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Well, can it, when it is in direct contact with the components and circuitry?

stevenvh
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slashmais
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  • Yes. The silicon is cited in wikipedia as best filler, better than epoxies. The proper answer should point to exact product. I can not name the product exactly –  May 28 '12 at 07:07
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    As a side note, don't expect it be fire-resistant to any degree - when set on fire it burns just great. This may be important for your design. – sharptooth May 28 '12 at 09:52
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    In researching materials for my project, I came across this site. Thanks for the information. I would like to share another source of comprehensive data that I discovered. It is the following file: "Silicone Materials for Electronic Devices and Component Assemblies.pdf". –  Feb 05 '14 at 14:51
  • @55online You need to provide a link, not just a title for the document. Please confirm that [this](https://www.momentive.com/workarea/downloadasset.aspx?id=24761) is the document you're referring to. – Dave Tweed Feb 05 '14 at 16:13
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    @DaveTweed linked to ["**Silicone materials for Electronic Devices and Component Assemblies"** - here](https://www.momentive.com/workarea/downloadasset.aspx?id=24761) . The link may not be readily visible. This seems to be a very relevant document - it is specific brand related but contains much useful information. – Russell McMahon Feb 05 '14 at 22:04
  • Link is dead but [archive.org has a copy](https://web.archive.org/web/20160918221901/http://www.momentive.com:80/WorkArea/DownloadAsset.aspx?id=24761) – Alex Hajnal Dec 22 '21 at 01:06

4 Answers4

52

The best generic term is "Silicone Rubber".
I'll refer to it as SR for brevity.

SR is a good to excellent encapsulant that has limitations (as does everything).

It is not essential to use an electronics grade SR - these are usually dearer and may have Mil Spec ratings which are not essential.
BUT see below for what IS essential.

For electronics and anything liable to be corroded you MUST use "neutral cure" SR.
You can buy acid cure SR which exudes acetic acid as it sets.
If it smells like vinegar then it's acid cure.

Do not use "acid cure" silicone rubber for electronics.

Neutral cure SRs will always say "neutral cure" or similar on the container. if they do not say this or similar they will be acid cure.

There are 2 main types of neutral cure SR (= NCSR) in common use. There are a number of other NCSRs but you will almost certainly not meet them.

Oxime cure is the cheaper and most common NCSR. It releases oximes and usually also methyl alcohol as it sets. Ventilation is needed and some people may get eczema skin reactions. The oximes can corrode bare bright copper during curing but this is usually not a major problem. Oxime cure NCSR does not bond to polycarbonate plastic.

Alkoxy NCSR is more costly and the better grades of NCSR are alkoxy. It releases methyl alcohol as it sets. This can be 5% - 10% by volume! So ventilation is an extremely good idea. It is good to work with - just be sensible.

ALL SRs that you meet are moisture cured !!!!!!!!!!!!!!!!!!!!
Atmospheric water reacts with the SR to cause cross linking of the SR rubber. If the air is relatively dry it takes longer. If the air is completely dry then SR will not set!!! SR tubes are water proof and water vapor proof. Once you open them they are NOT water vapor proof - a tube of set of SR that is carefully sealed will set completely hard on months to a year. Storage in 100% dry air MAY work.

To set right through, water vapor from the air must penetrate the SR. Penetration rates vary from about 1mm/day to 3mm/day. If you make a very thick blob of SR it can take many days to set in the middle. If you take two flat plates and overlap them and apply SR to the overlap the air path to the overlap is d/2 where d is the smallest overlap dimension and the path is through set SR and is very thin. SO an overlap SR join may take many many many days to set. Major SR makers recommend not using vast overlap in joins.

The 600 pound gorillas of the SR market are Dow Corning, Shinetsu (Japanese) and maybe BASF (BASF are the 600 pound gorilla of ANYTHING chemical but nobody notices). There are many other brands and many are good but if it's made by DC or Shinetsu you know it's good. Not all brands are good. Some people put large amounts of filler in their SRs to the extent that it works poorly.

DC do make some cheap lower performance NCSRs but even these work well for most purposes. DC and other large makers may sell specific grades in selected markets which are not available in all countries. For example they sell "Dow Corning Neutral Plus" oxime cure NCSR in Asia. Unlike most DC SR's, it has no product number and US sources do not know of its existence. It costs a few $US a tube ( e.g. in Hong Kong) and works well enough.

Many people do not know the following. Others will refuse to believe it:
Note that SR's are NOT water vapor proof. Water vapor will permeate through them but liquid water will not. So a container "sealed" with SR will have an internal relative humidity comparable to that outside it! SR is typically about 10x more water permeable than the EVA sealant/adhesive used to bond silicon "solar cells" & glass PV panels together. So a glass fronted PV panel and a "waterproof" backsheet is also not in fact sealed and inside humidity levels are ~+ outside ones. Keeping LIQUID water off your components is what is required to prevent major corrosion. Fortunately.
Corrosion still occurs with water vapour but at a vastly reduced rate due to the much lower concentration of reactants.
The other requirement is a void-free bond to the component. If there are voids then water vapour can condense to form liquid water and allow corrosion at greatly accelerated rates.

