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Even heating across a 4" x 4" PCB Heater

Anybody have some experiences they would like to share regarding the designof film heaters? I am working on (with?) one that is heating to a modest 37C but needs to be plus or minus half a degree Centigrade across the 4" x 4" surface. My contention is that the trace (and therefore heat density) needs to be higher at the edges than the center.

My other contention is that if you are going to design a heater a complete analysis of everything the heater is contact with is in order.

My contentions remain contentious if not in actual contempt as of this writing ;-)
 
G

George Herold

Jan 1, 1970
0
Anybody have some experiences they would like to share regarding the design of film heaters?  I am working on (with?) one that is heating to a modest 37C but needs to be plus or minus half a degree Centigrade across the 4" x 4" surface. My contention is that the trace (and therefore heat density) needs to be higher at the edges than the center.

My other contention is that if you are going to design a heater a complete analysis of everything the heater is contact with is in order.

My contentions remain contentious if not in actual contempt as of this writing ;-)

Hmm, OK I would guess that for a thin plate (pcb?) the heat loss would
not have much of an edge effect. So I'd try a uniform heater first.
A heat spreader might help... can you put 2 oz. copper on one side?
(Sorry, is this a pcb?) But we really need more info on the heat
loss... is the plate in air, vacuum, is there some differential heat
load from one side to the other?

George H.
 
You're mostly right, but heat flow is complex and peverse. Air

flow/convection is worse. 0.5C over that area will be difficult.



I assume that you are using the heater to heat some object, not just

heat itself.



You could do zone heating (multiple sensors and heaters) or bond the

heater to an aluminum block to reduce lateral thermal resistance.



https://dl.dropbox.com/u/53724080/Thermal/Brick/Brick1.JPG



https://dl.dropbox.com/u/53724080/Thermal/Brick/Brick2.JPG









--



John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

Custom timing and laser controllers

Photonics and fiberoptic TTL data links

VME analog, thermocouple, LVDT, synchro, tachometer

Multichannel arbitrary waveform generators

Perverse is a nice way of putting it ;-)

The heater is part of a PCB that has other functions that require not having electrically conductive material in certain regions because there are proximity sensors for things moving in the z with respect to the x y of the heater.

The heater sits below a 0.12" thick plastic plate which acts as a heat spreader but is really there to provide material for slots that engage rails onthe "thing" that gets heated. The "thing" is surrounded on three sides bymore plastic and a PCB on top that does "other stuff.

We are getting +or- 1.3C in the regions of interest. Would prefer to use asingle heater. I say FEA (Finite Element Analysis) by hand or in SW is the next (should have been first) step)

Thanks for having a look and sharing thoughts.

EdV
 
G

George Herold

Jan 1, 1970
0
The "no electrically conductive" rule kind of boxes you into a corner.

I agree that you need uneven heating to get even temperature -- to me,
your uneven temperature with even heating shouts that out.

Unless your board is going to sit in still air at a constant temperature,
and you can wait a good long time for things to settle to equilibrium, I
don't think a shaped heater is going to do the job -- I think you need
some sort of a zone heater.

Maybe go shopping for a plastic material that's more thermally conductive
than what you've got?  I don't think you'll find joy, but maybe.
Depending on your proximity sensor technology, maybe something that's
metal-filled, but still not conductive (if such things exist -- I'm not
sure if it would).

I assume a 4 x 4 x 0.12 plate of BeO would be out of the question, and
might not work for you anyway.

But perhaps a piece of aluminum oxide (Al2O3), see what Coorstek has
in stock. Is this a 'one of' or a production thing?
Can you put a metal heat spreader on the back side of your plastic block?

End the end, I think you're going to have to accept either failure or
zone heating.

Maybe a nested thermal box 'thingie' would work.

George H.
 
G

George Herold

Jan 1, 1970
0
Plastic is a terrible heat spreader. Get some metal in there.

