TECHNET Archives

September 2003

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Reply To:
Date:
Tue, 9 Sep 2003 07:49:31 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (278 lines)
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
-----------------------------------------------------

ATOM RSS1 RSS2