Electronic – Electrical insulation resistance of TO-220 package with SIL pad and plastic shoulder washer

From a unit-by-unit standpoint, if it passes hipot today, it should pass hipot tomorrow. As long as each unit passes hipot, you shouldn't have a safety or reliability concern.

From a product line standpoint, though, you can't be guaranteed that the fan will always be built the same way. It's possible different production runs will be built differently. Maybe a different plastic will be used, or a different geometry, or who knows what. If it's not a spec listed on the data sheet, you can't assume it will never change from batch to batch.

Since asking the question, I've found out about a case where exactly this happened to us. Many batches of fans passed hipot to our 3000VAC level, well past their spec, and we shipped units with no problems. Then a new batch came in, and every one failed 3000VAC hipot. We had products due out the door, and we had to scramble to insulate or isolate our fans.

For individual units, you can be fine using unstated specs that you test on a unit-by-unit basis. But as a matter of best practice for a product line, one should not rely on unstated part specifications.

The thermal conductivity of a dry vs. thermal grease is shown here for TO-264 package (from ON Semi application note AN-4166)

Looks like roughly 0.7 to 1.1 °C/W depending on the torque for dry mount.

Your particular thermal pads have no specifications for performance, so you really can't be sure what they are going to do. Maybe they are counterfeit made of some inappropriate material. The thickness isn't even specified! Buying such a product is a total crap-shoot, and not recommended. You can buy thin electrically conductive thermal pads with real datasheets for $18/1000 through distribution- they have excellent performance.

But anyway, if they were as good as the silicone pads used in the same application note, the performance would be as follows:

As you can see, the performance with the pads will be as bad as several times worse than dry mount, even with high quality silicone pads. If they are cheap cr*p and leave out the boron nitride (BN) filler from the silicone rubber to save a few pennies (likely, IMHO) they could be much, much worse.

Note: I deliberately did not use the data directly from the manufacturers of thermal pads for two reasons- the link is to unknown pads of unknown characteristics, and secondly the manufacturers have reason to specify their products in such a way as to make them look better than they may be in practice. The graphs are from actual measurements with semiconductors mounted to heat sinks, not bulk material measurements, and use the same methodology for each mounting type so A-B comparisons are more likely to be valid.

TL;DR: Don't use the pads, or get different ones that are thermally conductive and from a quality source.