Thermal conductivity calculation ?? for comparing thermal interface materials

[John]

My head is ready to explode trying to figure this out.
I have two electrically insulating materials being considered for use
with TO-247AC devices being mounted to a heat sink. They have the
following thermal conductivity and thickness specs:

- Berquist Sil-Pad K10 pad: 1.3 W/m-K conductivity, .006" thick.
- Aavid 4180 Alum. Oxide insulator: 15.06 W/m-K conductivity, 0.080"
thick.

In order to compare their thermal resistances, can I simply divide the
thermal conductivity by the thickness? The units and equations I can
find on the Web make this, what I thought to be very simple, decision
process very difficult.

- K10 thermal resistance = 1.3 / .006 = 216.67
- 4180 thermal resistance = 15.06 / 0.080 = 188.25

Therefore, even though the Aavid aluminum oxide insulator is 13.33
times thicker, it's thermally conductive enough to be the better
choice (thermally, that is)?

Essentially, I'm looking for the best (but still safe) TO-247AC,
electrically insulating, thermal interface material I can find. I'm
currently using Aavid's UltraStick compound (and love it), but it's
not electrically insulating and I need that for this application
(can't prevent touching of the heat sinks).

Thanks!
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[Phil Allison]

"John"

My head is ready to explode trying to figure this out.
I have two electrically insulating materials being considered for use
with TO-247AC devices being mounted to a heat sink. They have the
following thermal conductivity and thickness specs:

- Berquist Sil-Pad K10 pad: 1.3 W/m-K conductivity, .006" thick.
- Aavid 4180 Alum. Oxide insulator: 15.06 W/m-K conductivity, 0.080"
thick.

In order to compare their thermal resistances, can I simply divide the
thermal conductivity by the thickness? The units and equations I can
find on the Web make this, what I thought to be very simple, decision
process very difficult.

- K10 thermal resistance = 1.3 / .006 = 216.67
- 4180 thermal resistance = 15.06 / 0.080 = 188.25

Therefore, even though the Aavid aluminum oxide insulator is 13.33
times thicker, it's thermally conductive enough to be the better
choice (thermally, that is)?

Essentially, I'm looking for the best (but still safe) TO-247AC,
electrically insulating, thermal interface material I can find. I'm
currently using Aavid's UltraStick compound (and love it), but it's
not electrically insulating and I need that for this application
(can't prevent touching of the heat sinks).

** I think you are being tricked by figures that are non comparables.

The K10 material is soft and conforms to the shape of the device and
heatsink surface leaving no air gaps - so the thermal conductivity figure
is directly usable.

An Alumina insulator is hard needing thermal grease on both sides to fill
voids and work efficiently.

IME with high powered TO3 devices - thin mica with a smear of thermal
grease both sides is still the best.




......... Phil

 

[John Larkin]

My head is ready to explode trying to figure this out.
I have two electrically insulating materials being considered for use
with TO-247AC devices being mounted to a heat sink. They have the
following thermal conductivity and thickness specs:

- Berquist Sil-Pad K10 pad: 1.3 W/m-K conductivity, .006" thick.
- Aavid 4180 Alum. Oxide insulator: 15.06 W/m-K conductivity, 0.080"
thick.

In order to compare their thermal resistances, can I simply divide the
thermal conductivity by the thickness? The units and equations I can
find on the Web make this, what I thought to be very simple, decision
process very difficult.

- K10 thermal resistance = 1.3 / .006 = 216.67
- 4180 thermal resistance = 15.06 / 0.080 = 188.25

Right.

Therefore, even though the Aavid aluminum oxide insulator is 13.33
times thicker, it's thermally conductive enough to be the better
choice (thermally, that is)?
Yup.


Essentially, I'm looking for the best (but still safe) TO-247AC,
electrically insulating, thermal interface material I can find. I'm
currently using Aavid's UltraStick compound (and love it), but it's
not electrically insulating and I need that for this application
(can't prevent touching of the heat sinks).

Just be aware that the thermal conductivity of AlO is about as stated,
and that the Bergquist numbers are usually wildly optimistic, even if
you apply insane mounting pressures. Another factor of 2 might be
prudent.

The best thing, as a single insulator, would be aluminum nitride with
grease on both sides. It looks pretty much like AlO, isn't toxic, but
conducts heat about six times better. It's available in suitable
shapes.

