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
<jmuchow@SPAMMENOTcamlight.com> 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
<jmuchow@SPAMMENOTcamlight.com> 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
<jmuchow@SPAMMENOTcamlight.com> 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
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 10 Aug 2007 18:53:31 -0400, John
><jmuchow@SPAMMENOTcamlight.com> 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

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

On Sat, 11 Aug 2007 05:10:55 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>>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.

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

On Sat, 11 Aug 2007 12:40:17 GMT, NoSpam@daqarta.com (Bob Masta)
wrote:



>>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>
>

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

>>>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.

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)?

>>>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.

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.

On Sat, 11 Aug 2007 09:21:06 -0700, John Larkin wrote:

> On Sat, 11 Aug 2007 12:40:17 GMT, NoSpam@daqarta.com (Bob Masta)
> wrote:
>
>
>
>>>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>
>>
>
> 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.

On Aug 11, 5:10 am, John <jmuc...@SPAMMENOTcamlight.com> wrote:

> <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

>>>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.

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

>>>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.

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

On Sat, 11 Aug 2007 05:10:55 -0400, John wrote:
>>>>>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!
>
> 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

>>>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


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

On Wed, 15 Aug 2007 04:17:17 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>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
>
>
>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

>>>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.

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

>>>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.

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

On Thu, 16 Aug 2007 00:03:35 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>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.
>
>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.

Some alloys anodize a nicer black than others.


>
>But, that's one tough coating!

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.



>
>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?

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

On Wed, 15 Aug 2007 23:57:58 -0400, John
<jmuchow@SPAMMENOTcamlight.com> wrote:

>>>>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.
>
>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.
>

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

On Thu, 16 Aug 2007 08:52:19 -0700, John Larkin wrote:

> On Wed, 15 Aug 2007 23:57:58 -0400, John
> <jmuchow@SPAMMENOTcamlight.com> wrote:
>
>>>>>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.
>>
>>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.
>>
>
> 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?
>
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