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aluminum heatsink anodizing

J

Jamie Morken

Jan 1, 1970
0
Hi,

Does an anodized or painted aluminum heatsink perform better thermally
than a bare aluminum heatsink for the situation of to-220 components
attached to the heatsink with sil-pads?

cheers,
Jamie
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
Does an anodized or painted aluminum heatsink perform better thermally
than a bare aluminum heatsink for the situation of to-220 components
attached to the heatsink with sil-pads?

No simple answer.

I'd advise against paint since it forms a poor-thermally conductive layer.
Black finish only aids *radiative* heat transfer and is often the lowest
factor incolved.

If the heatsink is fan blown, then the main heat transfer mechanism is
conduction, not radiation or convection anyway.

'Sil-Pads' vary from utter CRAP in their thermal conductivity ratings to
utterly STUNNING.

So how about some specifics. Your question is far too wildly generalised
to make much sense.

Graham
 
E

Eeyore

Jan 1, 1970
0
John said:
Sil-pads are so bad, it doesn't matter how you finish the heat sink.

Try some of the Warth and Bergquist pads and take your words back ! NOT the
grey ones. Look at the white, green and 'rust' coloured ones. They are
STUNNING and conform *perfectly* to imperfect surface finishes, thus
ensuring even better heat conductivitity over the likes of Thermapath Al2O3
grease. Not sure about how they compare to the silver particle grease,
never done a comparison.

Read Motorola's AN1040.
http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF

Having said that I'd rather have a live heatsink with direct collector to
heatsink contact any day. Even so, you have to fill the micro-gaps.
Insulating a heatsink is easy.

Graham
 
E

Eeyore

Jan 1, 1970
0
Jamie said:
Does an anodized or painted aluminum heatsink perform better thermally
than a bare aluminum heatsink for the situation of to-220 components
attached to the heatsink with sil-pads?

Hint: Understanding good heatsinking practice and methods is science in
itself.

Few people get beyond starter level which is why so many goods fail early.

Graham
 
N

NoSPAM

Jan 1, 1970
0
Eeyore said:
If the heatsink is fan blown, then the main heat transfer mechanism is
conduction, not radiation or convection anyway.


I beg to differ here, You have forced convection with a fan rather than
natural convection without one, leading to improved heat transfer. If you
really need to dissipate an enormous amount of heat from a small area, look
at the Thompson CSF hypervapotron technology, a specialized form of
nucleate boiling, used to cool extremely large transmitting tubes. They
have achieved a heat transfer of 3000 watts per square centimeter surface
using water as the boiling liquid.

To go back to the original question, anodizing the surface of the heatsink
will have little effect on the heat transfer while providing a scratch
resistant, electrically insulating surface. Painting the heatsink will
only lessen the heat transfer. At the temperatures normally found with
semiconductor electronics, radiation heat transfer is usually negligible.
For example a perfectly emitting (black body) object at 150 C radiating to
a perfectly absorbing evacuated room at 20 C, the heat flux will be
approximately 0.14 watts per square centimeter. If you have to insulate
your TO-220 device from the heatsink, beryllium oxide provides an excellent
electrical insulator which has a thermal conductivity nearly that of
aluminum. Of course, BeO is extremely toxic if you should break one.

73, Dr. Barry L. Ornitz WA4VZQ
 
A

Archimedes' Lever

Jan 1, 1970
0
Sil-pads are so bad, it doesn't matter how you finish the heat sink.

John


I would use a level 3 hard anodize for electrical resistance, and use
silver filled IC chip bonding epoxy to nail the part to it. Less
serviceable, but passes the thermals as best as can be done while
maintaining no electrical contact.
 
A

Archimedes' Lever

Jan 1, 1970
0
Having said that I'd rather have a live heatsink with direct collector to
heatsink contact any day. Even so, you have to fill the micro-gaps.
Insulating a heatsink is easy.


Use silver filled hard setting chip bonding epoxy for the absolute best
result. That will fill all the gaps with a highly thermally conductive
metallic media.
 
P

Phil Allison

Jan 1, 1970
0
"Jamie Morken"
Does an anodized or painted aluminum heatsink perform better thermally
than a bare aluminum heatsink for the situation of to-220 components
attached to the heatsink with sil-pads?


** When you need good heatsink performance with TO220 pack devices - avoid
sil-pads entirely !!!!!!

Use thin, mica insulators with a smear of white (usually zinc oxide loaded )
thermal grease each side - not only does this conduct heat much better but
mica will not *compress* over time like sil-pads do and result in your
mounting bolts becoming so loose you can rotate them with your fingers.

