Give a nice flat mosfet package and a flat heat sink, I wonder how
much benefit accrues from adding silicone grease. It's really messy in
production and it's hard to confirm proper application. A little
googling didn't provide hard numbers.

I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
or mica insulator, no grease, 40 watts. I guess I'll have to try it.

On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
wrote:

>Give a nice flat mosfet package and a flat heat sink, I wonder how
>much benefit accrues from adding silicone grease. It's really messy in
>production and it's hard to confirm proper application. A little
>googling didn't provide hard numbers.
>
>I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>or mica insulator, no grease, 40 watts. I guess I'll have to try it.

The big problem is likely that one must design for it to be dry, so
when the grease drools away, the product won't melt.

Joe Gwinn

On 31/03/2024 5:14 am, John Larkin wrote:
> Give a nice flat mosfet package and a flat heat sink, I wonder how
> much benefit accrues from adding silicone grease. It's really messy in
> production and it's hard to confirm proper application. A little
> googling didn't provide hard numbers.
>
> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
> or mica insulator, no grease, 40 watts. I guess I'll have to try it.

I've used graphite cloth as gap filler. It seems to work and it isn't messy.

Meas. Sci. Technol. 7 (1996) 1653–1664.

--
Bill Sloman, Sydney

On 30/03/2024 18:14, John Larkin wrote:
> Give a nice flat mosfet package and a flat heat sink, I wonder how
> much benefit accrues from adding silicone grease. It's really messy in
> production and it's hard to confirm proper application. A little
> googling didn't provide hard numbers.
>
> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
> or mica insulator, no grease, 40 watts. I guess I'll have to try it.

ISTR on one of the overclocking hacker CPU cooling sites someone tried
everything from dry to cooking oil and engine oil. The marginal best was
some exotic "liquid metal" silver loaded brand I have never heard of and
the worst by a long way was dry.

The biggest change was from dry to some sort of heat exchange medium is
by preventing an air gap. It was a significant difference too.

The problem is that your flat surfaces are not exactly flat so that the
direct metal contact area can actually be quite small if there is any
surface roughness. Air is a rather good insulator and metals don't
radiate well at all. Silicon grease prevents air gaps and anything
similar will do the same job. It is just that silicon oils and greases
are less inclined to evaporate or go rancid and corrode your parts.

--
Martin Brown

On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
wrote:

>Give a nice flat mosfet package and a flat heat sink,

There is no such thing as flat. There is only different levels of
rough. Extremely flat is extremely expensive.

In theory, when you bolt two metal object together, they will only
contact at three microscopic points.

We always put some solder onto the tip of the soldering iron before
soldering. Otherwise the heat will simply not transfer to the
workpiece, even if both the soldering iron and the workpice are flat.
--
RoRo

On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
wrote:

>Give a nice flat mosfet package and a flat heat sink, I wonder how
>much benefit accrues from adding silicone grease. It's really messy in
>production and it's hard to confirm proper application. A little
>googling didn't provide hard numbers.
>
>I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>or mica insulator, no grease, 40 watts. I guess I'll have to try it.

Silpads. I used silpads since the 80's. I don't know if they are better
or worse than grease, but they are good enough and most importantly they
are consistant. What happens with testing on the prototypes, happens with
production units. Shorts were always with the little vinyl grommets, you
use to keep the screws from shorting to the part. You can squeeze those
down to tight and cause a short.

On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 30/03/2024 18:14, John Larkin wrote:
>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>> much benefit accrues from adding silicone grease. It's really messy in
>> production and it's hard to confirm proper application. A little
>> googling didn't provide hard numbers.
>>
>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>
>ISTR on one of the overclocking hacker CPU cooling sites someone tried
>everything from dry to cooking oil and engine oil. The marginal best was
>some exotic "liquid metal" silver loaded brand I have never heard of and
>the worst by a long way was dry.
>
>The biggest change was from dry to some sort of heat exchange medium is
>by preventing an air gap. It was a significant difference too.
>
>The problem is that your flat surfaces are not exactly flat so that the
>direct metal contact area can actually be quite small if there is any
>surface roughness. Air is a rather good insulator and metals don't
>radiate well at all. Silicon grease prevents air gaps and anything
>similar will do the same job. It is just that silicon oils and greases
>are less inclined to evaporate or go rancid and corrode your parts.

