Potting in thermally insulating foam

[Spehro Pefhany]

I learned it from some construction workers on a jobsite as a
teenager and never really cared how it work, just that it did. It
may be that the thin film over the skin keeps you from getting
microscopic cuts.

My supplier shifted to sheared from routed boards on a recent shipment
and I ended up with a fiberglass sliver stuck under a fingernail. We
call that "quality fade". Have to specify everything with those guys..

 

[John Devereux]

Yes, of course, cotton wool. Don't know why I am trying to make it
complicated.

[...]


Two thoughts:
1) As John said, *anything* solid conducts better than air. The goal
is to break up the convection currents with the minimum possible
mass. Cotton, fiberglass, and foam are all thermal bridges. So,
since it's a one-off, how about down?

Hmm, now we are getting back to Spheros raccoon fur again You'll be
making me skin kittens next!

2) OTOH, if your goal is isothermal pads, some copper heatspreading
may be in order too.

Yes, doing that too to an extent (groundplane on one side of
board). Still thinking about best arrangement.

 

[Spehro Pefhany]

At this point I thought you knew some osbscure detail about how raccoon
fur is the perfect insulator.

It keeps the little b*ggers warm enough in the winter, so it's
probably pretty good. I'd be happy to skin one for you, but the animal
rights folks get snippy about how exactly they are dispatched. The
hide can be tanned by spreading their brains on the inside.

Should be safe from the raccoon menace around here I think.

I imagine one of your bigger heat flows is going to be the ribbon
cable. Flex pcbs with narrow (like 4 mil) 1/2 oz copper traces are
pretty good thermally.

 

[[email protected]]

You should consider milling out slots to thermally isolate the
LTZ1000. One idea I've seen uses kind of a swastika design.

A copper accessory could equalize pad temps:
.--------------------.
| |
| .''''''.-. |
| / O O \ \ |
| . O O . . |
| | | |<~~~~1mm copper disc
| ' O O ' ' |
| \ O O / / |
| '------'-' |
| |
'--------------------'
kapton tape insulation


A stack of these could force isothermal lead temps too. Insulate the
leads, of course, or center them in the holes and fill with thermal
grease.

 

[John Devereux]

A copper accessory could equalize pad temps:
.--------------------.
| |
| .''''''.-. |
| / O O \ \ |
| . O O . . |
| | | |<~~~~1mm copper disc
| ' O O ' ' |
| \ O O / / |
| '------'-' |
| |
'--------------------'
kapton tape insulation


A stack of these could force isothermal lead temps too. Insulate the
leads, of course, or center them in the holes and fill with thermal
grease.

Yes that's quite cool. It's along the lines of what I am already doing
with the "thermal" groundplane, but more. That 30 oz copper thing you(?)
posted would make the groundplane way better.

 

[[email protected]]

Hi,

Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.

Anyone know any products that would let me pot things in something like
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.

One-off, hobbiest thing.

Thanks,

The spray-type expanding foam and the two-part stuff may have solvents
that outgas and cause problems. Styrofoam (or any type of plastic
foam) may cause a static problem. You might be able to use perlite or
vermiculite from the gardening center. It's very inexpensive, and a
fairly good insulator.

 

[[email protected]]

Yes that's quite cool. It's along the lines of what I am already doing
with the "thermal" groundplane, but more.

Sure. Just 2 oz (70um) copper vs. 1mm. Oh, I should mention the
"accessory" covers the pad-tops, in case that wasn't clear.

That 30 oz copper thing you(?)
posted would make the groundplane way better.

Yep, t'was me. Dunno how much they want for it, but searches showed
several pcb vendors offering "heavy copper".

 

[[email protected]]

Hi,
Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.
Anyone know any products that would let me pot things in somethinglike
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.
One-off, hobbiest thing.
Thanks,
Still air is a prety good insulator. Try putting a small plastic box around the
parts.
We make a couple of OCXOxs and temperature-controlled analog circuits, and we
put them under small deep-drawn aluminum covers. All the foam typethings that
we tried made the thermal insulation worse.
It's more thermal fluctuations due to air currents I am worried about,
than absolute performance in terms of insulation. Although more is
better there too.
I've seen the low frequency phase noise of a cheap XO go down by 5:1when a
small cover was added to block air currents. We used a plastic potting shell.
Opamp circuits can behave similarly.
Maybe some fiberglass insulation inside a box would help. The onlyfunction of
any foam or insulation is to break up convection. The material itself will be a
better thermal conductor than air.
Yes glass fibre / glass wool / rock wool sounds good. I assume theseare
all the same thing (I have only come across glass fibre over here I
think).
Try some cotton balls; my wife keeps them around for some mysteriousfemale
cosmetic use.
Yes, of course, cotton wool. Don't know why I am trying to make it
complicated.

[...]

Two thoughts:
 1) As John said, *anything* solid conducts better than air.  The goal
is to break up the convection currents with the minimum possible
mass.  Cotton, fiberglass, and foam are all thermal bridges.  So,
since it's a one-off, how about down?

Hmm, now we are getting back to Spheros raccoon fur again You'll be
making me skin kittens next!

Chic kittens wear down. Birds too.

 

[[email protected]]

Take some dishwashing liquid and rub it on your hands & foreams
before you start. Let it dry. It plugs the pores to keep the glass
out, and simply washes off when you're done.

I just insulated my basement with the pink stuff. I didn't have any
problems with itching this time. It seems they've changed the
formulation some. Still gotta wear a mask, though.

 

[mike]

Hi Mike, thanks for idea. Thermal resistance seems to be similar to the
expanded polystyrene I was looking for.

