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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 8 Mar 2002 11:19:24 +0200
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Gawd, Graham. I haven't been raised to the peerage, let alone to
sainthood during my lifetime. Not even Mother Teresa managed that :-)
Seriously, I'll have to watch my Ps and Qs when responding to TN posts.
I never realised that my words were recorded for posterity. You are
placing a heavy responsibility on my shoulders by publishing the
collected works of Brian Ellis (they would fill a library :-( , as they
go back 40 years or more) because I've BSed as much as anyone and events
have proved my forecasts and ideas wrong as much as they've been right.
(Remember the Plop-plop-Meter for detecting blowholing: it worked but
never took off!!!!)

Brian

Graham Naisbitt wrote:
>
> 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.
> > >
> > >
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