If there's an air gap between "thing" and the spreader, and between
thing and the PCB above, the temperature at any point on thing will be
some sort of weighted average of the bits above and below. Does thing
dissipate any power? Does the PCB above dissipate much power? Does it
have hot spots?

--

John Larkin                  Highland Technology Incwww..highlandtechnology.com  jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators- Hide quoted text -

- Show quoted text -

Hey, I was just remembering these ads by (panasonic?) selling
pyrolytic graphite sheets. That would make some nice thermal
spreaders... something like 3x copper along the plane of the sheets
(IIRC). I guess it might be too electrically conductive for the OP.
(It'd be fun to play with some.)

George H.
 
T

Tim Williams

Jan 1, 1970
0
Jeff Liebermann said:
The solution turned out to be thermally conductive alumina sand. The
potting shell was filled with the sand through a large hole in the
bottom PCB, shaken to distribute the sane, and sealed with epoxy. The
sand kept the temperature even across the devices on the PCB. An
added benefit was that the board could be easily de-potted, sand
removed, and repaired.

Seems like a bad idea: a few years of vibration later and it's
vibratory-tumbled its way out of existence!

I was hoping your story finished with "then soaked the sand with epoxy".
Maybe you tried, and it didn't work? Low viscosity epoxy (usually with
little to no filler of its own) does exist; vacuum would probably be
required to fully impregnate it. But vacuum probably would've come in
handy for the original problem (with potting alone), too.

Tim
 
T

Tim Williams

Jan 1, 1970
0
Have you tried aluminum core or heavy copper PCBs?

Aluminum core kind of sucks in some aspects, but it's my understanding
that manufacturers can make them with vias and multiple layers. Just
maybe not cheaply?

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Anybody have some experiences they would like to share regarding the
design of film heaters? I am working on (with?) one that is heating to a
modest 37C but needs to be plus or minus half a degree Centigrade across
the 4" x 4" surface. My contention is that the trace (and therefore heat
density) needs to be higher at the edges than the center.

My other contention is that if you are going to design a heater a complete
analysis of everything the heater is contact with is in order.

My contentions remain contentious if not in actual contempt as of this
writing ;-)
 
G

George Herold

Jan 1, 1970
0
Seems like a bad idea: a few years of vibration later and it's
vibratory-tumbled its way out of existence!

I was hoping your story finished with "then soaked the sand with epoxy".
Maybe you tried, and it didn't work?  Low viscosity epoxy (usually with
little to no filler of its own) does exist; vacuum would probably be
required to fully impregnate it.  But vacuum probably would've come in
handy for the original problem (with potting alone), too.

Tim

In the deep past I've mixed Al oxide with epoxy, (400 grit maybe?)
but mostly just to thicken it up. (We dripped it over cheap connectors
to make them leak tight... (10^-5 torr level))

George H.
 
N

Nico Coesel

Jan 1, 1970
0
Jeff Liebermann said:
Any chance the PCB will be potted?

In a past life, I was designing radios next to a group of engineers
working on "modular products", all of which were potted. At one
point, there was a problem getting uniform temperature distribution
across the PCB, which I vaguely recall was about the same size.
Filling the potting shell with epoxy was the standard method, which
did not work, due to bubbles, voids, and poor thermal characteristics.
Fluorinert worked, but nobody wanted to deal with the high cost and
leakage problem.

The solution turned out to be thermally conductive alumina sand. The
potting shell was filled with the sand through a large hole in the

Nowadays there are potting compounds with better thermal conductivity.
 
T

TheGlimmerMan

Jan 1, 1970
0
Nowadays there are potting compounds with better thermal conductivity.

You do not need to pot it. A thermal pad and proper case thickness
will homogenize and carry off the heat as best it can. Potting only
reduces serviceability and also usually changes some of the circuit
operational characteristics.
 
R

rickman

Jan 1, 1970
0
I did some digging for typical thermally conductive epoxy potting
compound. For comparison, solid aluminum oxide is 30 W/mK and copper
is 4.0 W/mK.
MG Epoxy 832TC 0.682 W/mK
Epoxy Set EC-1015 broken link
Epoxy Set EC-1006M-4 0.634 W/mK
TimTronics 8006M-4 0.63 W/mK
etc.
Methinks 0.6 W/mK should work. ....snip...

