Pros and Cons of Types of "Dielectric Grease"

Question

I have come across two different types of dielectric grease. One, the "usual" kind, is a silicone grease that has added insulators. Another, less common, is a hydrocarbon grease, typically polyalphaolefin (PAO) grease with silica additives.

I noticed that PAO Grease is not typically advertised as "dielectric grease" but we have a tube in the shop that the electricians have been using as such. I have also found some distributors who have listed it this way, but the manufacturer does not call it "dielectric grease" or even state the dielectric / conductive properties online (for reference, the grease in our shop is Nyogel 760G). Among it's listed uses is for lubricating (for connector insertion / removal force) and sealing electrical contacts. I have seen that PAO Coolant is listed as "dielectric" in military applications but I don't know if that's relevant to the grease form. I'm having a hard time finding information for it when used as a grease.

Is this grease a better or worse insulator than Silicone? What are it's pros and cons?

For my particular application, we have a connector seal that needs some kind of lubrication to prevent damage when the connection is made. If the connection is made well, the unit is sealed from the environment and we don't have problems with it. Sometimes the seal is damaged when it is connected, however, and then we have issues with water intrusion.

Additionally, the unit is not sealed without the connector in place. On the other side of the connector is an exposed PCB and I need to make sure nothing gets shorted out on the board. My concern is if we put the grease on the seal to make the connection better (some technicians have even suggested packing hole entirely with the grease), that some grease could ooze onto the PCB. If it's not a good dielectric, we could have shorts which would negate the reason for using it.

It's a poor design, but I'm stuck with it unfortunately. We have thousands of these units. I'm trying to find a good solution.

Answer 1

Sounds like those big old Canon circular Mil connectors.

Nyogel 760G looks like a good solution. It's a synthetic organic oil or "non-oil" with carbon-pairs between two hydrogen molecules. It is slippery, low viscosity and clean (when no dust is present) with low evaporation rates, good antioxidation qualities and good water rejection qualities, yet thin enough to make easy electrical contact from surface roughness. However, silicone is added just as a thickener and being an insulator both electrical and thermal would be a poor choice (unless lots of contact force, roughness, i.e. poor quality not ARINC quality). These oils generally have low k factors, less than or equal to epoxy in FR4 and thus better insulators than air if clean, but any liquid is worse than air with leakage resistance from conductive dust easily mobile and chargeable (e.g. salt spray would be worse after drying).

Water on the other hand is 20-30 times worse with a k factor of 80 for capacitance, but like oil better for insulation unless contaminated (than air.).

Any moisture and dust can result in a leaky stray capacitance, but oil is better than water and dry air with dust is better than anything wet with dust, and wet with salt is worse.

Pressing your finger on the PCB can tell you if a circuit is too sensitive and lacks conformal coating or solder mask or guard tracks. Interface connectors have no business being high impedance. So source at either end must be low impedance and if floating at least low common mode impedance capacitance to ground.

If there are no critical stray capacitance circuits using <50pF capacitors (light dry fingertip), then a silicone spray conformal coat is a good insulator and thin enough to reduce pF/cm^2 then the Nyogel for external contact lubricant or even WD40 thin spray or apply with a soaked plug inverted.