TCE problem may be reduced by
silica filling, but that high filling ratios may result in micro-cracking
during high temperature cycling. Otherwise, I don't think he devotes much
space to silicones as underfills, compared with other resin systems.
Hope this helps you all.......
Regards Graham Naisbitt
[log in to unmask] <mailto:[log in to unmask]>
Concoat Limited
Alasan House, Albany Park
Camberley GU16 7PH - UK
www.concoat.co.uk <http://www.concoat.co.uk>
Phone: +44 1276 691100
Fax: +44 1276 691227
Mobile: +44 79 6858 2121
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Brian Ellis
> Sent: 07 March 2002 07:32
> To: [log in to unmask]
> Subject: Re: [TN] Conformal coating: high humidity environment.
>
>
> Jan
>
> The order from good to bad, at temperatures < 40°C, but ONLY, say again,
> ONLY FOR MOISTURE RESISTANCE.
> paraxylylene (Parylene (R) ) 1
> acrylics 2
> epoxy 3
> polyurethane 4
> silicone 5
>
> For mechanical resistance, the order is 3, 1, 4, 2, 5
> For chemical resistance. 1, 3, 5, 4, 2
> For electrical properties (dry conditions) 2, 1, 5, 4, 3
> For ease/cost 2, (3, 4, 5), 1
> Repairability 2, 4, 5, 3, 1
>
> Obviously, these orders may vary somewhat with individual coatings, and
> are just a rough categorisation, but they will give you an idea
> according to which characteristics are most interesting for your
> application. Where there is a choice between single and 2-component
> products, go for solventless 2-component ones for best results, PROVIDED
> you can meter and mix them accurately, otherwise forget them. The reason
> there is such a choice is because there is no single ideal solution and
> you have to compromise somewhere along the line. If you draw up a
> "league table" from the above scores, you will find them all roughly
> equal, certainly within the tolerances between individual products.
>
> FWIW
>
> Brian
>
> Jan Satterfield wrote:
> >
> > Someone asked the questions regarding high humidity and the
> best conformal
> > coating to use and I read 4 different answers: Acrylics, urethanes,
> > paralyene and Silicone. I have always understood that acrylics were the
> > least moisture resistant, paralyene the most moisture resistant and
> > urethanes the most commonly used for moisture resistance on
> CCA's. Anyone
> > care to clarify?
> >
> > -----Original Message-----
> > From: [log in to unmask] [mailto:[log in to unmask]]
> > Sent: Wednesday, March 06, 2002 9:22 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Conformal coating: high humidity environment.
> >
> > Acrylics tend to have the best performance in high moisture
> environments, as
> > long as the temperatures are not extremely high.
> >
> >
> ------------------------------------------------------------------
> ----------
> > -----
> > 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 e-mail to
> [log in to unmask]: SET
> > Technet NOMAIL
> > To receive ONE mailing per day of all the posts: send e-mail to
> > [log in to unmask]: SET Technet Digest
> > 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 e-mail to
> [log in to unmask]: SET Technet NOMAIL
> > To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> > 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 e-mail to
> [log in to unmask]: SET Technet NOMAIL
> To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> 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 e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
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
---------------------------------------------------------------------------------
|