The original plan was to get 0.5C across a 16 sq-in area. That's not
easy. I suspect it can't be done except perhaps with liquid coolant.
For example, water is 0.6 W/mK. Getting a uniform head distribution
will probably be impossible unless the entire assembly is inside an
insulated box to prevent radiation losses. A silicon pad is great for
distributing the head on the circuit side of a PCB but not so great on
the component side. Since all the heat is generated on the component
side, putting a heat spreader on the opposite side is not going to be
very useful.


Whoa! If you can get 30 W/mK out of solid alumina, wouldn't that work
as a perfect heatspreader? Put the heatspreader between the heater and
the constant temp unit and that should provide the heat uniformly as
well as equalize the temperature of the unit needing the constant temp.

I guess we don't know exactly what it is that needs the constant
temperature. If it can be mated well to a solid block of alumina that
sounds great. But then I've never heard of an alumina heat sink for
computers so I expect there is something wrong with this idea.

Rick
 
R

rickman

Jan 1, 1970
0
Actually, you do. The ceramic substrate used in hybrids and power
amplifier blocks are aluminum oxide or aluminum nitride. Thick film
resistors and conductors are silk screened and fired onto the
substrate. Power devices are attached to the substrate with various
methods (reflow, glue, etc). The substrate carries away the heat to a
bonded aluminum heat sink.
<http://kokomogmch.com/kokomo-thick-film-printing/substrate-base-materials/>
Heat conduction is so good, that rework with a soldering iron is
difficult. Here's the not so wonderful results of my attempts to fix
a metal migration problem on a 50 watt VHF power amp:
<http://802.11junk.com/jeffl/pics/drivel/power-amp.jpg>

So does that mean I'm wrong twice? lol

Is Aluminum Nitride non-conductive? I know stainless is conductive.
How do they use that as a substrate?

Rick
 
T

TheGlimmerMan

Jan 1, 1970
0
I guess that explains why we don't see solid alumina heat sinks...

Rick

That is why it was mainly only HV insulative focus medium (LV stuff got
thin "pads") Used mostly as a "thin" "transfer interface" for HV because
it had a much higher punch through voltage than thin pads do, and had
enough thermal capability for the task across a thin cross sectional
thickness.

A REALLY good one is simple "hard anodization" on Aluminum. It is
electrically non-conductive, and quite good at passing thermal energy
though its matrix. There are three standardized levels of it, and the
thickest one can even do "HV" up to a certain voltage, but was never
trusted for that due to the occasional pin-hole, and that won't do for
mission critical designs.

I'll bet those new skillet coatings would be pretty cool (or hot)
interfaces. Maybe not.
 
Anybody have some experiences they would like to share regarding the design of film heaters? I am working on (with?) one that is heating to a modest 37C but needs to be plus or minus half a degree Centigrade across the 4" x 4" surface. My contention is that the trace (and therefore heat density) needs to be higher at the edges than the center.



My other contention is that if you are going to design a heater a complete analysis of everything the heater is contact with is in order.



My contentions remain contentious if not in actual contempt as of this writing ;-)

Thanks! I remember working with Minco in the late 80s on a "profiled" heater. I was sure they had a nice summary of the design process somewhere.

EdV
 
Anybody have some experiences they would like to share regarding the design of film heaters? I am working on (with?) one that is heating to a modest 37C but needs to be plus or minus half a degree Centigrade across the 4" x 4" surface. My contention is that the trace (and therefore heat density) needs to be higher at the edges than the center.



My other contention is that if you are going to design a heater a complete analysis of everything the heater is contact with is in order.



My contentions remain contentious if not in actual contempt as of this writing ;-)

Thanks much to all who have weighed in my topic. I think I have the info Ineed to move forward with a nice array of possible solutions.

EdV
 
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