There are many many grades of NCSR - setting times vary from minutes to hours. Viscosity varies from very pourable to thixotropic.

If you ask specific SR questions not covered here I may be able to answer them.

Added, February 2014:

@DaveTweed linked to
"Silicone materials for Electronic Devices and Component Assemblies" - here which is probably the document that user 55online mentioned.
This seems to be a very relevant document - it is specific brand related but contains much useful information.

__________________________________

Added 2018:

As a guide only:

Be wary of products that are relatively heavy and relatively light relative to competing brands. Heavier ones tend to be filled with CaCO3 or similar. Light ones (which I've mainly seen in "Asian market only" offerings are filled with ???.

SR when set will usually just float in water or perhaps sink slowly. Filled SR sinks more rapidly.

Jet Blue
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Russell McMahon
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  • Thanks for the comprehensive answer. (Your comment re acceptance is noted & I'll be less hasty in future.) – slashmais May 28 '12 at 12:46
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    @slashmais, I believe you have the ability for some time to unaccept the answer of which you have accepted, which is useful in these situations should you feel Russell's was more helpful to you. – Tom Paris May 30 '12 at 07:59
  • @Tom Paris - Thanks for the suggestion. I'm happy with acceptance of Martin's answer to remain, I don't need the points & his provided link was a good one. My comment on acceptance was more aimed at general principles - leaving it for a day or so at least is liable to end up with a better result for all. – Russell McMahon May 30 '12 at 09:10
  • @arielCo - thanks for the edit. Brain fade somewhere :-) - Alkoxy is indeed what I meant when I wrote acetoxy :-(. – Russell McMahon Mar 24 '13 at 22:37
  • "..humidity levels are ~+ outside ones. Keeping LIQUID water off.." - in this case, wouldn't condensation under the right conditions cause liquid water to come in contact with the components? – slashmais Sep 10 '17 at 07:59
  • @slashmais Yes, maybe, but ... :-). I have added a few more words to clarify what happens. The sealant must also be "void free" at the component surface. If it is then there is no place for liquid water t condense. If there are voids then liquid water will condense, as you suggest, and higher corrosion rate will occur at those locations. | Finding bonding materials that form void free contact is "the trick". EVA happens to work very well in this respect - leading to it being the main industry choice for this application for decades. – Russell McMahon Sep 11 '17 at 12:43
  • Any idea what to get in Europe? I just found 3145 90ML from Dow Corning but it costs $60 for 90ML. That's 8 times as much as "normal" sealants. Especially since this stuff goes bad once opened that's a lot of money to fix some small components. – Joris Mans Feb 19 '18 at 20:27
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    @JorisMans Where are you located? There will be a range of other related products - but, ebay has various 3145 offers. Be wary of dated product - it MAY be OK years after due date but varies with product. | [ebay Israel $US18](https://www.ebay.com/itm/Dow-Corning-RTV-3145-Silicone-Adhesive-3-oz/263497916765?hash=item3d59b3215d:g:-QUAAOSweO5ahVqO&autorefresh=true) or [17 Euro - Germany?](https://www.ebay.com/itm/Dow-Corning-3145-RTV-Clear-Silikon-Klebe-Dichtmittel-90ml-Silikonkleber/263230433752?hash=item3d49c1a9d8:g:D6kAAOSwn45Zwk~Z) note he says date unknown - care. – Russell McMahon Feb 22 '18 at 15:35
  • @JorisMans [5 for GBP40](https://www.ebay.com/itm/Dow-Corning-3145-RTV-clear-Silicone-Adhesive-Sealant-90ml-joblot-of-5-V/281976202576?epid=1630133906&hash=item41a7176550:g:V-AAAOSwHgVW8WU-) - expiry 2016 - may be OK. | ... – Russell McMahon Feb 22 '18 at 15:37
  • @JorisMans My 2012 answer (above) largely covers the range of Srs that are liable to be suitable. It's likely [tm] that any good quality NEUTRAL CURE silicon rubber will do well enough. Something sold as suitable for "wet area use" or roof & guttering or similar. But **MUST** be neutral cure. Dow Corning or Shin Etsu or most larger companies with known brands are liable to be OK. || As a guide only: Be wary of relatively heavy and relatively light product relative to competing brands. Heavier ones tend to be filled with CaCO3 or similar. ... – Russell McMahon Feb 25 '18 at 19:52
