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From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 9 Sep 2003 15:21:03 +0300
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Per-Erik

Unfortunately, I don't have time just now to discuss this further in
detail. I agree with you on most points, but let's not forget that
atmospheric humidity has four components:

- H2O in clumps (mist): this is relatively harmless to coatings as it
wets or sits on the surface, rather than penetrates
- H2O as isolated molecules (true humidity). This penetrates all
polymers ± easily
- H+ as a result of hydrolysis: a very reactive and unstable reducing
gas which can penetrate and react easily with many groups in and under
the coating.
- OH- as a result of hydrolysis: a very reactive and unstable oxidising
gas which can penetrate and react easily with many groups in and under
the coating.

The hydrogen radical and hydroxyl group may be just as dangerous as H2S
and are ever-present in air, constantly forming and recombining at
random, under the influence of ozone molecules. The hydroxyl group, in
particular, can play havoc with any loosely bonded hydrogen atoms in
organics it comes in contact with, as H-OH is a much stronger bond than
H-C (this is why many hydrocarbon vapours have a short atmospheric
residence times: the molecules are literally torn apart by hydroxyls,
during hydrolysis). As the global average tropospheric hydroxyl radical
concentration is estimated at 8.1 x 10^5 radicals.cm-3 at any single
moment, this is far from negligible.

I feel this must also be a consideration.

