Per-Erik,
Did you look at the interface between the coating and the board? The
bonding state of the materials may play a key role (need XPS study). If
the bonding is insufficient, the type of coating is secondary effect
compare to the interface characteristics.
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Tegehall Per-Erik
>Sent: Tuesday, September 09, 2003 2:33 AM
>To: [log in to unmask]
>Subject: Re: [TN] Silicone conformal coating
>
>
>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 begin 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. However, 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 days before an acrylic or a
>polyurethane coating has picked up as much wa
>
>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 has 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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