For reasonable voltages, we like to have aluminum heatsinks hard
anodized 1 mil thick, and then just use silicone grease. That will be
tens of times better than the Silpad or the thick AlO.

Best yet is to bolt the transistor to a copper heat spreader and
insulate *that* from the main heatsink.

John

 

[Pooh Bear]

My head is ready to explode trying to figure this out.
I have two electrically insulating materials being considered for use
with TO-247AC devices being mounted to a heat sink.

First off you should get Motorola's AN1040

https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Graham

 

[Pooh Bear]

Just be aware that the thermal conductivity of AlO is about as stated,
and that the Bergquist numbers are usually wildly optimistic, even if
you apply insane mounting pressures. Another factor of 2 might be
prudent.

I've heard this mentioened before.

I've had good results with Warth's synthetic 'rubber' insulators.

You don't have to worry about the grease 'drying out' either.

Graham

 

[Bob]

My head is ready to explode trying to figure this out.
I have two electrically insulating materials being considered for use
with TO-247AC devices being mounted to a heat sink. They have the
following thermal conductivity and thickness specs:

- Berquist Sil-Pad K10 pad: 1.3 W/m-K conductivity, .006" thick.
- Aavid 4180 Alum. Oxide insulator: 15.06 W/m-K conductivity, 0.080"
thick.

In order to compare their thermal resistances, can I simply divide the
thermal conductivity by the thickness? The units and equations I can
find on the Web make this, what I thought to be very simple, decision
process very difficult.

- K10 thermal resistance = 1.3 / .006 = 216.67
- 4180 thermal resistance = 15.06 / 0.080 = 188.25

Therefore, even though the Aavid aluminum oxide insulator is 13.33
times thicker, it's thermally conductive enough to be the better
choice (thermally, that is)?

Essentially, I'm looking for the best (but still safe) TO-247AC,
electrically insulating, thermal interface material I can find. I'm
currently using Aavid's UltraStick compound (and love it), but it's
not electrically insulating and I need that for this application
(can't prevent touching of the heat sinks).

Thanks!
-- remove SPAMMENOT for e-mail responses --

John,

The "m" in the thermal conductivity constant is really the ratio of the area
to the thickness. The units come out as meters, but it's really square
meters per meter.

Thermal resistance is in the form of degrees C (or Kelvin) per watt.

So, to get from thermal conductivity (let's call it therm_cond) to thermal
resistance (therm_resist):

therm_resist = thickness of the material in question [in meters] / ( contact
area of the material [in meters] * therm_cond )

For example, if your materials therm_cond is 1.3W/m-K, and the contact area
is 9mm * 8mm, and the thickness of the material is 2mm, then:

therm_resist = 2mm / (9mm * 8mm * 1.3W/m-K)
= 0.02 degrees C / watt

If your thermal conductivity is 15.06W/m-K and the material thickness and
contact area is the same as above:

therm_resist = 2mm / (9mm * 8mm * 15.06W/m-K)
= 0.0018 degrees C / watt


The temperature delta, for 15W transferred through the material, will be :

delta T = watts * therm_resist
= 15W * 0.02C/W
= 0.3 degrees C (for the 1.3W/m-K stuff)

= 0.03 degrees C (for the 15.06W/m-K stuff)

If you're putting the Bergquist and Aavid stuff in series, then the
temperature delta, for the combination, will be 0.33 degrees C.

In order to really find out what the silicon temperature is going to be
(this is the only thing that really matters), you need to know "theta jc"
for the IC or transistor package, you need to know the "theta sa" for the
heatsink (at the given amount of airflow), and you'll need to know the
ambient temperature of the airflow.

The Bergquist website shows you how to do these calculations (iirc).

Bob

 

[John]

Thanks guys!

Phil,
I forgot to mention that we would use our favorite phase-change
compound (Aavid's Ultra-Stick) between any hard insulator and the
device/heat sink.


John,
I looked high and low for aluminum nitride insulators but could only
find recommendations to use them. Do you know a source for TO-247AC
insulators?

I really like the hard anodizing idea. Top priority is low thermal
resistance, not cost. The Cool Innovations pin heat sinks we're using
in the prototypes are plain (no plating or anodizing) and aren't
offered with anodizing in the quantities we're looking for (100pc.
lots). Do you have a recommendation for company that does hard
anodizing?

