Aluminum oxide insulator safety precautions?

[John]

I'll be using TO-247 aluminum oxide insulators between some MOSFETs
and their heat sinks and wanted to get a safety-related reality check.

I've checked the MSDS for aluminum oxide and it seems that as long as
I avoid breathing any dust and wash thoroughly after touching the
insulators, I'm OK.

But, I want to do some measurements of the MOSFET's case temperature
and need to grind a hole in the insulator. I have the Dremel and the
grinding bits to do it, but before I started I wanted to make sure I
did this carefully enough.

Will simply doing this outside in a decent breeze, or thoroughly
venting the dust if inside, be enough? Assuming I have safety glasses
on, etc.

One other question...
Has anyone actually experienced skin irritation to aluminum oxide
insulators? I can how a powder could do that but was wondering just
how nasty these insulators were to handle.

Thanks!

 

[John Larkin]

I'll be using TO-247 aluminum oxide insulators between some MOSFETs
and their heat sinks and wanted to get a safety-related reality check.

I've checked the MSDS for aluminum oxide and it seems that as long as
I avoid breathing any dust and wash thoroughly after touching the
insulators, I'm OK.

But, I want to do some measurements of the MOSFET's case temperature
and need to grind a hole in the insulator. I have the Dremel and the
grinding bits to do it, but before I started I wanted to make sure I
did this carefully enough.

Will simply doing this outside in a decent breeze, or thoroughly
venting the dust if inside, be enough? Assuming I have safety glasses
on, etc.

One other question...
Has anyone actually experienced skin irritation to aluminum oxide
insulators? I can how a powder could do that but was wondering just
how nasty these insulators were to handle.

Thanks!

AlO2 is pretty benign. It's not toxic, so is as dangerous as any other
dust... sawdust, glass, sand.

It's not a very good heat conductor. AlN is much better. BeO is even
better, but BeO is seriously toxic.

John

 

[John]

AlO2 is pretty benign. It's not toxic, so is as dangerous as any other
Thanks John.

I was considering AlN until I saw that they were about $8 each
(TO-247, 100pc lots)! I was hoping that AlO2 was good enough to be
worth the 75-cents they cost.

At $8 each for the AlN, hard-anodizing the heat sink becomes the
better solution. And you can't beat the thermal impedance of
anodizing.

You wouldn't happen to have a source for AlN TO-264 insulators at a
decent price, would you? TO-247 insulators can work, but the MOSFETs
are TO-264.

John

 

[Rich Grise]

I was considering AlN until I saw that they were about $8 each
(TO-247, 100pc lots)! I was hoping that AlO2 was good enough to be
worth the 75-cents they cost.

At $8 each for the AlN, hard-anodizing the heat sink becomes the
better solution. And you can't beat the thermal impedance of
anodizing.

You wouldn't happen to have a source for AlN TO-264 insulators at a
decent price, would you? TO-247 insulators can work, but the MOSFETs
are TO-264.

If you have the option to hard anodize, then that's the best bet, as long
as it can take whatever voltage you're applying. Use good heatsink goo,
and you should be fine.

Failing that, what's wrong with mica?

Good Luck!
Rich

 

[John]

If you have the option to hard anodize, then that's the best bet, as long
Max voltage is 55VDC and we're using 1 mil Type III anodizing (0.5mil
penetration, 0.5mil surface).

Unfortunately, at the low volumes I'm working with (heat sinks) it's
pretty expensive to hard anodize. I did six for $30 each, with six
MOSFETs on each sink. If I can find a decent solution for a lot less
than $5 a MOSFET (with low thermal impedance as a priority), I'll take
it!

I thought mica had a pretty high thermal impedance? I did a quick
Google check and found a thermal conductivity number of 0.71W/mK.

Hmmm...using the 15.06W/mK value for the AlO2 insulators I purchased,
that makes the AlO2 a bit over 21 times more conductive. So, if I can
find a mica insulator at least 21 times thinner than the 80mil AlO2
insulators I have, that means that the mica would be a better choice?
That means the mica must be about 3.5mils or thinner (and still
insulate to 55V at that thickness).

John

 

[John Larkin]

Max voltage is 55VDC and we're using 1 mil Type III anodizing (0.5mil
penetration, 0.5mil surface).

Unfortunately, at the low volumes I'm working with (heat sinks) it's
pretty expensive to hard anodize. I did six for $30 each, with six
MOSFETs on each sink. If I can find a decent solution for a lot less
than $5 a MOSFET (with low thermal impedance as a priority), I'll take
it!