There's a lot of opinion on this but few or no numbers. Some people
seem to think that their music sounds better, or their gaming scores
improve, with some expensive grease.

A TO-220 footprint with a 100 micro-inch air gap, assuming zero
metal-metal contact to the heat sink, calculates to 0.65 K/W. I
wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1,
which is still fine for my application.

The TO-220 FullPak insulated transistors are appealing but have awful
thetas.

Digikey keeps getting worse. They mostly show gap-pad thetas that are
about 5x worse than reality. They seem to use the TO-3 specs for
TO-220 parts. And they show obsolete parts with zero stock if I
specify "usually stocked."

On Sun, 31 Mar 2024 14:38:44 +0200, Robert Roland <fake@ddress.no>
wrote:

>On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>wrote:
>
>>Give a nice flat mosfet package and a flat heat sink,
>
>There is no such thing as flat. There is only different levels of
>rough. Extremely flat is extremely expensive.
>
>In theory, when you bolt two metal object together, they will only
>contact at three microscopic points.

That "theory" assumes that both surfaces are infinitely rigid. The
force on those three microscopic contact points would be enough to
melt diamond.

>
>We always put some solder onto the tip of the soldering iron before
>soldering. Otherwise the heat will simply not transfer to the
>workpiece, even if both the soldering iron and the workpice are flat.

Wrap a bit of aluminum foil around your finger and touch it with a
hot, untinned soldering iron.

On 1/04/2024 1:59 am, John Larkin wrote:
> On Sun, 31 Mar 2024 14:38:44 +0200, Robert Roland <fake@ddress.no>
> wrote:
>
>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>>
>>> Give a nice flat mosfet package and a flat heat sink,
>>
>> There is no such thing as flat. There is only different levels of
>> rough. Extremely flat is extremely expensive.
>>
>> In theory, when you bolt two metal object together, they will only
>> contact at three microscopic points.
>
> That "theory" assumes that both surfaces are infinitely rigid. The
> force on those three microscopic contact points would be enough to
> melt diamond.

Diamond doesn't melt at room temperature. There is a
diamond/graphite/liquid carbon triple point at 12 GPa at 5000 K, but
your lump of metal wouldn't get hot enough to get anywhere near that.

The three microscopic points will distort so that that the compressive
force is spread over three finite - but small - areas.

With very clean surfaces under vacuum you get microwelding as the metal
at the microscopic contact points diffuses into the surface it is
squeezed up against. This can be a problem if you wan to dismantle your
gear later.

<snip>

--
Bill Sloman, Sydney

On Sun, 31 Mar 2024 07:53:05 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
><'''newspam'''@nonad.co.uk> wrote:
>
>>On 30/03/2024 18:14, John Larkin wrote:
>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>> much benefit accrues from adding silicone grease. It's really messy in
>>> production and it's hard to confirm proper application. A little
>>> googling didn't provide hard numbers.
>>>
>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>
>>ISTR on one of the overclocking hacker CPU cooling sites someone tried
>>everything from dry to cooking oil and engine oil. The marginal best was
>>some exotic "liquid metal" silver loaded brand I have never heard of and
>>the worst by a long way was dry.
>>
>>The biggest change was from dry to some sort of heat exchange medium is
>>by preventing an air gap. It was a significant difference too.
>>
>>The problem is that your flat surfaces are not exactly flat so that the
>>direct metal contact area can actually be quite small if there is any
>>surface roughness. Air is a rather good insulator and metals don't
>>radiate well at all. Silicon grease prevents air gaps and anything
>>similar will do the same job. It is just that silicon oils and greases
>>are less inclined to evaporate or go rancid and corrode your parts.
>
>There's a lot of opinion on this but few or no numbers. Some people
>seem to think that their music sounds better, or their gaming scores
>improve, with some expensive grease.

Most manuals on basic electronics will include tables of typical
Rth for various mounting media. This is drawn from empirical data
that you can duplicate on your own bench.