We had considered using a copper heat spreader between the four
TO-247AC devices and the 2.5" square heat sink (4 heat sinks per
prototype) but they were already pretty well spread out on each heat
sink. And we'd still need an insulator for the spreader.


Graham,
That On Semi app note is a nice one. Better than the ones from IR and
TI that we have. Thanks!

I'll check the specs on Warth's insulators too.

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[John]

The "m" in the thermal conductivity constant is really the ratio of the area
Ahhhhh....OK, now it's starting to make some sense.
We have the theta-jc for the devices, 0.32 degrees-C/W (IRFP2907),
We have the theta-sa for the sinks, 0.22 degrees-C/W (Cool Innovations
3-252517R) for our selected fan.

We were pretty comfortable with the equations for figuring out the
junction temperature for a given power level (just needed the
theta-cs), but we were getting dizzy trying to convert the thermal
conductivity numbers we had to thermal resistances (for the
insulators). Thanks for taking the time to explain that.

Time to do a bunch of calculations!

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[John Larkin]

Thanks guys!

Phil,
I forgot to mention that we would use our favorite phase-change
compound (Aavid's Ultra-Stick) between any hard insulator and the
device/heat sink.


John,
I looked high and low for aluminum nitride insulators but could only
find recommendations to use them. Do you know a source for TO-247AC
insulators?

These people seemed willing to do AlN insulators...

http://pmindustriesinc.com/

http://accumetmaterials.com

http://valleydesign.com

I really like the hard anodizing idea. Top priority is low thermal
resistance, not cost. The Cool Innovations pin heat sinks we're using
in the prototypes are plain (no plating or anodizing) and aren't
offered with anodizing in the quantities we're looking for (100pc.
lots). Do you have a recommendation for company that does hard
anodizing?

I'll check when I get to work. If it's a cast pin-fin sink, you should
probably machine it flat first, too, as they tend to be wavy.

Most anodized heatsinks are cosmetic soft anodize, not a reliable
insulator. Make sure it's "hard" anodized, 0.5 to 1 mil thick, and
burr-free.

We had considered using a copper heat spreader between the four
TO-247AC devices and the 2.5" square heat sink (4 heat sinks per
prototype) but they were already pretty well spread out on each heat
sink. And we'd still need an insulator for the spreader.

2.5" square is pretty small. Do all four transistors always heat
equally? If not, there's a big advantage to going to one bigger
heatsink, rather than four small ones.

You do need an insulator for the spreader, but the footprint is
potentially a lot bigger so theta is correspondingly less.

John

 

[[email protected]]

John,

The "m" in the thermal conductivity constant is really the ratio of the area
to the thickness. The units come out as meters, but it's really square
meters per meter.

Thermal resistance is in the form of degrees C (or Kelvin) per watt.

So, to get from thermal conductivity (let's call it therm_cond) to thermal
resistance (therm_resist):

This equation isn't quite right - though it used more or less correctly
below.

therm_resist = thickness of the material in question [in meters] / (
contact
area of the material [in SQUARE meters] * therm_cond )

For example, if your materials therm_cond is 1.3W/m-K, and the contact area
is 9mm * 8mm, and the thickness of the material is 2mm, then:

therm_resist = 2mm / (9mm * 8mm * 1.3W/m-K)
= 0.02 degrees C / watt

Pay attention to the units - either

therm-resist = 0.002m /(0.009m * 0.008 m * 1..3W/m.K)
= 21.37 degrees Kelvin per Watt

or therm_resist = 2mm / (9mm * 8mm * 0.0013W/mm-K)
= 21.37 degrees Kelvin per Watt

In fact the Berquist material is only 0.156 mm thick (0.006 inches), so
the correct answer would be

= 1.67 degrees Kelvin per Watt

if Berquist's figures were to be trusted

If your thermal conductivity is 15.06W/m-K and the material thickness and
contact area is the same as above:

therm_resist = 2mm / (9mm * 8mm * 15.06W/m-K)
= 0.0018 degrees C / watt

By the same argument, actually 1.8 degrees C/watt

The temperature delta, for 15W transferred through the material, will be :

delta T = watts * therm_resist

Using the corrected numbers

= 15W * 1.67C/W
= 25.05 degrees C (for the 1.3W/m-K stuff)

= 27 degrees C (for the 15.06W/m-K stuff)

If you're putting the Bergquist and Aavid stuff in series, then the
temperature delta, for the combination, would be 52 degrees C.

