Aluminum oxide insulator safety precautions?

I totally agree.

I've gotten the Aavid Ultrastick phase-change goop down to what I thought was about 1-2mils after clamping. It flowed like water when hot but never got any thinner than that. And that's with belleville washers, etc.

The grease I'm using now starts at that thickness (if I put it on with care) and then just gets thinner with pressure. Their thermal conductivity specs are pretty close so I'm going with the grease.

I'm using the Wakefield 126 Series stuff now but Dow TC-5022 looks great. Several times the conductivity of the 340 grease. I have a sample on order and will see if there's any difference. At these thicknesses though, I can probably use mayonnaise and still be OK. :-)

John

Reply to
John
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I just got back my first 1mil hardcoat anodized heat sinks and was pretty surprised to see that the black I ordered was only a light gray. Oh well.

But, that's one tough coating!

I checked it with a 10x loupe and the hardcoat on the main mounting surface of the heat sink seems "cracked". Kind of like the surface of a really old oil painting. Lots and lots of tiny cracks that form the hardcoat into tiny crystals/plates. It doesn't flake though and I don't see this effect on any of the vertical pins coming off the base plate of the sink.

Do you know if this effect is typical or a possible problem?

John

Reply to
John

Some alloys anodize a nicer black than others.

Get a dvm and a pair of sharp probes. Try to measure ohms as you poke the heatsink. If it's good anodize, you will have to apply real muscle, grinding the probes into the coating, before you see continuity.

It does usually have character, partly because of the surface finish before anodizing. Sometines we specify "heavy etch, 1.0 mil hard black anodize". The etch smooths the surface and removes any tiny burrs.

But if your coating passes the probe test, it's probably fine.

John

Reply to
John Larkin

Yeah, any sort of flowable grease will probably be about the same. I suspect that the thermal filler materials don't help much; what's important is that it's not air and that it squashes down very thin.

Vaseline maybe? Axle grease? Grey Poupon?

John

Reply to
John Larkin

Nah, Grey Poupon has those little ground-up seeds.

But what about RTV? It's silicone, and you wouldn't even need bolts! ;-)

I've found that Vaseline makes dandy vacuum grease, and it's almost impossible to clean it off any surface it's been applied to, especially where you _don't_ want it. ;-)

Cheers! Rich

Reply to
Rich Grise

Epoxy is good. If you load it with glass beads, you don't even need an insulator.

John

Reply to
John Larkin

[snip]

OK, I'm feeling better about the coating. With a 10x loupe, the hardcoating looks very much like a windshield after being shattered...lots of tiny "platelets" sitting on the surface of the metal, all adjoining each other. But, I grabbed a pair of pretty sharp probes and went to town. I had to very visibly dig/scratch into the coating to get any reading.

Would the heavy etch possibly have any impact on the case-to-sink thermal impedance (by increasing surface roughness)?

We have several 6-32 holes for mounting the MOSFETs. I'm thinking it can't etch away enough metal to affect the tightness of those, could it? I guess it depends on how big a burr the etching could remove.

John

Reply to
John

LOL, brilliant. The bead size could even be spec'd to get the desired dielectric strength.

John

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John

Check out Cataphote.

John

Reply to
John Larkin

I can't believe I can buy 2nm size nanospheres! :-) Cataphote doesn't seem to have a web site but I did find other companies. Just an amazing selection of sizes and materials!

John

Reply to
John

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They make the bulk glass beads used for retroflectors. Here in California, at every stop sign they also lay down a STOP stencil in the street, spray on white paint, the toss on handsfull of Cataphote beads to make the lettering reflective. Some blow away and leave little sparkles in the neighborhood here and there, until the rain washes them away.

John

Reply to
John Larkin

If you want it flatter, get some diamond grit and lap it. ;-)

Otherwise, I wouldn't worry if you've got good heatsink goo.

Cheers! Rich

Reply to
Rich Grise

The etch does improve the burr situation. Even better is to clamp-mount the transistors.

The hard anodize does tend to gunk up threads tapped into the heatsink. If the hole is a bit countersunk before it's tapped, you're less likely to have burrs, and you can chase the threads with a tap if necessary, although that's real hard on taps. I think you can get oversize taps, which allow for the buildup from anodizing and don't need chasing.

We've done thousands of power amps with transistors bolted or clamped to anodized heatsinks, up to 200 volts, and only had a few problems, mostly from burrs or post-anodize dings as I recall.

John

Reply to
John Larkin

Thats' the site I tried. It's only an entrance to someone's search portal.

John

Reply to
John

The Wakefield stuff we have seems to be working out very well. We also ordered a sample of the Dow TC5022 (I think) goop and they're sending us a 1kg can!

I'm going to have to go out and give some of this stuff away. We'll never use that much in 100 years. :-)

John

Reply to
John

You read our minds. If we make any more of these things, we're going to switch over to clamping.

Oversize taps...that sounds familiar. So far, with the couple of sinks we've tested, it only took using a machine screw to chase the threads.

Currently, the holes are countersunk before tapping. So far, so good. I hate having to grab the loupe and check each sink though. I guess if we don't find any problems after checking a bunch, we'll just assume all is OK until one of the sinks tests hot. Hmm...actually, it'll take less time to 100% check the batch rather than swap out a heat sink that has a burr. :-)

John

Reply to
John

Hmmm, yes, it is strange. They *used* to have a real web site.

John

Reply to
John Larkin

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