TECHNET Archives

June 2001

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 15 Jun 2001 11:32:43 +0300
Content-Type:
text/plain
Parts/Attachments:
text/plain (104 lines)
Steve

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!)

Brian

"Stephen S. Schiera" wrote:
>
> What are the advantages of a silicon coating over an acrylic?
>
> In what environmental situations would silicone outperform the acrylic?
>
> I had the understanding that silicone materials are hydroscopic and could
> absorb and retain moisture.  If this is the case would long exposure to
> moist environments create a greater potential for circuit failure?
>
> Thanks in advance.
>
> Steve
>
> ---------------------------------------------------------------------------------
> 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
---------------------------------------------------------------------------------

ATOM RSS1 RSS2