If isolation is not required, I've had excellent results introducing
soft copper wafers into the junction. These seem to deform to
reduce issues with surface flatness. Also useful when efficient
high-current conduction is required between the two surfaces.

Properly-mixed silicon oil won't make a mess if properly applied.
If you recheck older junctions, you'll see that they are fairly
hard to break, due to their air-free composition.

This is old tech.

RL

On 31/03/2024 3:53 pm, John Larkin wrote:
> There's a lot of opinion on this but few or no numbers. Some people
> seem to think that their music sounds better, or their gaming scores
> improve, with some expensive grease.
>

Numbers can be found. For instance see page 8 of Philips AN10384

piglet

On Sun, 31 Mar 2024 13:12:13 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sun, 31 Mar 2024 07:53:05 -0700, John Larkin <jl@997PotHill.com>
>wrote:
>
>>On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
>><'''newspam'''@nonad.co.uk> wrote:
>>
>>>On 30/03/2024 18:14, John Larkin wrote:
>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>> production and it's hard to confirm proper application. A little
>>>> googling didn't provide hard numbers.
>>>>
>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>
>>>ISTR on one of the overclocking hacker CPU cooling sites someone tried
>>>everything from dry to cooking oil and engine oil. The marginal best was
>>>some exotic "liquid metal" silver loaded brand I have never heard of and
>>>the worst by a long way was dry.
>>>
>>>The biggest change was from dry to some sort of heat exchange medium is
>>>by preventing an air gap. It was a significant difference too.
>>>
>>>The problem is that your flat surfaces are not exactly flat so that the
>>>direct metal contact area can actually be quite small if there is any
>>>surface roughness. Air is a rather good insulator and metals don't
>>>radiate well at all. Silicon grease prevents air gaps and anything
>>>similar will do the same job. It is just that silicon oils and greases
>>>are less inclined to evaporate or go rancid and corrode your parts.
>>
>>There's a lot of opinion on this but few or no numbers. Some people
>>seem to think that their music sounds better, or their gaming scores
>>improve, with some expensive grease.
>
>Most manuals on basic electronics will include tables of typical
>Rth for various mounting media. This is drawn from empirical data
>that you can duplicate on your own bench.

I'll do some testing: mica with grease, mica without, and a silicone
gap-pad. The literature is inconsistent and often wrong.

>
>If isolation is not required, I've had excellent results introducing
>soft copper wafers into the junction. These seem to deform to
>reduce issues with surface flatness. Also useful when efficient
>high-current conduction is required between the two surfaces.

The black graphite pads have very low thetas, but I need insulation.

>
>Properly-mixed silicon oil won't make a mess if properly applied.
>If you recheck older junctions, you'll see that they are fairly
>hard to break, due to their air-free composition.
>
>This is old tech.
>
>RL

On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:

>On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>wrote:
>
>>Give a nice flat mosfet package and a flat heat sink, I wonder how
>>much benefit accrues from adding silicone grease. It's really messy in
>>production and it's hard to confirm proper application. A little
>>googling didn't provide hard numbers.
>>
>>I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>
>Silpads. I used silpads since the 80's. I don't know if they are better
>or worse than grease, but they are good enough and most importantly they
>are consistant. What happens with testing on the prototypes, happens with
>production units. Shorts were always with the little vinyl grommets, you
>use to keep the screws from shorting to the part. You can squeeze those
>down to tight and cause a short.

Right, I want to avoid the mess and uncertainty of grease.

Bergquist has a TO-220 pad, their 1500ST material, that should be
about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
which ain't great but is survivable.

I'm considering using good 6-32 plastic screws to mount the fets to
the cooler.

On 3/31/24 12:33, John Larkin wrote:
> On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>
>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>>
>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>> much benefit accrues from adding silicone grease. It's really messy in
>>> production and it's hard to confirm proper application. A little
>>> googling didn't provide hard numbers.
>>>
>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>
>> Silpads. I used silpads since the 80's. I don't know if they are better
>> or worse than grease, but they are good enough and most importantly they
>> are consistant. What happens with testing on the prototypes, happens with
>> production units. Shorts were always with the little vinyl grommets, you
>> use to keep the screws from shorting to the part. You can squeeze those
>> down to tight and cause a short.
>
> Right, I want to avoid the mess and uncertainty of grease.
>
> Bergquist has a TO-220 pad, their 1500ST material, that should be
> about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
> which ain't great but is survivable.
>
> I'm considering using good 6-32 plastic screws to mount the fets to
> the cooler.
>
>
I expect they will stretch when heated, so you might see increasing
failures over time. Not an easy problem...

