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Arcing in Thermal Adhesive

D

D from BC

Jan 1, 1970
0
I'm gluing a part to a heat sink with a non-conductive thermal
adhesive.
After cure, testing shows no conduction between the part and the heat
sink. Good..

Problem: After power up, a short occurs between the part and the
aluminum heat sink. After power down, the part is still shorted to the
heatsink!! Bizarre..
There is no testing mistake.
(200VDC exists between part and heat sink during normal operation.)

What's going on?
Is it possible that an small arc develops in the adhesive . The arc
vaporizes some aluminum. Then when the power is cut off the aluminum
vapor cools and develops a conductive bridge (a short).
Possible?
Or maybe the arc burns the adhesive and the burnt adhesive is
conductive..

Anybody encounter something like this?
The shorting effect doesn't happen when I increase the bond line.
However, I'm trying to keep my bond line to a minimum.
D from BC
 
J

John Larkin

Jan 1, 1970
0
I'm gluing a part to a heat sink with a non-conductive thermal
adhesive.
After cure, testing shows no conduction between the part and the heat
sink. Good..

Problem: After power up, a short occurs between the part and the
aluminum heat sink. After power down, the part is still shorted to the
heatsink!! Bizarre..
There is no testing mistake.
(200VDC exists between part and heat sink during normal operation.)

What's going on?
Is it possible that an small arc develops in the adhesive . The arc
vaporizes some aluminum. Then when the power is cut off the aluminum
vapor cools and develops a conductive bridge (a short).
Possible?
Probable.

Or maybe the arc burns the adhesive and the burnt adhesive is
conductive..

Possible. What's the resistance?
Anybody encounter something like this?
The shorting effect doesn't happen when I increase the bond line.
However, I'm trying to keep my bond line to a minimum.
D from BC

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
 
D

D from BC

Jan 1, 1970
0
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
 
P

Phil Allison

Jan 1, 1970
0
"D from BC"
I'm gluing a part to a heat sink with a non-conductive thermal
adhesive.
After cure, testing shows no conduction between the part and the heat
sink. Good..

Problem: After power up, a short occurs between the part and the
aluminum heat sink. After power down, the part is still shorted to the
heatsink!! Bizarre..
There is no testing mistake.
(200VDC exists between part and heat sink during normal operation.)

What's going on?


** 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
 
E

Eeyore

Jan 1, 1970
0
D said:
I'm gluing a part to a heat sink with a non-conductive thermal
adhesive.
After cure, testing shows no conduction between the part and the heat
sink. Good..

Problem: After power up, a short occurs between the part and the
aluminum heat sink. After power down, the part is still shorted to the
heatsink!! Bizarre..

Not really.

There is no testing mistake.
(200VDC exists between part and heat sink during normal operation.)

What's going on?

Your thin layer of adhesive can't insulate 200V is what's going on ! You need a
physical mechanical re-inforcing barrier. A very thin piece of paper would
likely do it.

Graham
 
J

James Arthur

Jan 1, 1970
0
Even better, hard anodize the heat sink first.

John

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

James Arthur
 
D

D from BC

Jan 1, 1970
0
"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


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
 
D

D from BC

Jan 1, 1970
0
Sure, why not? Type II anodize isn't quite as good, but is easily
done at home.

James Arthur

Type II?? Whats that?

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

John Larkin

Jan 1, 1970
0
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?
Yup.


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?

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
 
D

D from BC

Jan 1, 1970
0
On Mon, 05 Mar 2007 17:09:24 -0800, John Larkin

[snip]
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

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
 
J

Jim Yanik

Jan 1, 1970
0
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

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.
 
J

James Arthur

Jan 1, 1970
0
What could be bad about hard anodizing? If the oxide layer is thick
,it should be ok.

It seems to work well in electrolytic capacitors...

James Arthur
 
J

James Arthur

Jan 1, 1970
0
Type II?? Whats that?

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

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:
http://www.caswellplating.com/kits/lcd_ano.pdf
http://www.focuser.com/atm/anodize/anodize99.html

HTH,
James Arthur
 
D

D from BC

Jan 1, 1970
0
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:
http://www.caswellplating.com/kits/lcd_ano.pdf
http://www.focuser.com/atm/anodize/anodize99.html

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
 
J

James Arthur

Jan 1, 1970
0
Is it the longer it's anodized the thicker the layer?

Yes, up to a limit, whereafter the coating dissolves as quickly as
it's grown.
I'm just wondering why the type III requires cooling.

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...
I'll check out the links...

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

You bet.

James Arthur
 
R

Robert Latest

Jan 1, 1970
0
D said:
I'm gluing a part to a heat sink with a non-conductive thermal
adhesive.

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
 
D

D from BC

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
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
 
E

Eeyore

Jan 1, 1970
0
John said:
Thermal fillers, like the stuff in silicone grease, tends to be very
small particles, below 100 microinches, so don't provide much
insulation spacing.

I like the idea of little (glass?) beads that'll maintain a specific separation.
I only came across the idea for glueing core halves together for a reliable gap.

Graham
 
D

D from BC

Jan 1, 1970
0
I like the idea of little (glass?) beads that'll maintain a specific separation.
I only came across the idea for glueing core halves together for a reliable gap.

Graham

There's all types of glass. Finding one that conducts the most heat
would be a interesting.
D from BC
 
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