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- >> --------------------------------------------------- > >--------------------------------------------------- >Technet Mail List provided as a service by IPC using LISTSERV >1.8e 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 or (re-start) delivery of Technet >send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) >To receive ONE mailing per day of all the posts: send e-mail >to [log in to unmask]: SET Technet Digest Search the archives of >previous posts at: http://listserv.ipc.org/archives Please >visit IPC web site >http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for >additional information, or contact Keach Sasamori at >[log in to unmask] or 847-509-9700 ext.5315 >----------------------------------------------------- > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------