On 31/03/2024 15:53, John Larkin wrote:
> On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
> <'''newspam'''@nonad.co.uk> wrote:
>
>> On 30/03/2024 18:14, John Larkin wrote:
>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>> much benefit accrues from adding silicone grease. It's really messy in
>>> production and it's hard to confirm proper application. A little
>>> googling didn't provide hard numbers.
>>>
>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>
>> ISTR on one of the overclocking hacker CPU cooling sites someone tried
>> everything from dry to cooking oil and engine oil. The marginal best was
>> some exotic "liquid metal" silver loaded brand I have never heard of and
>> the worst by a long way was dry.
>>
>> The biggest change was from dry to some sort of heat exchange medium is
>> by preventing an air gap. It was a significant difference too.
>>
>> The problem is that your flat surfaces are not exactly flat so that the
>> direct metal contact area can actually be quite small if there is any
>> surface roughness. Air is a rather good insulator and metals don't
>> radiate well at all. Silicon grease prevents air gaps and anything
>> similar will do the same job. It is just that silicon oils and greases
>> are less inclined to evaporate or go rancid and corrode your parts.
>
> There's a lot of opinion on this but few or no numbers. Some people
> seem to think that their music sounds better, or their gaming scores
> improve, with some expensive grease.

It was quite a simple setup.

Same heatsink, same stress test and note down the CPU core temperature
at equilibrium. CPUs are convenient in already being well instrumented -
the biggest difference was nothing vs anything else.

There is an 80:20 rule at work here - you get 80% of the improvement by
eliminating the tiny air gap by wetting it out with a heat transfer
medium and the rest is incremental using ever more exotic materials.

In the extreme they still use the near lethal BeO ceramic material in
some high power RF transistors since it is second only to diamond for
thermal conductivity whilst being an electrical insulator.

https://materion.com/-/media/files/ceramics/articles/beo-still-a-force-in-rf-power-transistor-packaging.pdf

In the bad old days you used to have to be careful of TO-3 can
transistors that had blown their top for that stuff. These days they use
inferior but much safer alternatives like alumina and aluminium nitride.

You say that there are no numbers. Where have you been looking?

> A TO-220 footprint with a 100 micro-inch air gap, assuming zero
> metal-metal contact to the heat sink, calculates to 0.65 K/W. I
> wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1,
> which is still fine for my application.

If you are prepared to de-rate accordingly then there isn't really a
problem but if you want to run them at full power then they need to be
in intimate contact with their heat sink and that means wetted by some
sort of heat transfer medium. I was quite impressed with the bluetack
like stuff that came with my Raspberry Pi passive aluminium heatsink.

I found the pads more annoying to handle than silicone grease YMMV.
Getting them on square was much harder than just adding a dab of goo.

--
Martin Brown

On 1/04/2024 7:45 am, wmartin wrote:
> On 3/31/24 12:33, John Larkin wrote:
>> On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>>
>>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>>> wrote:
>>>
>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>> production and it's hard to confirm proper application. A little
>>>> googling didn't provide hard numbers.
>>>>
>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>
>>> Silpads. I used silpads since the 80's. I don't know if they are better
>>> or worse than grease, but they are good enough and most importantly they
>>> are consistant. What happens with testing on the prototypes, happens
>>> with
>>> production units. Shorts were always with the little vinyl grommets, you
>>> use to keep the screws from shorting to the part. You can squeeze those
>>> down to tight and cause a short.
>>
>> Right, I want to avoid the mess and uncertainty of grease.
>>
>> Bergquist has a TO-220 pad, their 1500ST material, that should be
>> about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
>> which ain't great but is survivable.
>>
>> I'm considering using good 6-32 plastic screws to mount the fets to
>> the cooler.
>>
> I expect they will stretch when heated, so you might see increasing
> failures over time. Not an easy problem...

