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Re: glob-top thermals
On Tue, 12 Feb 2019 11:55:58 -0500, Phil Hobbs

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Depends on how hot the resistor gets!

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My thought is that the area exposed to air is increased a bit by the
spreading of the epoxy. A universe that is a hemisphere of epoxy is
one extreme, a thin conformal coat is the other.


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Underfill would be messy for production. If I annoy them they withhold
donuts.

I have assigned a scut bunny to do experiments, using a 1206 thinfilm
RTD as the combined DUT and thermometer.

The Vishay aluminum nitrides are an option but are hard to actually
buy. AVX and some others have AlN, also hard to buy.

High speed design often requires pushing small parts hard. I need to
know how hard is safe.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On 2/12/19 1:39 PM, John Larkin wrote:
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Well, 0.85 W/m/k is the same as ordinary paste.  You could squirt that  
underneath after soldering.

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Does his/her business card say "Scut Bunny 1st Class"?

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Well, you could make the baby board out of alumina. ;)

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Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: glob-top thermals
On Tue, 12 Feb 2019 14:18:22 -0500, Phil Hobbs

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FR4 is about 0.3 w/mk. There are PCB laminates that are 3.

But that's too easy.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
John Larkin wrote...
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 Move all bottom traces away from the resistors, etc.
 Then fatten the pads into copper-pour areas, with
 via to a pour on the bottom.  Maybe you can use a
 guard to reduce capacitance.


--  
 Thanks,
    - Win

Re: glob-top thermals
On Monday, February 11, 2019 at 9:07:59 AM UTC-8, John Larkin wrote:

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A watt for a square inch is troublesome.    Surely the board MOUNTS to
something, can you position the hot thing near a (heat-conductive) structural
element?    

Re: glob-top thermals
wrote:

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It will be a baby board that surface-mounts on a bigger board.

1 watt isn't bad for the whole baby board. It is a potential hot spot
on the little board.



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On Monday, February 11, 2019 at 12:07:59 PM UTC-5, John Larkin wrote:
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It seems to me that a glue I have an interesting circuit that unfortunately
 dumps 0.8 watts into
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It seems  to me that gluing a heat sink to the resistor would increase the  
air surface area more than the surface area of glob of epoxy .  A quick sea
rch on Aliexpress found some heat sinks that are 9 mm square  by 12 mm high
.  But there are likely problems with the glue de laminating.

                                        Dan


Re: glob-top thermals
On Wednesday, 13 February 2019 02:00:46 UTC, snipped-for-privacy@krl.org  wrote:
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ly dumps 0.8 watts into
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e air surface area more than the surface area of glob of epoxy .  A quick s
earch on Aliexpress found some heat sinks that are 9 mm square  by 12 mm hi
gh.  But there are likely problems with the glue de laminating.
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I was thinking little aluminium rectangles bent into a |__| shape, but just
 a bit of very thin flat ali would help, with a stiff insulating glue layer
. Doesn't need much conduction to improve things.

Or you could go another way & solder a loop of copper wire over the top thr
ough hole. That might even work.


NT

Re: glob-top thermals
On Tuesday, February 12, 2019 at 6:00:46 PM UTC-8, snipped-for-privacy@krl.org wrote:
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True; a glob, too, might encourage streamline airflow (which is BAD for heatsinking).
The 'big parts' nearby could create eddies at their corners, which would
help, and a bristle of fins adds conduction offboard to the mix.

Re: glob-top thermals
On Wednesday, 13 February 2019 11:03:46 UTC, whit3rd  wrote:
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a top heatsink would also conduct laterally, moving a bit of heat from the centre hotspot to the ends, resulting in better board heatsinking.


NT

Re: glob-top thermals
On Wed, 13 Feb 2019 03:08:03 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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An aluminum nitride slab could be glued to the top side of the parts
and suck a lot of heat out. That would make the board into sort of a
mysterious module.

That would be cool but a lot of work. Well, maybe not so much work.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: glob-top thermals
John Larkin wrote...
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 What glue would you use?  Removeable?


--  
 Thanks,
    - Win

Re: glob-top thermals
On 13 Feb 2019 07:41:38 -0800, Winfield Hill

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No, real epoxy. The idea of the little GaN modules is that they are
throw-away components.

I have a source for cheap custom AlN pieces.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: glob-top thermals
On Wednesday, 13 February 2019 16:31:21 UTC, John Larkin  wrote:
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I agree that a bit of ali would make more sense. Use a copper wire loop & you don't need to glue anything on.

I still don't know why you're not leaving more cu on the board. I guess you need ultra low C.


NT

Re: glob-top thermals
On Wed, 13 Feb 2019 13:47:24 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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This is picosecond stuff. On a 4-layer board, with L2 ground plane,
each square inch of L1 trace will be about 50 pF. And fast parts are
tiny and get hot.




--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
onsdag den 13. februar 2019 kl. 23.43.59 UTC+1 skrev John Larkin:
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abuse an smd connector as heatsink?  

http://zierick.com/pages/sm_ipct_1307.php




Re: glob-top thermals
On Wednesday, February 13, 2019 at 10:32:17 AM UTC-5, John Larkin wrote:
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Interesting approach.  Look at one end of the spectrum by using a thermally insulating epoxy as a very ineffective heat sink then careening in the other direction to consider using a rather more exotic and expensive material.  

Oh well, he'll come back to center eventually.  Aluminum is pretty nice stuff really.  Thermal pad material is easy to use.  

Rick C.  

Re: glob-top thermals
On Mon, 11 Feb 2019 09:07:49 -0800, John Larkin

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Our mechanical guy wants to get one of these syringe-fed dispensing
pumps

http://www.gpd-global.com/co_website/pcdpumpseries.php

and chuck it into our Tormach n/c milling machine, and glob-top a
whole panel of tiny boards at once. Or several panels.



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On Thursday, 14 February 2019 14:52:41 UTC-5, John Larkin  wrote:
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You could dispense clear epoxy, which has some interesting applications.  

Eg.
https://5.imimg.com/data5/PK/AW/MY-992394/dome-labels-500x500.jpg

--Spehro Pefhany  



Re: glob-top thermals
wrote:

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I've seen that on something; it looks cool.

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The really good glob-top 1-part epoxy is black and shiny and very
hard.

We want to hide things too!


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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