Aluminum oxide insulator safety precautions?

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!

On Fri, 10 Aug 2007 18:53:31 -0400, John
<(E-Mail Removed)> wrote:

>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

>>>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.

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

On Fri, 10 Aug 2007 21:20:04 -0400, John wrote:

>>>>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.
>
> 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

>>>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

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

On Fri, 10 Aug 2007 22:19:03 -0400, John
<(E-Mail Removed)> wrote:

>>>>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
>
>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

>>>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 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.


>>>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

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

On Fri, 10 Aug 2007 23:24:24 -0400, John
<(E-Mail Removed)> wrote:

>>>>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 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.
>
>
>>>>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
>
>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

>>>>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

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

On Fri, 10 Aug 2007 17:49:26 -0700, John Larkin
<(E-Mail Removed)> wrote:

>On Fri, 10 Aug 2007 18:53:31 -0400, John
><(E-Mail Removed)> wrote:
>
>>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.

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

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