It's not stretching that's the problem, but cold flow or creep "the
tendency of any solid material to move or deform over a period of time
under the influence of persistent mechanical stress, with no recovery of
shape when the stress is removed."

--
Bill Sloman, Sydney

On 31/03/2024 20:33, John Larkin wrote:
> On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>
>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>>
>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>> much benefit accrues from adding silicone grease. It's really messy in
>>> production and it's hard to confirm proper application. A little
>>> googling didn't provide hard numbers.
>>>
>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>
>> Silpads. I used silpads since the 80's. I don't know if they are better
>> or worse than grease, but they are good enough and most importantly they
>> are consistant. What happens with testing on the prototypes, happens with
>> production units. Shorts were always with the little vinyl grommets, you
>> use to keep the screws from shorting to the part. You can squeeze those
>> down to tight and cause a short.
>
> Right, I want to avoid the mess and uncertainty of grease.
>
> Bergquist has a TO-220 pad, their 1500ST material, that should be
> about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
> which ain't great but is survivable.
>
> I'm considering using good 6-32 plastic screws to mount the fets to
> the cooler.
>
>

The recommended mounting torque is about 6lb-in and the plastic screws I
am familiar with have max rated torques much less than half that.

Even if you find a plastic screw reliable for that torque including a
conical belleville compression washer might be good to accommodate
slippage and creep.

I suspect a good spring clip may be more consistent and quicker in
production.

piglet

Martin Brown <'''newspam'''@nonad.co.uk> wrote:

[..]
> If you are prepared to de-rate accordingly then there isn't really a
> problem but if you want to run them at full power then they need to be
> in intimate contact with their heat sink and that means wetted by some
> sort of heat transfer medium.

Designing audio amplifiers, it often worked out more economical to use
two output devices run gently, rather than a single one pushed to its
limits. The two thermal paths between die and heatsink are effectively
in parallel and carrying half the power each, so you can run the die
cooler [preferred] or the heatsink hotter [if you really must].

You also keep the device well below its maximum current rating where the
current gain is higher and it needs less drive power. Economies in the
drive stage and current protection may well compensate for the cost of
an extra lump of silicon and a couple of resistors.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On Mon, 1 Apr 2024 09:32:20 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 31/03/2024 15:53, John Larkin wrote:
>> On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>>
>>> On 30/03/2024 18:14, John Larkin wrote:
>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>> production and it's hard to confirm proper application. A little
>>>> googling didn't provide hard numbers.
>>>>
>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>
>>> ISTR on one of the overclocking hacker CPU cooling sites someone tried
>>> everything from dry to cooking oil and engine oil. The marginal best was
>>> some exotic "liquid metal" silver loaded brand I have never heard of and
>>> the worst by a long way was dry.
>>>
>>> The biggest change was from dry to some sort of heat exchange medium is
>>> by preventing an air gap. It was a significant difference too.
>>>
>>> The problem is that your flat surfaces are not exactly flat so that the
>>> direct metal contact area can actually be quite small if there is any
>>> surface roughness. Air is a rather good insulator and metals don't
>>> radiate well at all. Silicon grease prevents air gaps and anything
>>> similar will do the same job. It is just that silicon oils and greases
>>> are less inclined to evaporate or go rancid and corrode your parts.
>>
>> There's a lot of opinion on this but few or no numbers. Some people
>> seem to think that their music sounds better, or their gaming scores
>> improve, with some expensive grease.
>
>It was quite a simple setup.
>
>Same heatsink, same stress test and note down the CPU core temperature
>at equilibrium. CPUs are convenient in already being well instrumented -
>the biggest difference was nothing vs anything else.
>
>There is an 80:20 rule at work here - you get 80% of the improvement by
>eliminating the tiny air gap by wetting it out with a heat transfer
>medium and the rest is incremental using ever more exotic materials.
>
>In the extreme they still use the near lethal BeO ceramic material in
>some high power RF transistors since it is second only to diamond for
>thermal conductivity whilst being an electrical insulator.
>
>https://materion.com/-/media/files/ceramics/articles/beo-still-a-force-in-rf-power-transistor-packaging.pdf
>
>In the bad old days you used to have to be careful of TO-3 can
>transistors that had blown their top for that stuff. These days they use
>inferior but much safer alternatives like alumina and aluminium nitride.
>
>You say that there are no numbers. Where have you been looking?
>
>> A TO-220 footprint with a 100 micro-inch air gap, assuming zero
>> metal-metal contact to the heat sink, calculates to 0.65 K/W. I
>> wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1,
>> which is still fine for my application.
>
>If you are prepared to de-rate accordingly then there isn't really a
>problem but if you want to run them at full power then they need to be
>in intimate contact with their heat sink and that means wetted by some
>sort of heat transfer medium. I was quite impressed with the bluetack
>like stuff that came with my Raspberry Pi passive aluminium heatsink.
>
>I found the pads more annoying to handle than silicone grease YMMV.
>Getting them on square was much harder than just adding a dab of goo.

