Steve 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!) Brian "Stephen S. Schiera" wrote: > > What are the advantages of a silicon coating over an acrylic? > > In what environmental situations would silicone outperform the acrylic? > > I had the understanding that silicone materials are hydroscopic and could > absorb and retain moisture. If this is the case would long exposure to > moist environments create a greater potential for circuit failure? > > Thanks in advance. > > Steve > > --------------------------------------------------------------------------------- > Technet Mail List provided as a free service by IPC using LISTSERV 1.8d > 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 delivery of Technet send the following message: SET Technet NOMAIL > Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 > --------------------------------------------------------------------------------- --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------