This loss of crucial pressure ruins the performance of the heatsinking.

That said, a black anodised heatsink radiates IR energy much better than a
silvery one - so it will generally be a few degrees cooler.



..... Phil
 
E

Eeyore

Jan 1, 1970
0
John said:
No way. Sil-pads are organics, rotten thermal conductors

You're talking SHIT ! You're out of your depth. Where insulation is required,
the high tech polymer pads outperform EVERYTHING.

Graham
 
E

Eeyore

Jan 1, 1970
0
NoSPAM said:
I beg to differ here, You have forced convection with a fan rather than
natural convection without one, leading to improved heat transfer.

**** YOU !

I designed audio 2 kW amps in 2u Rack units. I know WTF I'm talking about.

Case temp rarely exceeded 80C for metal can TO-3.

Now do the sums kiddie ! Heatsinking is an expert area that clearly no-one else
here has much of a clue about. I've been studying it for over 30 years.

Graham
 
E

Eeyore

Jan 1, 1970
0
Phil said:
"Jamie Morken"

** When you need good heatsink performance with TO220 pack devices - avoid
sil-pads entirely !!!!!!

Use thin, mica insulators with a smear of white (usually zinc oxide loaded )
thermal grease each side - not only does this conduct heat much better but
mica will not *compress* over time like sil-pads do and result in your
mounting bolts becoming so loose you can rotate them with your fingers.

This loss of crucial pressure ruins the performance of the heatsinking.

That said, a black anodised heatsink radiates IR energy much better than a
silvery one - so it will generally be a few degrees cooler.

Better hint. Don't use TO-220 for serious power and use pressure clips instead
of screws. Screws used with devices with a single mounting hole lead to the
awful scenario documented in AN1040 Fig 1 !
http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF


Graham
 
A

Archimedes' Lever

Jan 1, 1970
0
You're talking SHIT ! You're out of your depth. Where insulation is required,
the high tech polymer pads outperform EVERYTHING.

Graham


It doesn't outperform hard anodized aluminum. On that, you do not even
need a pad.
 
E

Eeyore

Jan 1, 1970
0
Archimedes' Lever said:
It doesn't outperform hard anodized aluminum. On that, you do not even
need a pad.

Hard anodised aluminium is hard to control and may crack. Any post anodising
machining
operations will render it void.

You're better off with Al2O3 washers in fact.

Graham
 
E

Eeyore

Jan 1, 1970
0
John said:
I've shipped thousands of high-performance amps and controllers that
use transistors bolted or clamped directly to hard-anodized heat sinks
with just a dab of grease. If you do it right, it works every time,
and thermal performance is superb for the price. Junction capacitance
matters in our products, so we have to push the fets thermally to get
the maximum power per pF.

Hint: smart people machine the heat sink *before* they anodize it.

Who said mine need any machining ? It's a rubbish way to get a flat surface.

Graham
 
C

Capt. Cave Man

Jan 1, 1970
0
Now do the sums kiddie ! Heatsinking is an expert area that clearly no-one else
here has much of a clue about. I've been studying it for over 30 years.

Graham


You're an idiot. My last large sink product was a 1500W supply for a
CAT scanner for Philips. We used a custom extrusion and I outlined and
trained the assemblers on the proper method of assembly to achieve
perfect co-planarity across 25 installed components on a 16.5" wide sink.

The best thermal interface in the world doesn't work worth a shit if
every single device is not perfectly mated flat and evenly pressured.

My methodologies are so good that we can use spring clips to hold down
all the devices, and every unit was high reliability. They were almost
all TO-247s and there were others as well, all perfectly aligned.

Anybody with brains knows that a wave solder op is not possible for
such assemblies for these reasons. So a good product will be hand
assembled as it relates to the big guns and their heat sinks.

I believe that there are more folks here that know a lot about sinking
a heat source than you think, DonkTard.

You should not be so full of yourself. You could end up getting
flushed.
 
N

NoSPAM

Jan 1, 1970
0
----- Original Message -----
From: "Eeyore" <[email protected]>
Newsgroups: sci.electronics.design
Sent: Thursday, December 18, 2008 10:29 PM
Subject: Re: aluminum heatsink anodizing

**** YOU !

I designed audio 2 kW amps in 2u Rack units. I know WTF I'm talking
about.