Yeah, that might need a fixture or something in production.

I'm getting quotes on custom AlN insulators, which would still need
grease.

I eyeballed several of my candidate TO-220 mosfets, The bottoms are
mirror finished but, sadly, not very flat. Placed on a flat surface,
and held edgewise with bright light in the background, I estimate
about 2 mils of daylight in places. I'll ask my machinist if he can
quantify that better, but it looks like I'll need grease or a
compliant pad. 2 mils is a lot of air.

On Mon, 1 Apr 2024 12:35:37 +0100, piglet <erichpwagner@hotmail.com>
wrote:

>On 31/03/2024 20:33, John Larkin wrote:
>> On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>>
>>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>>> wrote:
>>>
>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>> production and it's hard to confirm proper application. A little
>>>> googling didn't provide hard numbers.
>>>>
>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>
>>> Silpads. I used silpads since the 80's. I don't know if they are better
>>> or worse than grease, but they are good enough and most importantly they
>>> are consistant. What happens with testing on the prototypes, happens with
>>> production units. Shorts were always with the little vinyl grommets, you
>>> use to keep the screws from shorting to the part. You can squeeze those
>>> down to tight and cause a short.
>>
>> Right, I want to avoid the mess and uncertainty of grease.
>>
>> Bergquist has a TO-220 pad, their 1500ST material, that should be
>> about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
>> which ain't great but is survivable.
>>
>> I'm considering using good 6-32 plastic screws to mount the fets to
>> the cooler.
>>
>>
>
>The recommended mounting torque is about 6lb-in and the plastic screws I
>am familiar with have max rated torques much less than half that.
>
>Even if you find a plastic screw reliable for that torque including a
>conical belleville compression washer might be good to accommodate
>slippage and creep.
>
>I suspect a good spring clip may be more consistent and quicker in
>production.
>
>piglet
>

I've used spring clips before, but they won't work in our current
situation. I'm bolting eight TO-220 mosfets to a copper CPU cooler and
I don't have the area or the height for clips.

The Bergquist pads look OK at 20 PSI, which is only 5 pounds force on
a TO-220 footprint.

On Mon, 01 Apr 2024 09:12:38 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Mon, 1 Apr 2024 12:35:37 +0100, piglet <erichpwagner@hotmail.com>
>wrote:
>
>>On 31/03/2024 20:33, John Larkin wrote:
>>> On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>>>
>>>> On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
>>>> wrote:
>>>>
>>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>>> production and it's hard to confirm proper application. A little
>>>>> googling didn't provide hard numbers.
>>>>>
>>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>>
>>>> Silpads. I used silpads since the 80's. I don't know if they are better
>>>> or worse than grease, but they are good enough and most importantly they
>>>> are consistant. What happens with testing on the prototypes, happens with
>>>> production units. Shorts were always with the little vinyl grommets, you
>>>> use to keep the screws from shorting to the part. You can squeeze those
>>>> down to tight and cause a short.
>>>
>>> Right, I want to avoid the mess and uncertainty of grease.
>>>
>>> Bergquist has a TO-220 pad, their 1500ST material, that should be
>>> about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
>>> which ain't great but is survivable.
>>>
>>> I'm considering using good 6-32 plastic screws to mount the fets to
>>> the cooler.
>>>
>>>
>>
>>The recommended mounting torque is about 6lb-in and the plastic screws I
>>am familiar with have max rated torques much less than half that.
>>
>>Even if you find a plastic screw reliable for that torque including a
>>conical belleville compression washer might be good to accommodate
>>slippage and creep.
>>
>>I suspect a good spring clip may be more consistent and quicker in
>>production.
>>
>>piglet
>>
>
>I've used spring clips before, but they won't work in our current
>situation. I'm bolting eight TO-220 mosfets to a copper CPU cooler and
>I don't have the area or the height for clips.
>
>The Bergquist pads look OK at 20 PSI, which is only 5 pounds force on
>a TO-220 footprint.