Think I am going to try the glass wool first but might get some of that
if does not work.

Thanks,

Just make sure you get the right one. The cheapest version sets HARD
and exerts enough force to bend window frames.
The slightly more expensive version is much softer.

 

[Spehro Pefhany]

Or find someone who doesn't constantly cut corners.

It's a national pastime in some countries.


Best regards,
Spehro Pefhany

 

[[email protected]]

[email protected] writes:

Hi,
Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.
Anyone know any products that would let me pot things in something like
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.
One-off, hobbiest thing.
Thanks,
Still air is a prety good insulator. Try putting a small plastic box around the
parts.
We make a couple of OCXOxs and temperature-controlled analog circuits, and we
put them under small deep-drawn aluminum covers. All the foam type things that
we tried made the thermal insulation worse.
It's more thermal fluctuations due to air currents I am worried about,
than absolute performance in terms of insulation. Although more is
better there too.
I've seen the low frequency phase noise of a cheap XO go down by 5:1 when a
small cover was added to block air currents. We used a plastic potting shell.
Opamp circuits can behave similarly.
Maybe some fiberglass insulation inside a box would help. The only function of
any foam or insulation is to break up convection. The material itself will be a
better thermal conductor than air.
Yes glass fibre / glass wool / rock wool sounds good. I assume these are
all the same thing (I have only come across glass fibre over here I
think).
Try some cotton balls; my wife keeps them around for some mysterious female
cosmetic use.
Yes, of course, cotton wool. Don't know why I am trying to make it
complicated.

[...]

Two thoughts:
 1) As John said, *anything* solid conducts better than air.  The goal
is to break up the convection currents with the minimum possible
mass.  Cotton, fiberglass, and foam are all thermal bridges.  So,
since it's a one-off, how about down?

Hmm, now we are getting back to Spheros raccoon fur again You'll be
making me skin kittens next!

Yes, doing that too to an extent (groundplane on one side of
board). Still thinking about best arrangement.

Double oven!

Double fudge brownies!

Hmm. Ideally, if the ovens and their contents were isothermal there
wouldn't be any convection. But life being what it is, a chunk of
copper in the inner box might be necessary to ensure that.

Adequately insulated, the outer box forces the inner box to the same
temp.

Interesting choices. I guess fastest-settling is an inner oven at
something less (cooler) than the LTZ die, and an outer oven cooler
than the inner, all actively controlled.

I'm not sure what matters most to John though--stability of temp, or
isothermal environment.

 

[John Devereux]

[...]

Double oven!

Double fudge brownies!

Hmm. Ideally, if the ovens and their contents were isothermal there
wouldn't be any convection. But life being what it is, a chunk of
copper in the inner box might be necessary to ensure that.

Adequately insulated, the outer box forces the inner box to the same
temp.

Interesting choices. I guess fastest-settling is an inner oven at
something less (cooler) than the LTZ die, and an outer oven cooler
than the inner, all actively controlled.

I only want to run the die at 10-15K above the max ambient (about
30'C). Otherwise long term stability is degraded. So there might not be
scope for too many levels here.

I'm not sure what matters most to John though--stability of temp, or
isothermal environment.

Isothermal environment (pads) I think, at the moment. The die has its
own integrated heater and sensor and there is an external control
circuit to accurately stabilise the die temperature. The function of the
insulation is more to stop air currents, which could result in
fluctuating pad/lead temperatures. And cause large errors due to 35uV/K
kovar-copper thermocouples. The insulation also reduces the power
requirements and should prevent external thermal gradients appearing
across the critical pads the way I am doing it.

 

[josephkk]

Take some dishwashing liquid and rub it on your hands & foreams
before you start. Let it dry. It plugs the pores to keep the glass
out, and simply washes off when you're done.

Thanks. That will be useful for handling a variety of materials.

?-)

 

[josephkk]

Hi Mike, thanks for idea. Thermal resistance seems to be similar to the
expanded polystyrene I was looking for.

Think I am going to try the glass wool first but might get some of that
if does not work.

Thanks,

Perhaps you can use rock wool in close, wrap in a plastic film, foam the
outside, and cut back to target dimensions. Better potential
reparability.

?-)

 

[josephkk]

It's more thermal fluctuations due to air currents I am worried about,
than absolute performance in terms of insulation. Although more is
better there too.


Yes glass fibre / glass wool / rock wool sounds good. I assume these are
all the same thing (I have only come across glass fibre over here I
think).

I suppose I can pull a tuft out of my ceiling just above the bench, come
to think of it!

Depends, that might be NaCo3 treated paper shreds.

?-)

 

[John Devereux]

Depends, that might be NaCo3 treated paper shreds.

Nah, it's definitely fibre glass or "rock wool" I suppose, I put it in!

And yes got rash all over my arms the first time.

 

[[email protected]]

Were you hot & sweating, or was it a cold day? Makes a big
difference.

Sure it makes a difference but I was sweating at times. When I was in
NY, I did it in the middle of the Winter and still itched for days. I
think they've changed the fiber, perhaps it doesn't break as easily,
or perhaps breaks in larger pieces. There is some marketing blurb on
the packaging about it but doesn't explain it at all.

 

[Jasen Betts]

Now you tell me. A week ago I went into the attic to add a backing
plate to an anchor that had pulled out of the ceiling on a fluorescent
in the laundry. Had to fish around in 8" batts to find it :-( After
much hand lotion and a prescription from the Doc, the itching has
finally subsided.

newer formulations have bio-soluble fibres, the itch goes in a few hours.