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May 2002

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Subject:
From:
Graham Naisbitt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 24 May 2002 11:54:29 +0100
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Tim,

I can only add to what others have said, but first, another reminder of
the message from Brian Ellis:

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).

....and my 2 pennorth.....

Silicone materials do not readily stick to anything, including other
silicones. Furthermore, de-wetting will be evidenced when other coating
types are applied over a silicone of almost any type: underfills,
adhesives, pre-pregs, buffer and staking compounds.

I do recall that some years ago, 10,000 military assemblies had to be
scrapped because of a silicone pre-preg used to bond the board to an
aluminium heat sink causing massive de-wetting. Cost of each assembly -
$15,000 each.....and that was in 1984!

Most military users to my experience, now use physical straps for large
components. I believe that Loctite have some good silicone free staking
compounds. You might care to investigate further....

Hope this helps.

Regards Graham Naisbitt

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> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Tim McQuay
> Sent: Thursday, May 23, 2002 18:45
> To: [log in to unmask]
> Subject: [TN] Conformal coating / gluing
>
>
> Hi Folks:
> We have a debate regarding sequence of operations when we
> glue down components and apply conformal coating to the same
> board. The IPC-A-610C does not cover this and I have not been
> able to locate any other info. Basicly we want to know, do we
> glue first or do we coat first. Can someone please tell us if
> the bond between silicone glue and conformal coating is as
> strong as between silicone glue and PCB uncoated surface. We
> are concerned that heavy components, such as the large caps
> on some of our boards, would not be well supported if bonding
> is done after conformal coating. We have our own bias but we
> wanted to hear an outside opinion and/or see a standard on
> the topic. The adhesive and coating we use are listed below.
>         3-1744 (DOW), SILICONE ADHESIVE - for staking big components
>         KE3421 (SHIN-ETSU), SLCN BRD COAT - conformal board coating
>
> Thanks
> Tim McQuay
> Training Coordinator
> Vansco Electronics
> Fax: 204-474-1674
> e-mail: [log in to unmask]
>
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