The Bergquist pads I was looking at are theortetically 1.8 W/mK
material, but the actual TO-220 theta suggests about half that in real
life.

T-global makes pads that are rated 5 W/mK.

On Mon, 01 Apr 2024 08:16:14 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Mon, 1 Apr 2024 09:32:20 +0100, Martin Brown
><'''newspam'''@nonad.co.uk> wrote:
>
>>On 31/03/2024 15:53, John Larkin wrote:
>>> On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
>>> <'''newspam'''@nonad.co.uk> wrote:
>>>
>>>> On 30/03/2024 18:14, John Larkin wrote:
>>>>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>>>>> much benefit accrues from adding silicone grease. It's really messy in
>>>>> production and it's hard to confirm proper application. A little
>>>>> googling didn't provide hard numbers.
>>>>>
>>>>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>>>>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>>>>
>>>> ISTR on one of the overclocking hacker CPU cooling sites someone tried
>>>> everything from dry to cooking oil and engine oil. The marginal best was
>>>> some exotic "liquid metal" silver loaded brand I have never heard of and
>>>> the worst by a long way was dry.
>>>>
>>>> The biggest change was from dry to some sort of heat exchange medium is
>>>> by preventing an air gap. It was a significant difference too.
>>>>
>>>> The problem is that your flat surfaces are not exactly flat so that the
>>>> direct metal contact area can actually be quite small if there is any
>>>> surface roughness. Air is a rather good insulator and metals don't
>>>> radiate well at all. Silicon grease prevents air gaps and anything
>>>> similar will do the same job. It is just that silicon oils and greases
>>>> are less inclined to evaporate or go rancid and corrode your parts.
>>>
>>> There's a lot of opinion on this but few or no numbers. Some people
>>> seem to think that their music sounds better, or their gaming scores
>>> improve, with some expensive grease.
>>
>>It was quite a simple setup.
>>
>>Same heatsink, same stress test and note down the CPU core temperature
>>at equilibrium. CPUs are convenient in already being well instrumented -
>>the biggest difference was nothing vs anything else.
>>
>>There is an 80:20 rule at work here - you get 80% of the improvement by
>>eliminating the tiny air gap by wetting it out with a heat transfer
>>medium and the rest is incremental using ever more exotic materials.
>>
>>In the extreme they still use the near lethal BeO ceramic material in
>>some high power RF transistors since it is second only to diamond for
>>thermal conductivity whilst being an electrical insulator.
>>
>><https://materion.com/-/media/files/ceramics/articles/beo-still-a-force-in-rf-power-transistor-packaging.pdf>
>>
>>In the bad old days you used to have to be careful of TO-3 can
>>transistors that had blown their top for that stuff. These days they use
>>inferior but much safer alternatives like alumina and aluminium nitride.
>>
>>You say that there are no numbers. Where have you been looking?
>>
>>> A TO-220 footprint with a 100 micro-inch air gap, assuming zero
>>> metal-metal contact to the heat sink, calculates to 0.65 K/W. I
>>> wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1,
>>> which is still fine for my application.
>>
>>If you are prepared to de-rate accordingly then there isn't really a
>>problem but if you want to run them at full power then they need to be
>>in intimate contact with their heat sink and that means wetted by some
>>sort of heat transfer medium. I was quite impressed with the bluetack
>>like stuff that came with my Raspberry Pi passive aluminium heatsink.
>>
>>I found the pads more annoying to handle than silicone grease YMMV.
>>Getting them on square was much harder than just adding a dab of goo.
>
>Yeah, that might need a fixture or something in production.
>
>I'm getting quotes on custom AlN insulators, which would still need
>grease.
>
>I eyeballed several of my candidate TO-220 mosfets, The bottoms are
>mirror finished but, sadly, not very flat. Placed on a flat surface,
>and held edgewise with bright light in the background, I estimate
>about 2 mils of daylight in places. I'll ask my machinist if he can
>quantify that better, but it looks like I'll need grease or a
>compliant pad. 2 mils is a lot of air.