I thought mica had a pretty high thermal impedance? I did a quick
Google check and found a thermal conductivity number of 0.71W/mK.

Hmmm...using the 15.06W/mK value for the AlO2 insulators I purchased,
that makes the AlO2 a bit over 21 times more conductive. So, if I can
find a mica insulator at least 21 times thinner than the 80mil AlO2
insulators I have, that means that the mica would be a better choice?
That means the mica must be about 3.5mils or thinner (and still
insulate to 55V at that thickness).

The advantage of mica is that it's usually very thin. 3 mils is
common, and 80/3 is 27, so the mica wins over the 80 mil alumina. 1
mil hard anodize would of course be a lot better than either, and
would easily stand the voltage.

I did get a quote, and some sammples, on some AlN insulators, and I
recall numbers closer to $2. I'll see if I still have the info.

Are the fet drains common? The best insulator is no insulator!

John

 

[John]

The advantage of mica is that it's usually very thin. 3 mils is
I just found 3mil mica from Keystone, so there's hope! But I remember
that mica had even worse thermal ratings than Sil-Pads (and equivalent
insulators) and those pads had wayyyyy too high a resistance for my
uses.

Uh-oh, I just realized that if typical mica was about 3mil thick and
that mica's specs weren't good enough for the power levels I want to
reach with these MOSFETs (acting as an electronic load) then the AlO2
I have may have too high a resistance too.

LOL, even with the cost, hard anodizing is looking better.


Alas, each drain has current sense resistor for the "servo loop"
controlling each MOSFET (as it acts as an electronic load). :-(

$2 or so is a great price. If you had access to that AlN info, that
would be great! Thanks.

John

 

[John Larkin]

I just found 3mil mica from Keystone, so there's hope! But I remember
that mica had even worse thermal ratings than Sil-Pads (and equivalent
insulators) and those pads had wayyyyy too high a resistance for my
uses.

Sil-pads are garbage. Even with huge mounting pressure, they never
meet the specified thermal conductivity. And they're thick, so thermal
resistance is high.

Uh-oh, I just realized that if typical mica was about 3mil thick and
that mica's specs weren't good enough for the power levels I want to
reach with these MOSFETs (acting as an electronic load) then the AlO2
I have may have too high a resistance too.

LOL, even with the cost, hard anodizing is looking better.



Alas, each drain has current sense resistor for the "servo loop"
controlling each MOSFET (as it acts as an electronic load). :-(

Move the resistor to the source!

John

 

[John]

Alas, each drain has current sense resistor for the "servo loop"
Hmm...good idea. I could do that (dealing with some high common mode
voltages, manageable though) but I'd still need to electrically
insulate the heat sink and prevent any touching. An enclosure can
solve that problem but there's a LOT of heat to remove, cost of
enclosure, etc.

LOL, there's never an easy way!

I think I'm seeing why hardcoat anodizing is used. It's keeps me from
dealing with pads and grease, speeds up assembly, already provides the
insulation and it looks good. Probably worth calling around for
better prices.

I'll still test out those AlO2 pads I have and perhaps some AlN pads
too (if you find that info for the ~$2 pads). Gotta' get me some
learnin' about those things and their effect, even if the anodizing
looks like the best solution

<deep sigh> I did try Dremeling out a hole for a thermocouple in one
AlO2 pad earlier today. Hah! I thought that 2 of my grinder/cutter
bits were carbide but all they did was leave a steel-color smear on
the surface of the pad. Time to go shopping for something a little
harder.

John

 

[Bob Masta]

AlO2 is pretty benign. It's not toxic, so is as dangerous as any other
dust... sawdust, glass, sand.

There are big differences in safety when these things are respirable
dusts. Sawdust, and probably alumina (aluminum oxide or corundum)
are typically in the "nuisance dust" category, although alumina at
high concentrations and/or long exposures is known to cause scarring
leading to Shaver's disease. Silica (silicon dioxide), however, is a
major component of sand and glass and is a much more serious problem.
When the particles are fine they cause permanent scarring, enough of
which leads to slicosis. Those who work with mineral dusts wear
respirators rated to remove this stuff. (NOT the silly little
"nuisance dust" masks sold to painters and woodworkers.)

It's not a very good heat conductor. AlN is much better. BeO is even
better, but BeO is seriously toxic.

Yeah , BeO is scary stuff! For airborne dust, it's 50 times more
toxic than Arsenic. Before I got into pottery and started
reading up on all this toxicity info, I acquired some TO3 BeO pads.
Now I think I'll just let them sit in their little plastic bags in
their little plastic parts drawer.... <g>

Best regards,


Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!