In order to really find out what the silicon temperature is going to be
(this is the only thing that really matters), you need to know "theta jc"
for the IC or transistor package, you need to know the "theta sa" for the
heatsink (at the given amount of airflow), and you'll need to know the
ambient temperature of the airflow.

The Bergquist website shows you how to do these calculations (iirc).

It's pretty easy to make conceptual slips in working out these sorts of
equations - a reality check is often a very good idea.

 

[Bob]

This equation isn't quite right - though it used more or less correctly
below.


Pay attention to the units - either

therm-resist = 0.002m /(0.009m * 0.008 m * 1..3W/m.K)
= 21.37 degrees Kelvin per Watt

Damn! Looks like I picked the wrong week to stop beating my dog.

What's a factor of a thousand between friends, anyway?

Thanks for the correction.

Bob

 

[John]

Damn! Looks like I picked the wrong week to stop beating my dog.
I was running the equations last night and was wondering why the
numbers were looking so incredibly great until I noticed that the K10
thermal resistance was almost exactly 1000 times smaller than all the
other silicone pad numbers I had gotten. I added some zeros after
that.

Here are the numbers for my original two top choices for a TO-247AC
device with a thermal pad area of 206 sq. mm:

Bergquist K10 silicone pad = 0.57 degrees-C/W
Aavid 4180 Aluminum Oxide pad = 0.66 degrees-C/W

That Bergquist pad is pretty impressive.

Warth (now Laird Technologies) has an even more impressive pad, the
KTP series (particularly the KTP127, 0.127mm thick), but it's been
tough trying to find someone here in the States to get a quote from.
Another day or two should do it.

The numbers for Aluminum Nitride insulators are incredible, but you
gots to pay for it. About $7 a device...ouch. Pretty sure the
performance won't be worth it, especially if we can hard anodize the
heat sinks.
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[John]

These people seemed willing to do AlN insulators...
<deep sigh>
I didn't think of looking into buying a sheet of AlN (perhaps the size
of the heat sink). I just concentrated on finding TO-247 shaped pads.
Thanks for the wake-up call.

I checked into those three sites....
Valley Design had some prices, it would cost about $6-$10 per device.
Wow. The thermal resistance of AlN is incredibly low but I have to
look hard at the numbers to see if it's worth the price. Good to know
about though.



They're forged, according to Cool Innovations. Amazing that they can
do that with the pins so close together. The sinks are sent already
machined on the base and the tops of the pins.

I did a LOT of checking around and found dozens and dozens of places
that do "Type III hardcoat anodizing" (I think that's what I'm looking
for) with standard thicknesses of 1 mil surface coating, 1 mil
penetration. Or thicker if I wanted. Lots of good info on the
American Anodizers Council web site (http://www.anodizing.org/).

But, a recommendation for an anodizer would be great, thanks.
I'm not looking forward to cold calling a bunch of places looking for
someone who understands what's needed.



The four transistors on each heat sink are spaced as best as we can
and all run to within 1% of each other (power dissipation).



I should probably get out the calculator again. The spreader would
get us cloer to the "ideal" heat source (spread evenly across the heat
sink) but there would be another thermal interface between the
spreader and sink and I'd want to make sure the cost and performance
would be worth it.

Hmm...a large 1/4" thick copper spreader, the size of the 4 heat
sinks, might be a good idea for mounting though. It would be easier
to bolt the 4 sinks to the spreader and just have one solid assembly
to attach the transistors to. And I'd get that lower theta too.

Maybe even thermal epoxy to attach the sinks to the spreader? Have to
check those numbers too. <another deep sigh> Weren't computers
supposed to make my life easier by now? All I'm doing every day is
banging away at my HP calculator, working thermal equations!!

Time to hit the McMaster-Carr web site for Alloy 110 prices!

Thanks again
John
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[[email protected]]

John wrote:

Maybe even thermal epoxy to attach the sinks to the spreader?

Be careful about thermal epoxy. Most of the stuff I looked at was
loaded with little glass balls to guarantee enough epoxy filled gap to
provide a - fairly high - level of insulation.

I've used an elastomeric thermal adhesive which didn't insulate
particularly effectively, but was thin enough to offer a low thermal
resistance.

