Dielectric Grease vs Conductive Grease
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Use of greases in connection protection seems to be a controversial topic.
Some complaints claim dielectric grease is conductive or abrasive, containing silica that increases wear. (It doesn't contain silica).
The most frequent Internet complaint is that dielectric grease insulates connections, making connections less conductive. The general basis for this claim is in the word "dielectric" used in the name. The word dielectric is assumed to mean the connection will have future problems because "dielectrics" are insulators. Generally, authors predict greases with powdered metal (in slang "conductive greases") will improve or maintain connection quality over time while dielectric greases will isolate connections because "that is what dielectrics do".
History
My first experience with silicone grease was in the 1960's as a lubricant in record turntables. It was also commonly used as a lubricant and protectant on turret-type television tuners, where channel coil packs or "strips" were mounted in a rotary turret. Rotating the turret moved different channel strips over stationary contacts to select each channel, and the clear silicon grease lubed the contacts and kept air off the plated surfaces. Later, RCA, Motorola, and Magnavox, facing field failures from bad electrical contact connections, recommended pure silicon grease as a contact protectant for modules in modular TV sets. The grease was used to prevent loss of connection between circuit modules, pins, and sockets. Hundreds of thousands of TV sets with hundreds of connections in each one were living with silicone grease on modules with signal and high voltages, and using silicone grease for lubrication on frequently-switched gold or silver plated contacts, while a few people seemed to have problems right off the bat with much lower voltage systems. Silicone grease was also used in high voltage CRT anode connectors to prevent or reduce corona.
My second experience was in the CATV industry. As a systems engineer, I was drawn into radiation and ingress problems with CATV/MATV systems. The problems centered around dry connections that corroded, and around aluminum trunk cable shield connections protected by Noalox, a grease people often call "conductive". All of these problems were eliminated by "non-conductive" grease. The initial grease and sealer I brought into the systems was a white Teflon-silicone grease from a company in Elyria, Ohio. While that grease solved problems, it was expensive to apply to tens of thousands of fittings. It also was unsightly, service personal would leave white fingerprints everywhere. After consulting with several grease manufacturers, I switched to a GE 100% pure silicone dielectric grease in connections. We used that grease without incident for many years, completely flooding F connectors that were directly exposed to snow or rain.
I continue to use silicone dielectric grease today. I use it as a lubricant on coaxial connector O-rings and threads. I use it to lube stainless bolts and nuts, to prevent galling. I use it for plug-in connections, in particular in my automotive hobby. I also use silicone dielectric grease for battery terminal connection preservation, coating it directly on the battery post. I use it in liberal amounts on ground connections, to inhibit corrosion on stainless-to-zinc (galvanized), lead-to-lead, stainless-to-copper, and stainless-to-aluminum electrical connections.
I have never found a problem with silicone dielectric compound increasing resistance or increasing wear. We use it in new equipment production to lubricate and preserve contact plating in very low current meter switches. It has never caused shorts across insulation, I use it on spark plug HV boots on race engines and in high voltage connectors.
Silicone vs. Petroleum Grease
Petroleum grease (Vaseline) was recommended (and was apparently used) on low power antenna installations years ago. While people report using it without problems, I'll never use it in my installations. The primary shortfall of Vaseline is the very low melting point. Most brands or types liquefy at around 100 degrees F, just above human body temperature. While this may be a medical benefit when coating human skin, it is a serious problem with connector applications, unless we do not care about grease running where we do not want or we do not care about drying over time.
The second petroleum jelly issue is Vaseline's release of flammable vapor, even at low temperatures. A cotton ball soaked in Vaseline will burn a very long time, and actually makes a good fire starter. Since connectors are often near insulation or other things that can act like wicks, petroleum jelly is not the best thing. This is especially true when the grease migrates in warm temperatures.
Typical Applications for Greases
Heat sinks
Car forums repeat a few myths about greases. Forums usually claim dielectric grease thermally insulates connections. Forums also claim dielectric grease electrically insulates connections, such as in connectors and on battery posts. Neither is true.