 

[John Larkin]

Hmm...good idea. I could do that (dealing with some high common mode
voltages, manageable though) but I'd still need to electrically
insulate the heat sink and prevent any touching. An enclosure can
solve that problem but there's a LOT of heat to remove, cost of
enclosure, etc.

LOL, there's never an easy way!

I think I'm seeing why hardcoat anodizing is used. It's keeps me from
dealing with pads and grease, speeds up assembly, already provides the
insulation and it looks good. Probably worth calling around for
better prices.

We do hard anodize and as I recall it's not real expensive. It's
probably worth it when you consider how many fets and associated
driver/sense circuits you may be able to eliminate. Make sure the
heatsink is *flat* first... most extrusions are wavy and need to be
fly-cut flat if you want the lowest theta.

You could also consider heat spreaders: a small, chunky aluminum or
even copper plate between each fet and the heatsink. The fet is in
direct contact with the spreader, and the spreader is insulated from
the sink. The bigger insulator footprint reduces theta.

I'll still test out those AlO2 pads I have and perhaps some AlN pads
too (if you find that info for the ~$2 pads). Gotta' get me some
learnin' about those things and their effect, even if the anodizing
looks like the best solution

<deep sigh> I did try Dremeling out a hole for a thermocouple in one
AlO2 pad earlier today. Hah! I thought that 2 of my grinder/cutter
bits were carbide but all they did was leave a steel-color smear on
the surface of the pad. Time to go shopping for something a little
harder.

What is really cool to measure is true junction temperature. To do
that, characterize the substrate diode drop vs temp at some small
current, 10 mA maybe. Then arrange to power up the fet and then
suddenly, not too many microseconds, disconnect the regular stuff and
reverse bias 10 mA, and measure the substrate diode again. You can
even plot the cooloff curve and estimate various thermal time
constants.

John

 

[John Larkin]

Yeah , BeO is scary stuff! For airborne dust, it's 50 times more
toxic than Arsenic. Before I got into pottery and started
reading up on all this toxicity info, I acquired some TO3 BeO pads.
Now I think I'll just let them sit in their little plastic bags in
their little plastic parts drawer.... <g>

Apparently it's fine as long as you don't grind it up or anything. You
can still buy BeO slabs fabbed to your specified dimensions. I can't
imagine how they manufacture and slice up the stuff, at least how they
would do that in the USA.

John

 

[John]

We do hard anodize and as I recall it's not real expensive. It's
Good tip on the heat sink flatness. The mounting surface is milled to
a Rmax of 6.3 (I'm assuming mils). Not a mirror finish but it's not
bad. I couldn't find any other info on how much my theta-case-to-sink
could be lowered with a smoother surface.

I was wondering earlier about using a spreader (100mms square) or
individual spreaders.

I'm using six TO-264 FETs on a 100mm square sink mounting surface with
less than .5" between each FET and between each FET and the edge of
the sink. A bit over 34% total coverage. Do you think that a
spreader (or spreaders) could lower the total theta-case-to-sink
enough to be worth the theta-case-to-sink penalty from the extra
greased surface (of the spreader)?

Whoa, that's a great idea!

OK, so I'd measure the body diode voltage drop at a low current level
at several different temperatures representative of where the MOSFET
would be operating (probably by having another MOSFET on the sink
running as a load and waiting several minutes for thermal stability
for each step).

I could set up a LM317 for the constant-current 10mA supply. I'd then
use two MOSFETs to switch the main current in/out and the LM317 supply
out/in. A little bit of care in the timing to not blow anything up
and I think I could do this. And I have a nice shiny new Natl.
Instruments DAQ module just itching to plot the body diode voltage!

Thanks again for your help John.

 

[Stephen J. Rush]

Apparently it's fine as long as you don't grind it up or anything. You
can still buy BeO slabs fabbed to your specified dimensions. I can't
imagine how they manufacture and slice up the stuff, at least how they
would do that in the USA.

It's possible to work safely with any damn thing if you throw enough money
and paranoia at it. In petroleum refining, heavy hydrocarbons are
catalytically 'cracked' and lighter ones are polymerized to make gasoline.
The catalyst for the first process is silica; no problem. The catalyst
for the second is *hydrofluoric acid*. I like to think that any bean
counter who suggests cutting back the multilayered percautions around the
polymerization plant is shown a safety video that gives him nightmares.

 

[redbelly]

<deep sigh> I did try Dremeling out a hole for a thermocouple in one
AlO2 pad earlier today. Hah! I thought that 2 of my grinder/cutter
bits were carbide but all they did was leave a steel-color smear on
the surface of the pad. Time to go shopping for something a little
harder.