Two mils sounds quite good for such a package. Your machinist can
definitely make a more precise measurement. The official definition
of the package type likely has a flatness requirement, and it's this
that you should design for or at least be prepared to deal with.

Joe Gwinn

On 2024-04-01, john larkin <jl@650pot.com> wrote:
> On Mon, 01 Apr 2024 09:12:38 -0700, John Larkin <jl@997PotHill.com>
> wrote:
>
> The Bergquist pads I was looking at are theortetically 1.8 W/mK
> material, but the actual TO-220 theta suggests about half that in real
> life.
>
> T-global makes pads that are rated 5 W/mK.

Honeywell does 8.5 with PTM7950 etc.

It's a phase change material

--
Jasen.
🇺🇦 Слава Україні

On Sat, 6 Apr 2024 11:53:25 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:

>On 2024-04-01, john larkin <jl@650pot.com> wrote:
>> On Mon, 01 Apr 2024 09:12:38 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>>
>> The Bergquist pads I was looking at are theortetically 1.8 W/mK
>> material, but the actual TO-220 theta suggests about half that in real
>> life.
>>
>> T-global makes pads that are rated 5 W/mK.
>
>Honeywell does 8.5 with PTM7950 etc.
>
>It's a phase change material

I need reliable electrical insulation, so phase-change or graphite
won't do. That's another reason not to use mica... it's fragile.

All the gap-pads that we have tested have been awful, numbers like 5
degc/watt for our TO-220. None come within a factor of three of the
theta that we calculate based on their specified material thermal
conductivity. Bummer.

What really works is an AlN insulator with grease, like 0.35 K/W for
the TO-220. The silicon itself is 0.74 j-c. I guess we'll do that,
messy but effective.

At 40 watts/fet, Tj = 80 (= cooler temp) * 40 * (0.74 + 0.35) = 124c

We can't anodize the copper cooler. Hard anodize and phase change
could be another option.

On Sat, 06 Apr 2024 08:39:04 -0700, John Larkin <jl@997PotHill.com>
wrote:

>On Sat, 6 Apr 2024 11:53:25 -0000 (UTC), Jasen Betts
><usenet@revmaps.no-ip.org> wrote:
>
>>On 2024-04-01, john larkin <jl@650pot.com> wrote:
>>> On Mon, 01 Apr 2024 09:12:38 -0700, John Larkin <jl@997PotHill.com>
>>> wrote:
>>>
>>> The Bergquist pads I was looking at are theortetically 1.8 W/mK
>>> material, but the actual TO-220 theta suggests about half that in real
>>> life.
>>>
>>> T-global makes pads that are rated 5 W/mK.
>>
>>Honeywell does 8.5 with PTM7950 etc.
>>
>>It's a phase change material
>
>I need reliable electrical insulation, so phase-change or graphite
>won't do. That's another reason not to use mica... it's fragile.
>
>All the gap-pads that we have tested have been awful, numbers like 5
>degc/watt for our TO-220. None come within a factor of three of the
>theta that we calculate based on their specified material thermal
>conductivity. Bummer.
>
>What really works is an AlN insulator with grease, like 0.35 K/W for
>the TO-220. The silicon itself is 0.74 j-c. I guess we'll do that,
>messy but effective.
>
>At 40 watts/fet, Tj = 80 (= cooler temp) * 40 * (0.74 + 0.35) = 124c

Oops,

Tj = 80 + 40 * (0.74 + 0.35) = 124c

Delayed coffee effect.