 

[John Larkin]

I did a LOT of checking around and found dozens and dozens of places
that do "Type III hardcoat anodizing" (I think that's what I'm looking
for) with standard thicknesses of 1 mil surface coating, 1 mil
penetration. Or thicker if I wanted. Lots of good info on the
American Anodizers Council web site (http://www.anodizing.org/).

We use Santa Clara Plating, in Santa Clara California; they do
consistantly good work.

Fab drawings should say "Finish: deburr, heavy etch, 1 mil hard black
anodize."

We pay something like a $75 lot fee plus a few dollars per part for
small stuff.

Hey, these are slick:

http://www.mouser.com/index.cfm?&ha...tt=*532maxclip07*&Dk=1&Ns=SField&N=0&crc=true

John

 

[John]

We use Santa Clara Plating, in Santa Clara California; they do
Thanks for the reference John!

You read our mind with those clips. The heat sinks we have in mind
don't support the standard clips (in a slot) but we were thinking of
using the MAX07 (screw mount) clips. With 18lbs of force, they look
good.

But, how necessary are clips for mounting a TO-247AC case? With the
screw going thru the body of the TO-247, it's not going to pivot up
like using the tab on a TO-220, is it?

Oh, wait, with the clip I can use the Super-TO247 (or whatever it's
called) case that doesn't have the screw hole and gain another 10%
decrease (or so IIRC) decrease in thermal resistance due to a bigger
thermal pad.

Soooooo much to consider when you're trying to pack as mamny watts as
you can into a given space.
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[John Larkin]

Thanks for the reference John!

You read our mind with those clips. The heat sinks we have in mind
don't support the standard clips (in a slot) but we were thinking of
using the MAX07 (screw mount) clips. With 18lbs of force, they look
good.

But, how necessary are clips for mounting a TO-247AC case? With the
screw going thru the body of the TO-247, it's not going to pivot up
like using the tab on a TO-220, is it?

Oh, wait, with the clip I can use the Super-TO247 (or whatever it's
called) case that doesn't have the screw hole and gain another 10%
decrease (or so IIRC) decrease in thermal resistance due to a bigger
thermal pad.

Soooooo much to consider when you're trying to pack as mamny watts as
you can into a given space.
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We use both the regular and super 247's with clips. I figure not
having a hole in the heatsink reduces theta even more! Hmmm, have to
do the math on that claim some day.

The clips don't press nearly as hard as a screw, so aren't appropriate
if you use a sil-pad. These clips *will* squash a big blob of regular
Dow Corning-type thermal grease down below 100 microinches thickness,
if the surfaces are that flat.

Oh, the solid phase-change things, the ones that look like sil-pads,
suck.

Hard anodize does sort of mess up tapped holes. I think we tap them a
tad oversize to prevent the screws from galling. You can chase the
holes with a tap afterwards, but it wrecks taps.

This thermal stuff is a lot more complex than it first appears.

What's the gadget you're building?

John

 

[John]

The clips don't press nearly as hard as a screw, so aren't appropriate
That's somethng important for us to consider. If we didn't hard
anodize, we'd go with the Bergquist K10 pads or AlN (expensive!)
sheets. The K10 pads would then need to be screw mounted.



I wasn't really impressed with the specs myself, especially
considering how great the Aavid UltraStick phase-change compound is.

What were your experiences with the phase-change pads?



I was wondering about that yesterday. We buy taps by the box full
(and can afford to go through a few of them) but it would save a lot
of time if we didn't have to.



An active load used for testing power sources. I have two of them
going at 350W each, but I really need to get up to the 1KW+ range.
350W in a 5" cube wasn't too hard. Tripling that power level gets
interesting! It might not be possible, without liquid cooling, etc.,
but a goal nonetheless.

John


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[John Larkin]

I wasn't really impressed with the specs myself, especially
considering how great the Aavid UltraStick phase-change compound is.

What were your experiences with the phase-change pads?

They don't ever flow out from below the device, so they stay several
mils thick. Theta galore.

John

 

[Pooh Bear]

John wrote:

But, how necessary are clips for mounting a TO-247AC case? With the
screw going thru the body of the TO-247, it's not going to pivot up
like using the tab on a TO-220, is it?

The hole isn't central. The header can indeed 'lift' slightly.

I like to use a *large* washer ( commonly generically called a bicycle washer ) when mounting TO-247 to distribute the pressure.

Graham