Here is a test using a 35-watt dissipation resistor. The resistor is mounted to the heatsink with a stack of Belleville washers. These washers are "spring washers". They are used to maintain a constant pressure when partially collapsed. This ensures compression or pressure against the heatsink is essentially the same between tests, eliminating cap screw torque as a factor in results.
diectric grease test bare rough heatsink base temperature
Here is measured data of a few sample greases with 30 watts heat dissipation:
Type Sink F Device F Delta degrees Percent (aprox)
Bare scratched 62.0 63.9 1.9 3.1%
Heat sink compound thick 55.9 57.6 1.7 3.0%
Bare polished 61.6 63.0 1.4 2.3%
Vaseline 62.5 63.1 0.6 1.0%
Dielectric grease 62.3 62.8 0.5 0.8%
Heat sink compound thin 56.2 56.6 0.4 0.7%
Best result is at bottom. All greases were tested under "scratched" conditions by roughing the heatsink with ~300 grit paper.
Using too much dense grease, like thick heatsink compound, greatly increased thermal resistance. This occurred because compression pressure was not enough to force excess grease out of the area between the heatsink and the resistor tab. When the layer was thinned to a light "wipe" of grease, thermal resistance fell off significantly.
There is essentially no difference between Permatex Dielectric Tune Up Grease and a special heatsink compound used on high-power transistors. Even Vaseline, at 1%, is better than bare metal-on-metal.
dielectric compound base temperature
dielectric compound on heatsink measurement
Connectors
In radio frequency low power installations, in particular at low frequencies and/or when the connector has very little air gap, completely flooding the connector is perfectly acceptable. Flooding a connector is not acceptable at high power, because most greases will carburize when subjected to an arc. They will also change the dielectric constant, lowering the dielectric constant in the connector. This will create an impedance bump at very high frequencies, the problem's effect on the system being entirely dependent on the length of the bump in electrical degrees and the amount of the bump. (Not all things that show on a TDR are meaningful.)
In regular low voltage multiple-pin circuit connectors, such as automotive applications, flooding with a proper insulating grease of low-viscosity dielectric grease is perfectly acceptable unless a manufacturer recommends against it. The grease should have good stability and not contain metals in any form, and be specifically designed for use as a dielectric grease. This generally is a silicone dielectric grease, although some Teflon based greases are acceptable.
In single low-voltage terminals or connections, such as metal-to-metal joints, grounds, or battery posts, almost any pure grease of light viscosity will be acceptable. Caution should be used with greases containing metallic powders to be sure any metal is compatible with the embedded grease metal. Connection enhancement from embedded metal powder is very minor, if it exists at all, and there is increased risk of bad connections if the metal powder has any interaction with the base metals.
In single high voltage connections, such as spark plug boots or other high voltage connectors (x-ray, neon sign, or HV power lines), only pure dielectric silicone greases should be used. Generally a light coating or wipe is all that is required. Dielectric grease will actually increase voltage breakdown across insulators, especially in the presence of moisture. Never use or allow a metalized grease around HV connections.
The important physical characteristic is that any grease must have low enough viscosity to push out of the way at contact points, be water or liquid resistant, and be stable enough to remain in place as a protectant against moisture and air for a long time. It will not do any good to apply a grease that does not do required functions of excluding air and moisture, and lubricating the interface to prevent galling or fretting, for extended periods of time.
Contrary to silly Internet rumors and articles, silicone dielectric grease will NOT insulate pressure connections. Silicone dielectric grease will prolong connection life as well as, and have just as good conduction performance, as a properly selected metallic powder grease (conductive grease). On the other hand, and improperly selected "conductive" grease can actually cause connection problems.
Switches, Movable Contacts, and Relays
Unless you are absolutely sure what you are doing is OK, do only what a switch or relay manufacturer suggests. There are some cases where very high current contacts can be lubricated, or should be lubricated, to prolong or extend life. There are many cases where lubricating contacts accelerates failure.
As a general rule, low-viscosity greases can be directly applied to low-voltage contacts. Low-voltage generally would include consideration of opening or closing transient voltages, such as opening arcs from inductance back-pulse.
Contact arcs have the ability to alter composition of greases. Silicone greases can be converted by arcs to silicone carbide, which is highly abrasive. For this reason, silicone grease should be avoided when contacts are "hot switched" and have any chance of arcing.
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