No you don't. A fan blowing air across a heatsink provides for greater
heat transfer from the sink to the aiir than does the natural convection
moving the heated air away slowly. Obviously Eeyone has never blown air
across a spoonful of hot soup to cool it faster than just letting the spoon
sit in still air!
Now do the sums kiddie ! Heatsinking is an expert area that clearly
no-one else
here has much of a clue about. I've been studying it for over 30 years.

Graham


You haven't learned much or else you are using incorrect terminology and
trying to cover it up by bluster and profanity. Conduction provides most
of the heat transfer from the part _to_the_heatsink_. The heatsink
transfers its heat to the air by _convection_.

As far as experience with heat transfer, I have been designing heat
exchangers and such over 40 years. Heat transfer is a subject rarely ever
taught to electrical engineers and what little there is taught is so
simplified. Have you ever designed with heat pipes which use boiling
liquids to carry away heat using the latent heat of vaporization? I have.
And I have designed with the Thompson CSF Hypervapotron cooling technology
for a 1.5 Megawatt rotating arc reactor for coal gasification. I'l like to
see Eeyore try to get this much heat out of a box around a cubic foot in
volume.

Without looking it up via Google or other online search engine, try to
explain Nusselt and Prantl numbers and how they relate to heat transfer.
Extra credit: explain Rayleigh and Grashof numbers.

By now, Graham, you should have figured out that at least one of my degrees
is in chemical engineering, and that heat transfer is studied more by
chemical engineers than any other engineers. Mechanical engineers do study
quite a bit of heat transfer too, but they are usually only concerned with
just air and water. For some strange reasons, mechanical engineering heat
transfer texts often include dimensions to an equation based on
dimensionless numbers.

Go ahead and keep showing off you temper and ignorance. You are
reinforcing the view of most readers here that your posts are usually not
worth reading.

Barry L. Ornitz, PhD
 
P

Phil Allison

Jan 1, 1970
0
"NoSPAM"
"Eeyore"
No you don't. A fan blowing air across a heatsink provides for greater
heat transfer from the sink to the aiir than does the natural convection
moving the heated air away slowly.

** You seem to have seriously MISREAD what he said.

Obviously Eeyone has never blown air across a spoonful of hot soup to cool
it faster than just letting the spoon sit in still air!


** That is EXACTLY what his post says.

" If the heatsink is fan blown, then the main heat transfer mechanism is
conduction ...."

This refers to the heat energy being conducted into the forced air stream
and removed.

You haven't learned much or else you are using incorrect terminology and
trying to cover it up by bluster and profanity.


** Terminology often varies from one area of technology to another, one
place to another plus it varies over time as well.

In the world of *audio power amplifiers*, ones that have no fans are called
" convection cooled " while those that have installed fans are often called
" force air cooled " or just " fan cooled ".

If a number of amplifiers * without fans* are fitted into a cabinet (
called a " rack ") then normally several fans are fitted to the lower part
of that rack and force air up and through the amplifiers and out the top .

This is sometimes called " forced convection cooling " or more simply " a
fan cooled rack" .

Blowing an air stream over a hot surface might be called " assisted
convection " in some quarters - but that is no more "correct" than
saying the fast flowing air conducts the heat away.



...... Phil
 
A

Archimedes' Lever

Jan 1, 1970
0
Hard anodised aluminium is hard to control and may crack.

There are three levels of hard anodizing from a mil grade POV, and if
the metal shop offers it, they are capable of supplying it, and any
machining gets done prior to the operation.
Any post anodising
machining
operations will render it void.

Pretty dumb to even consider machining a hard anodized Al surface.
You're better off with Al2O3 washers in fact.

Absolutely not.
 
A

Archimedes' Lever

Jan 1, 1970
0
Here are 32 TO-247-like packages *surface mounted*! The nickel plated
things are actually solid copper heat spreaders. No insulators at all
of course... the heatsink is hot to the load. This is in a 17KW
peak-output MRI gradient driver. We alternate the P and N-channel
fets, which spreads the heat best for the kinds of waveforms we drive.
Three 120 CFM fans blast air through the fins.

ftp://jjlarkin.lmi.net/Amp.jpg

Ignore donkeypoop. The wronger he is, the nastier he gets.

John

Those are the same type of spring clip hold downs that we used.
 
A

Adrian Tuddenham

Jan 1, 1970
0
NoSPAM said:
... At the temperatures normally found with
semiconductor electronics, radiation heat transfer is usually negligible.

A point not often appreciated is the possible heat gain when a heatsink
is operated in bright sunshine. A surface finish which is a poor
radiator/absorber of radiant heat will work better in those
circumstances.

I have expeienced this problem with portable P.A. amplifiers used
outdoors.
 
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