Arcing in Thermal Adhesive

Probable.

Possible. What's the resistance?

What's a bond line?

The glued parts have to be flat, with no burrs or such. Spacers can define the epoxy gap; small mylar discs (from a paper punch), a bit of monofilament, or a tiny amount of sand or Cataphote bead filler mixed into the epoxy.

Even better, hard anodize the heat sink first.

John

The short is below 1ohm. I just can't imagine arc fried non-conducting adhesive getting that conductive.

Bond line: thickness of the adhesive

I'm just temporarily stuck with thermal adhesive to do prototype testing.. A hard anodized Al heat sink is going to be used when available. D from BC

"D from BC"

** The metal part is lightly touching the aluminium heatsink at some point.

No conduction occurs at low voltage due the oxide layer always formed on surface of aluminium.

All sane folk use mica or other insulators for voltages like 200 volts.

Even "hard anodising" will not be reliable at 200 volts.

....... Phil

Sure, why not? Type II anodize isn't quite as good, but is easily done at home.

James Arthur

I can understand the Al oxide layer breaks down and an arc occurs at the closest point..

But after power down, a new short still persists. I don't know the physics of very small spark gaps.. Does the spark weld the part to the Al heat sink? Or maybe there's some sort of metal vapor deposit on the thermal adhesive after the power is removed.

What could be bad about hard anodizing? If the oxide layer is thick ,it should be ok. Long ago I think I saw a microscopic picture of the cross section of anodized aluminum.. The cells had random anodization depths.. Is that the problem? D from BC

Type II?? Whats that?

I was thinking of anodizing in sulfuric acid (battery acid concentration). D from BC

Yup.

1.5 mil hard anodize is reliable at 200 volts with just silicone grease, provided the surfaces are smooth/flat and it's a serious hard anodize. With epoxy, it should be even better.

Test hard anodize by poking it with a pair of sharp ohmmeter probes. You should read infinity until you start seriously pushing and digging into the surface.

John

[snip]

I was thinking of making a simple capacitance meter cct to test for a target anodization thickness. I haven't figured a test frequency yet... 1khz, 10khz, 100khz..?? That'll be a new post if I get stuck. D from BC

D from BC wrote in news: snipped-for-privacy@4ax.com:

maybe you're getting an arc thru a small VOID in the adhesive? Then it "carbonizes" the surrounding adhesive,making it conductive,or the arc vaporizes aluminum and plates it's own short.

--
Jim Yanik
jyanik
at
kua.net

It seems to work well in electrolytic capacitors...

James Arthur

The three most popular aluminum anodizing processes produce coatings called Type I,II, and III. The difference is in process variables, particularly the process temperature and current density applied to the work. Type III requires cooling, is the densest and hardest, and is what most people mean by "hard anodize." Type II can be done at room temperatures, is pretty decent, and is called "hard anodizing" by some.

Battery acid diluted 2-or-3:1, lowish currents, and room temps will get you a low-hassle Type II coating.

I've done it using currents much lower than usually recommended, e.g.

20-50mA/in^2 from a wall-wart, and was naive enough to be pleased with the results. ;)

You might want to scan these excellent guides:

HTH, James Arthur

Is it the longer it's anodized the thicker the layer? I'm just wondering why the type III requires cooling. I'll check out the links... Thanks D from BC

Yes, up to a limit, whereafter the coating dissolves as quickly as it's grown.

The coating is porous, a hexagonal, honeycomb-like structure. Type III anodize uses high current densities, which decreases the size of the pores but also makes a lot of heat. Heat, in turn, dissolves the coating faster...

The Caswell plating guide has cool pictures in the front, and a technical summary at the end.

You bet.

James Arthur

Is this epoxy filled with anything? I've had good experience with Epotek's Al2O3-filled H77. Just glued two flat metal surfaces together, good to

1000V (tested).

robert

I'm using an acylic polymer with a 1.25 W/(mK) thermal conductivity. The filler material is not listed on the data sheet. It's grey. D from BC

Thermal fillers, like the stuff in silicone grease, tends to be very small particles, below 100 microinches, so don't provide much insulation spacing.

John

When I was epoxying LEDs, I set up a multimeter to measure ohms as I was adjusting spacing to make sure there was no low ohm shorts. I don't know how the Lumileds are spaced when they are assembled to make stars, but it seems epoxy is the only spacer and insulator. greg