  • @JorisMans ... Light ones (which I've mainly seen in "Asian market only" offerings are filled with ???. SR when set will usually just float in water or perhaps sink slowly. Filled SR sinks more rapidly. – Russell McMahon Feb 25 '18 at 19:53
  • Russel thank you for your answer. It was very informative. However here in the UK i am really struggling to find an alcoxy based NCSR from a major manufacturer such as one of the few larger suppliers you mentioned. It would be helpful therefore to know of specific product name(s). From Dow Corning, Shinetsu or BASF. I also happened to notice something else from a different manufacturer. Which was MG Chemicals: 422B silicone modified conformal coating. So I was also wondering (perhaps) if there is some way to mix in something to lower the viscocity of NCSRs to make them like a conformal coating – Dreamcat4 Dec 05 '18 at 23:09
  • Well, shortly after posting that, found the GE SSG4000. Perhaps it might be the one I'm after. Because it's a bit stiffer (medium-modulus +25% elasticity instead of +50%). – Dreamcat4 Dec 05 '18 at 23:27
  • @Dreamcat4 [**This web serach**](https://www.google.co.nz/search?num=40&source=hp&ei=rdoLXM56k6b1A9imvpAE&q=ebay+uk+silicone+rubber+neutral+cure&btnK=Google+Search&oq=ebay+uk+silicone+rubber+neutral+cure&gs_l=psy-ab.3..33i160.2123.13310..14188...1.0..0.506.10584.0j7j10j17j2j1......0....1..gws-wiz.....6..0j35i39j0i131j0i22i30j33i22i29i30j33i21.JE1EBw37n5E) turns up numerous products that SEEM to be potentially useful. | ... – Russell McMahon Dec 08 '18 at 14:57
  • ... [Maybe?](https://www.ebay.co.uk/itm/Soudal-Silirub-2-Colour-Silicone-Sealant-High-Quality-Low-Modulus-Neutral-Cure/281611910779?var=580617127365&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160323102634%26meid%3D695b9d2f66bc4eb5a3f3ec3334af7c56%26pid%3D100623%26rk%3D1%26rkt%3D2%26mehot%3Dag%26sd%3D131194753156%26itm%3D580617127365&_trksid=p2047675.c100623.m-1) – Russell McMahon Dec 08 '18 at 14:57
  • ... [or this?](https://www.ebay.co.uk/itm/Silicone-Sealant-300ml-Low-modulus-White-Neutral-Cure-use-with-polycarbonate/132234653663?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D20160323102634%26meid%3D695b9d2f66bc4eb5a3f3ec3334af7c56%26pid%3D100623%26rk%3D2%26rkt%3D2%26sd%3D131194753156%26itm%3D132234653663&_trksid=p2047675.c100623.m-1) – Russell McMahon Dec 08 '18 at 14:58
  • ... ALLEGEDLY [**Dow Corning, neutral cure, low modulus**](https://www.ebay.co.uk/itm/Dow-Corning-C60-Sealant/123489017632?hash=item1cc084df20:g:tVAAAOSwyXVbZFBd:rk:6:pf:0) - choose any three :-). – Russell McMahon Dec 08 '18 at 15:01
  • ... and the "related items" section [**here**](https://www.ebay.co.uk/p/Soudal-Silirub-2-Neutral-Cure-Low-Modulus-Silicone-Sealant-300ml-Cartridge-Black/3003781765) has a number of allegedly LMNCSRs . – Russell McMahon Dec 08 '18 at 15:03
  • ... Also, [**Dow Corning's website may help**](https://consumer.dow.com/en-us.html) , [ANTALA](https://www.antala.uk/brands/dow-corning-silicone/) appear to be a major UK DowSil supplier, plus here is an 18 page Dow Corning [Adhesives and sealants Selection Guide](https://consumer.dow.com/en-us/document-viewer.html?ramdomVar=3033390994421520867&docPath=/content/dam/dcc/documents/en-us/catalog-selguide/11/11-39/11-3921-01-advanced-silicone-adhesives-and-sealants.pdf) in an annoying window - but it seems to be downloadable. – Russell McMahon Dec 08 '18 at 15:10
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Yes but beware that many "bathroom" grade sealants exude acetic acid when they set which will eat into copper. This link describes different types of sealant. Use an "electronics" grade sealant to avoid this problem. Alternatively you might want to look into potting compound.

Above link is broken as at September 2014 - may have been this one

Russell McMahon
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Martin
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3

Yes, you can use many materials that aren't conductive or corrosive. Silicone has a slightly higher relative permittivity than air (3.6), but I don't expect this will cause problems, except maybe slight offsets at very high frequencies.

Note that sealing will thermally isolate your product, so it might operate at a higher temperature. Be sure to allow for sufficient cooling for power devices.

stevenvh
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There are products specifically designed for waterproofing electronics called conformal coatings.

enter image description here

They come in different varieties

Comparison of Silicone vs Urethane
Comparison of Acrylic vs Urethane

spuder
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