John

Aluminum oxide is an extremely hard material. I remember trying
something similar back in grad school, and getting that same grayish
metal deposit on the previously-white ceramic.

Time to shop is right, for new carbide bits to replace the ones you've
dulled. If any thing is going to work with the Dremel I think you'll
need diamond tooling, but even then you may just rub off the diamond
grit before it cuts a hole through the aluminimum oxide.

Regards,

Mark

 

[John]

Sil-pads are garbage. Even with huge mounting pressure, they never
I never drilled through the AlO2 pad as the Dremel bits were wayyyy
too expensive to even consider using, but I got some testing done.

I couldn't test the case temperature of the FET thru the AlO2 pad but
I did measure the drain lead temperature (as it exited the case) and
using that, along with case temp. w/o the pad, derived the approx.
junction temperature for the FET with the pad and without.

It appears that the TO-247 Aavid AlO2 insulators I'm using have
approx. 0.165 degrees-C/W thermal resistance using a good grease. This
is about what a K-10 Silpad gives me. Not good.

Double checking my results by using the thermal conductivity of AlO2
(from Aavid) and that of a K-10 pad verifies the results above.

AlO2 conductivity = 15.06 W/m-K, 80 mil thickness
K-10 pad cond. = 1.1 W/m-K, 6mil thickness

AlO2 pad is 13.3 times thicker than K-10.
AlO2 pad is 13.69 times more thermally conductive.

The numbers balance out almost perfectly.

I would love to try AlN as it's, IIRC, about 15-20 times more
thermally conductive than AlO2. Gotta' find it for a decent price
though (<<<$5/pad for TO0264) or hardcoat anodizing wins. I can't
afford the thermal penalty of the pads so far.

John

 

[John]

Time to shop is right, for new carbide bits to replace the ones you've
I think you're right.
I went shopping for bits and the diamond ones were $15 each! IMHO,
not worth the risk of just stripping the diamond off before I get
through the AlO2.

Using the drain lead temperature of the MOSFET with and w/o the pad in
place and the case temperature w/o the pad, I think I was able to
extrapolate the junction temperature for both scenarios pretty
accurately. Well, at least accurately enough for my purposes.

John

 

[Rich Grise]

Hmm...good idea. I could do that (dealing with some high common mode
voltages, manageable though) but I'd still need to electrically
insulate the heat sink and prevent any touching. An enclosure can
solve that problem but there's a LOT of heat to remove, cost of
enclosure, etc.

LOL, there's never an easy way!

I think I'm seeing why hardcoat anodizing is used. It's keeps me from
dealing with pads and grease,

No, No, No! You still need the goo. I prefer the white pasty stuff to
just plain silicone grease - in fact I like it so much that when I buy
a new CPU and heatsink, I clean their goo off the heatsink and use my
white stuff. One number that comes to mind is Dow Corning DC-340, but
the little tube of the stuff I have has lasted so long that I can't
read the label any more. )-;

Good Luck!
Rich

 

[John]

No, No, No! You still need the goo. I prefer the white pasty stuff to

Thanks Rich! I should have said "grease AND pads". I was definitely
going to use grease.

Currently I have Aavid's phase-change UltraStick and Wakefield's
non-silicone Series 126 grease.

The UltraStick has slightly better thermal specs but is hard to get
thin enough to get those specs. The Wakefield grease is that great
white pasty stuff you mentioned and goes on incredibly thin. A little
messier than the UltraStick, but it's probably what I'll stick with.

I'll check out DC-340 too. I only have a small amount of the
Wakefield grease so another tub of something else won't be a problem.

Thanks again!
John

 

[John Larkin]

Thanks Rich! I should have said "grease AND pads". I was definitely
going to use grease.

Currently I have Aavid's phase-change UltraStick and Wakefield's
non-silicone Series 126 grease.

The UltraStick has slightly better thermal specs but is hard to get
thin enough to get those specs. The Wakefield grease is that great
white pasty stuff you mentioned and goes on incredibly thin. A little
messier than the UltraStick, but it's probably what I'll stick with.

I'll check out DC-340 too. I only have a small amount of the
Wakefield grease so another tub of something else won't be a problem.

Thanks again!
John

I don't trust the phase-change crayon stuff. It's likely to leave air
pockets, and I'm not sure if it will ever really flow thin. I've
measured the white Dow grease as squashing down to below 100
micro-inches under moderate pressure, about my resolution limit.

The phase-change pads are definitely garbage.

John