Is heat sink grease needed??

[zupta82]

Hello everyone, I was wondering if heat sink grease is needed on the aluminum side it does not look like the white oily compound, I was thinking maybe aluminum oxidation. There is a paper insulator between the transistors and heat sink. Here are some pics. I’m just going to shift some gears and take a break from the scope I’m trying to fix. It took 3 weeks for my power transistors to come in the mail
Thank You

[zupta82]

Sorry I forgot to upload a pic of the aluminum heat sink side

[T3sl4co1l]

Yes.  Looks crusty and dry?  Wipe it off (the plastic as well) and re-grease.

Tim

[wraper]

It is not supposed to be used. Likely it's a silicone oil leaked from thermal pad. If thermal pad became non elastic, the proper way would be to replace it. Or you could add thermal paste to salvage the thing.

[T3sl4co1l]

That looks to be polyimide sheet, which is used just like mica -- with grease.

If it is silicone or other rubber, it won't need grease.

Tim

[TheMG]

Just so you know, old thermal grease can dry up and turn into an almost chalk-like powdery substance over time.

Note that in operation that doesn't really affect its effectiveness but once disturbed, should be cleaned off and fresh thermal paste applied.

[zupta82]

I was wondering do I need to grease the transistor side as well, sorry for the double post I didn’t realize I was eluding to my previous project I started

[T3sl4co1l]

Yes, both sides.

[zupta82]

Thank you.

[zupta82]

I forgot to mention, when I reassembled the board I spread a thin layer of grease over the entire rectangular heat sink surface and then stuck the paper down. I then finally applied a thin layer over just the transistors before I anchored them down. I found this to be easier because I wiped clean the old grease impression shadows and not new every transistor is sitting in straight

[zupta82]

Here are some pics of the transistors reassembled

[zupta82]

Here is the transistor side

[zupta82]

I was afraid that I might short the adjacent transistors or interfere with the proper heat transfer. Isn’t heat sink grease slightly conductive

[SeanB]

Depends on the composition, you get metal loaded ones, which are either silver or copper, but the majority are white zinc oxide powder finely ground with a silicone oil grease as the binder and transfer agent. The silicone does evaporate out with time and heat, so does need periodic refreshing. The metal loaded are somewhat conductive, but the white is generally not, but still is in need of the insulator to provide the actual insulation, as it can break down with a thin film and voltage applied to the thin film.

I have made my own before, using zinc oxide powder and some pink Dow corning silicone grease, because I was using 5 or 6 tubes per job changing UPS transistors. For a few recent ones I refreshed the existing compound using a tiny amount of Beyer silicone oil, as I have a bottle I use to apply to phone screen protectors, to get rid of edge bubbles. 100ml is a lifetime supply of that, and as it is so thick it acts well for thermal transfer. For a CPU interface with a lid there is no difference between the cheap common white, and any of the fancy ones, the biggest loss is the internal transfer compound in the CPU.

[zupta82]

It s nearly impossible to align the transistors to individual pre-applied sections of grease on the aluminum, this is why I smeared a thin coat over the entire aluminum. The board is wedged in a notch in the heat sink and has to be flexed somewhat to align the stand-off bolts. Here is a pic of it fully assembled with the retaining finger-like device holding the transistors down.