Best regards,

Brian



Tegehall Per-Erik wrote:
> Brian
>
> What you are saying is that the function of the conformal coating is to act as an impervious barrier against humidity. Thus, a low permeability is crucial and the protection afforded is directly related to the permeability of the coating. Let's assume to begin with, that absorption of humidity will endanger the reliability of an assembly (which I do not agree with if the cleanliness is adequate). Then, why should permeability be more important than the water absorption level at equilibrium (that is, saturation level)? I guess that many assume that a high permeability also automatically means a high water absorption level but there is no such relation. Silicone materials have a very high permeability compared to most other coating materials but it is also a fact that the saturation level of water is lower for silicone materials than for other coatings. This means that the amount of water absorbed by a silicone coating likely is somewhat higher than for other coatings to beg
in with, but after some time the other coatings will have absorbed more water. Which is then most dangerous, a high permeability or a high saturation level? As in many other discussions on TechNet, the answer is: It depends. In this case, among other things, on how long it will take to reach saturation.
>
> About ten years ago I did an experiment to prove that permeability of a coating is not crucial for the protection afforded. I coated a comb pattern on thoroughly cleaned HASL boards with one acrylic, one polyurethane and one silicone coating and Parylene, each with three different thicknesses. I then put the boards in a humidity cabinet and increased the temperature to 85 degree C but without any added humidity. When the temperature had stabilised, the humidity was increased to 85 % RH. The surface insulation resistance (or rather interface insulation resistance in this case) for the comb patterns on the boards coated with the silicone coating decreased instantaneously and almost as fast and to almost the same level as on a non-coated reference board even for thicknesses of up to 150 um. So you are right, the permeability is very high for silicone coatings and the insulation resistance will be affected and, if that had been important, silicon coatings would be useless. How
ever, the results very practically identical for the boards coated with the acrylic and the polyurethane coatings. The SIR decreased a little slower on the boards coated with Parylene but after about one hour the SIR was on about the same level on a board with 50 um Parylene as on the boards coated with the other conformal coatings. In fact, after longer exposure to humidity, the SIR on the Parylene coated boards actually decreased to the same level as for the noncoated board. That is, it decreased more for the Parylene coated boards than for the boards coated with the other coatings. These results show that SIR beneath a coating is affected very quickly even when Parylene is used as coating but they do not answer the question how long it will take to reach saturation level. I haven't measured that but the problem with pop-corning of plastic components shows that epoxy materials pick up quite large amounts of water in a few days, so my guess would be that it is a matter of da
ys before an acrylic or a polyurethane coating has picked up as much water as a silicone coating.
>
> Is then a high uptake of water to the level of saturation dangerous? Of course not (under condition that the cleanliness is adequate). If had been, a coated board would not pass a humidity test.
>
> If the cleanliness is not adequate prior to the application of the coating, then the situation is different. Hygroscopic contamination will pick up water through the coating and cause blistering and electrochemical migration. I think that you agree that this will also happen if you apply Parylene to a contaminated surface. Electrochemical migration requires three things to occur; contamination, humidity and a bias. Thus, if Parylene had been an impermeable barrier against humidity, it would not had been necessary to clean the assembly. I agree that hygroscopic contamination will be picked up much faster if you have a silicone coating. But on the other hand, it is only when you have a bias that electrochemical migration will occur. If heat is generated when the equipment is used, the assembly will dry up much faster if you have a coating with high permeability and low moisture uptake. Therefore, a silicone coating may be preferable under such conditions (although it should
verified). If heat is not generated, silicone is perhaps not the best choice (but again it should be verified). In any case, reliability problem due to contamination needs to be solved by improving the cleanliness, and not by finding a coating that will delay failures somewhat.
>
> What is then the function of a conformal coating? It is to eliminate the third factor causing electrochemical migration (and blistering) from the surface beneath the coating by preserving a cleanliness that have been achieved by cleaning prior to application of the conformal coating. Thus it needs to be an impermeable barrier, but towards ionic and hygroscopic contamination from the field environment and not towards humidity. It may also need to protect the circuitry from being in direct contact with a condensed water film on the assembly. (Thus, air is disqualified as replacement for a conformal coating.) I have not seen any data of the permeability of various contaminants in conformal coatings but I guess that most coating have very low permeability of ionic contaminants. Organic contaminants may be a problem since they may be absorbed. I also have to admit that if you have corrosive gaseous pollutions such as hydrogen sulfide, silicone coating is a bad choice since it h
as a high permeability for most gaseous pollutions.
>
> Then, why does Parylene generally afford a much better protection than other coatings? It is because it affords better protection against contamination from the field environment, not because better barrier properties but because of better sealing properties due to better coverage. With Parylene, you will get a coating with even thickness on all surfaces, even on the tip of a needle. It is very difficult to get good coverage with other types of coatings since the coverage of protruding parts such as solder joints and component leads due to gravitation, surface tension and capillary forces.
>
> Per-Erik Tegehall
> IVF
>
> -----Ursprungligt meddelande-----
> Från: Brian Ellis [mailto:[log in to unmask]]
> Skickat: den 6 september 2003 09:48
> Till: TechNet E-Mail Forum.; Tegehall Per-Erik
> Ämne: Re: [TN] Silicone conformal coating
>
>
> Per-Erik
>
> An interesting idea. Briefly, you are proposing a more permeable coating
> so that humidity can leave, as well as enter. I suggest that if these
> conditions apply, then the best coating would be the most permeable.
> Probably 90% or more of electronic assemblies use such a coating. It is
> called air. It is extremely successful for the conditions under which
> such circuits are normally used. Conformal coating is usually used when
> air-coating is unsuitable, for whatever reason. Therefore, we need
> something as far removed from the conditions of air as possible, such as
> a theoretically impervious skin. The nearest we have to this in our
> armoury of weapons of mass construction is para-xylylene
> (Paralene/Parylene). This is near-ideal, but expensive and difficult to
> apply, so we usually look to other materials. I would suggest that
> acrylics are closest to this ideal, except that they have poor
> temperature and chemical resistances. So the choice, IMHO is either air,
> with maximum permeability, or a product with minimal permeability.
> Compromises between the two are probably second-best.
>
> Brian
>
> Tegehall Per-Erik wrote:
>
>>After having thought a little more on this, I will expound my answer. It
>>seems that all assume that high permeability is a bad thing but in my
>>opinion it is not that simple and a high permeability may even be
>>preferable. If you have adequate cleanliness on the assembly prior to
>>applying the coating, the moisture penetrating the coating will not be a
>>problem and the permeability will then not be an issue. It is when you
>>have to much hygroscopic and ionic contamination beneath the coating
>>that you may get problem with delamination and electrochemical
>>migration. Even if you use Parylene, quite a lot of moisture will be
>>picked up by hygroscopic contamination beneath it in a few days in a
>>humid environment. If you then turn the euipment on and the board is
>>heated due to power dissipation, it will take much longer time to dry up
>>the assembly if you have a coating with low water permeability. And it
>>is during this time period electrochemical migration will occur.
>>Therefore, a silicone coating may actually be better than Parylene in
>>this case.
>>
>>Per-Erik Tegehall
>>IVF
>>
>>    -----Ursprungligt meddelande-----
>>    Från: Tegehall Per-Erik
>>    Skickat: den 5 september 2003 08:38
>>    Till: [log in to unmask]
>>    Ämne: Re: [TN] Silicone conformal coating
>>
>>    Steve,
>>
>>    It depends on what you mean with good moisture resistance. All
>>    conformal coatings absorbs moisture but the diffusion rates of water
>>    in the coatings and the saturation level varies. Water molecules
>>    have a high diffusion rate in silicone coatings but the saturation
>>    level is lower than in most other coatings. Which is to be prefered?
>>    It probably depend on the application.
>>
>>    However, a general statement of good moisture resistance, I
>>    interpret as that the coating properties are not degraded by moisture.
>>
>>    Per-Erik Tegehall
>>    IVF
>>
>>        -----Ursprungligt meddelande-----
>>        Från: Steve Gregory [mailto:[log in to unmask]]
>>        Skickat: den 4 september 2003 21:35
>>        Till: [log in to unmask]
>>        Ämne: [TN] Silicone conformal coating
>>
>>        Hi All!
>>
>>        I know when you look up properties for Silicone conformal
>>        coating, it usually states that it has good moisture resistance,
>>        but is it really that good?
>>
>>        I've heard that it is somewhat permeable to moisture, that if
>>        you were worried about moisture, and the assembly were going to
>>        see the outside environment, you would be better off with an
>>        acrylic, or urethane coating.
>>
>>        Any thoughts?
>>
>>        Thanks!
>>
>>        -Steve Gregory- ---------------------------------------------------
>
>
>
>
>

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