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I've used silicone RTV that's intended for use on PCBs and have also used 100% silicone purchased from the hardware store to use on PCBs. I was not able to tell a difference between the two. Everything looked and smelled exactly the same. I know there could still be differences, but I'm wondering if they're just selling it specifically for electronics so they can charge a premium. The cost of RTV for electronics is around $30 while the stuff at the hardware store is $6.

Obviously not all silicone/caulk from the hardware store is the same as silicone RTV. The question is if some of it is and if people are wasting money on silicone RTV.

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    \$\begingroup\$ You get no electrical specification for the cheap silicone. \$\endgroup\$
    – Uwe
    Apr 22 at 18:29
  • \$\begingroup\$ @Uwe That is a very valid point for some applications. Most times though, how much do people really care or even bother to look. Other applications where it's used to secure an inductor for example, is almost never going to matter what the electrical specification is. \$\endgroup\$ Apr 22 at 18:45
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    \$\begingroup\$ It all comes down to who is going to be mad if there's a failure and the RTV is a culprit. If you can point to a datasheet and say that the material you chose is suitable for the requirements as you understood them, you're in a much better position than if all you have is a $6 tube of generic gasket maker from the hardware store and a shrug. \$\endgroup\$
    – vir
    Apr 22 at 18:56
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    \$\begingroup\$ @vir I'm going to have to agree with you on that. At the end of the day, it may not matter most of the time. You just never know when it will matter, and when it finally does matter, the cost for will quickly exceed the couple bucks you saved buying silicone from the hardware store. \$\endgroup\$ Apr 22 at 19:12
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    \$\begingroup\$ You want "neutral cure" RTV. If you just need a small amount, there's somebody selling single-use tubes for $5 on Amazon. Look for "Chip Quik NCS10G-20G" It's a small tube for sure, but enough for a single-use PCB application. Problem with silicone RTV is once you open it, it goes bad really quick so you definitely don't want to buy a huge amount. \$\endgroup\$
    – Kyle B
    Apr 23 at 0:32

4 Answers 4

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Cheap silicone is acidic (like vinegar) while GE's Room Temperature Vulcanizing (RTV) is not, and is suitable for electronic anti-corrosive insulation on metals. The main features are thermal insulation and high voltage insulation in addition to the expected dust & water HV protection.

However, the application is only as good as the cleanliness of the contact surfaces, to avoid creepage effects.

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    \$\begingroup\$ I read that RTV was non corrosive, which is why I decided to buy it for a project. It made sense at the time because the silicone from the hardware store had a vinegar smell. It was when I opened the silicone RTV and smelled the same vinegar smell that I began questioning it. \$\endgroup\$ Apr 22 at 18:37
  • \$\begingroup\$ Check the datasheet, my guess is clones of the GE product from 40 ys ago have changed en.m.wikipedia.org/wiki/RTV_silicone \$\endgroup\$ Apr 22 at 18:43
  • \$\begingroup\$ asaptec.com/wp-content/uploads/2016/04/RTV120.pdf \$\endgroup\$ Apr 22 at 18:47
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    \$\begingroup\$ To the OP: My understanding, which could be wrong, is that if it smells like vinegar it is not suitable for electronics. I think at the very least you should contact the manufacturer and ask them whether the product has a vinegar smell and whether it is suitable for electronics. Maybe it was mispackaged or there is some kind of mis-understanding. You could also avoid a LOT of confusion and speculation by adding links to the actual products you are talking about. These links should be in the original question (edit the original question). \$\endgroup\$
    – mkeith
    Apr 22 at 18:56
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    \$\begingroup\$ @mkeith You were right. I was looking at other silicone RTVs from Digikey. Then when I looked at my purchase history, I found the one I actually purchased. I specifically says do not use for electronics applications. So it was purely just a my fault for assuming that buying RTV from Digikey meant that it was okay for electronics. This also goes along with a comment I made in the question where I said "Most times though, how much do people really care or even bother to look." This is a perfect example of why you need to look. \$\endgroup\$ Apr 22 at 19:03
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For electronics work you want "platinum cure", "addition-cure" or electronic grade silicone. "RTV" means "room temperature vulcanizing" and is a generic term that includes various types of curing. So is "silicone", it can refer to materials from oils and greases to relatively hard silicone rubber. Some silicone rubber is vulcanized at raised temperature. As part of summer work when I was in high school we fabricated silicone nuclear power plant seals using a large heated mold and a horse trough filled with water to cool it. The raw milled rubber came as kind of a cord.

The typical big-box store construction grade silicone caulking has acetic acid cure, and acetic acid is both corrosive and quite electrically conductive. You can smell the vinegary odour quite easily.

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  • \$\begingroup\$ When you say conductive, do you mean conductive before or after it's cured? \$\endgroup\$ Apr 22 at 18:47
  • \$\begingroup\$ There can be a residue of acid on the surface of the cured rubber which is conductive. It probably will get conductive any time moisture appears even if it can somehow dry out (difficult if it's a potted assembly). \$\endgroup\$ Apr 22 at 19:33
  • \$\begingroup\$ Is there a reason consumer-grade neutral (alkoxy or in particular oxime) cure silicone chemistries aren't acceptable for electronics use? \$\endgroup\$ Apr 24 at 16:17
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GE Silicone from hardware stores is always labeled either Silicone I or Silicone II. You'll find the type in very small print, often in an out-of-the-way location on the label. Silicone I releases acetic acid as it cures. It is corrosive to copper and is not ideal for electronics. Silicone II releases methanol as it cures. It does not corrode copper, it is non-conductive and it works fine for electronics, though without any electrical specification.

All GE silicone in hardware stores is RTV, i.e., Room Temperature Vulcanizing.

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The difference is "acid cure" vs "neutral cure"

RTV silicones use a few different, but related, cure chemistries. The most common one is what's called an "acetoxy" or "acid" cure, which releases acetic acid as it cures. This explains both the vinegar odor, and why you want to avoid this chemistry for electronics applications, where said acetic acid will corrode copper parts.

The second one is what's called an "alkoxy cure". These release methanol, ethanol, or isopropanol as they cure, and are a type of "neutral cure" silicone. This means they don't have the corrosion risks associated with acid cure silicone products, and are suitable for electronics applications. However, they have some cost and performance disadvantages in applications where acetic acid corrosion is a non-issue, so they are relatively uncommon formulations nowadays.

As a result, most "neutral cure" RTV silicones available on the consumer market these days use a different cure chemistry, a so-called "oxime" cure that releases a funky chemical called methyl ethyl ketoxime during the cure process. This is not an acid, and thus doesn't pose quite the same risks of corrosion that an acid cure product poses, but may not be acceptable for critical applications because any potential influence MEKO might have on copper corrosion is not as well known.

TL;DR: for non-critical applications, any consumer neutral cure RTV product is acceptable -- just avoid products that don't say "Neutral Cure" on the label, and you'll be fine.

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