glob-top thermals

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I was wondering what might be the thermal effects of lumping a heap of
glob-top epoxy on a surface-mount part.

I have an interesting circuit that unfortunately dumps 0.8 watts into
a 1206 resistor. That's pushing things a bit even by my standards.

I might be able to push stuff around to fit a 1210 or even something
bigger, but not much. The entire board is 0.77 square inches and there
are some big parts wasting space already.

The board will be glob-topped, so maybe the epoxy will spread the heat
around some.  

Experiments may be required. I could glob-top a 1206 platinum RTD and
let it self-heat and measure itself maybe.

This is nice epoxy:

https://www.dropbox.com/s/jmhyros7en61tnd/T852B_Glob_2.jpg?dl=0

PANACOL STRUCTALIT 5891 C

but the thermal conductivity is only 0.4 w/mK, still a lot better than
air.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


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

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It even has its own Youtube show.


https://www.youtube.com/watch?v=rS4TUQDpUsE





--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: glob-top thermals
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Good tunes, too.  Panacol The Disco!

Tim

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Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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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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Yeah it's hard to know if a layer of 'insulation' would help.  
I was once thinking that several layers of heat shrink tubing  
around a through hole R's might allow for higher powers.  
And though the 'insulator/ goop' is better than still air, is it better  
than convection in air?  Who knows, and may be orientation dependent.

I never tried the resistor + heat shrink tubing.

George H
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Re: glob-top thermals
mandag den 11. februar 2019 kl. 18.07.59 UTC+1 skrev John Larkin:
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http://www.vishay.com/docs/31098/rcp.pdf

Re: glob-top thermals
On Mon, 11 Feb 2019 10:18:51 -0800 (PST), Lasse Langwadt Christensen

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I've used them, but they are not stocked in the value I need (lots of
50 ohms available) and a 1206 would get awfully hot. They need lots of
copper to cool the end caps, and I don't have space for that, nor can
I afford the capacitance.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On Monday, 11 February 2019 19:04:14 UTC, John Larkin  wrote:
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FWLIW I once used electrolytic caps to cool some small germanium transistors. Just tied them together. It worked :)

Dunno if you can get some copper powder in that epoxy - not too much of course.


NT

Re: glob-top thermals
On Monday, 11 February 2019 20:08:50 UTC, snipped-for-privacy@gmail.com  wrote:

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Even better would be diamond grit. It is a good insulator, very
good thermal conductor and is surprisingly cheap.
Ideally use a mix of course and fine grades to get the best
packing density and blend it with a small amount of low-viscosity
resin.

John

Re: glob-top thermals
On Tue, 12 Feb 2019 11:41:25 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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I got some diamond-filled epoxy once. It wasn't a radically good
thermal conductor. The diamond grains are still mostly separated by
epoxy.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On 2/11/19 12:07 PM, John Larkin wrote:
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There are 1-W 0612 resistors.  I'm using these ones:

R15-100H-00, RES,, FIXED, 100.0m, 1.0%, 1.0W, 100.0p, THICK, 200.0,,
Wide 1206 hi pwr                      , 0612     , ERJ-B2CFR10V      ,
PANASONIC   , $0.1174  , 1000.0 , @EACH, DIGIKEY

Cheers

Phil

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Dr Philip C D Hobbs
Principal Consultant
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Re: glob-top thermals
On Mon, 11 Feb 2019 15:59:28 -0500, Phil Hobbs

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That only comes in 0.1R. The 0612 orientation does allow better
conduction cooling, hot spot to end caps.

Susumu has a 1 watt 1206

https://www.digikey.com/product-detail/en/susumu/HRG3216Q-20R0-D-T1/408-1893-2-ND/5762557

but it looks like they just have more pad area. I can do that with any
1206.



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On Mon, 11 Feb 2019 13:14:08 -0800, John Larkin

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There are some 0612s in my resistance range, rated 3/4 watts.

https://www.digikey.com/products/en/resistors/chip-resistor-surface-mount/52?k=&pkeyword=&sv=0&pv2085=u19.6+Ohms&pv2085=u20+Ohms&pv2085=u20.5+Ohms&pv2085=u21+Ohms&pv2085=u21.5+Ohms&pv2085=u22+Ohms&pv2085=u22.1+Ohms&pv2085=u22.6+Ohms&pv2085=u23.2+Ohms&pv2085=u23.7+Ohms&pv2085=u24+Ohms&sf=1&FV=ffe00034%2C142c199b%2C142c0367&quantity=&ColumnSort=0&page=1&pageSize25%


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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On Mon, 11 Feb 2019 13:18:24 -0800, John Larkin

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Now I'm thinking that two 0805s in parallel will be a lot better than
one 1206.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
John Larkin wrote...
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 Or series.  Maybe you can separate them.
 I'm using CRCW1210-HP e3, rated 0.75W.


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 Thanks,
    - Win

Re: glob-top thermals
On 11 Feb 2019 14:06:28 -0800, Winfield Hill

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Good idea, series: less capacitance.


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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: glob-top thermals
On 2/11/19 6:09 PM, John Larkin wrote:
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Needs a bunch of copper in the middle though.

Cheers

Phil Hobbs


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Dr Philip C D Hobbs
Principal Consultant
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Re: glob-top thermals
On Tue, 12 Feb 2019 10:11:08 -0500, Phil Hobbs

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How about this:

https://www.dropbox.com/s/ahk32q2qrzuv0lx/Z458_0805s.jpg?dl=0

That roughly multiplies the pad areas by 5x. I'm thinking that the
glob-top epoxy will pull heat out of the resistors and the topside
traces and spread it out some to the surface. Heat will also go down
through the FR4 to the ground plane, layer 2.

Capacitances shouldn't be bad. L2 helps isolate things.

Using two 0805s spreads out the heat a bit too.

I still wonder what glob-top does to the hot spot on a resistor. Maybe
I'll do the thinfilm RTD experiment, or get a scut bunny to do it.






--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: glob-top thermals
On Tuesday, 12 February 2019 15:54:11 UTC, John Larkin  wrote:
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There's a lot of copper missing on layer 1 that could remain there. And on other layers IIRC.

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There needs to be a tiny heatsink one can pick & place onto those sm parts.  


NT

Re: glob-top thermals
On Tuesday, February 12, 2019 at 10:54:11 AM UTC-5, John Larkin wrote:
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Why not use a thermally conductive layer of glob or a thermal pad spreading
 the heat to a larger amount of glob.  You've used thermal pads before.  Bu
t then I guess they need to be pressed into good contact which would requir
e some sort of clamp.  How about a small aluminum sheet on top mounted to y
our screw holes?  

  =================  Al Sheet
    =============   Thermal Pad
     [===] [===]     Resistors
  =================     PWB

Yeah, instead of a very, very easy glob top that doesn't conduct well, use  
something a little harder that conducts very well.  

Or you could use a little math to calculate the thermal conductivity of the
 glob top.  I expect this is not a case where more is more since the conduc
tivity is so poor, but the math will tell all.  

Rick C.

Re: glob-top thermals
On 2/12/19 10:54 AM, John Larkin wrote:
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I don't think I'd want to rely on it, because it seems likely to  
delaminate at some stage on account of the thermal shear stress.

The alumina substrate of the resistor is about 35 W/m/K, so going to  
0.85 for lateral heat conduction on the top probably won't change the  
temperature much unless there's some aluminum or something on the other  
side.

Maybe you could extend the pads towards the middle of the resistor and  
use the epoxy as underfill.

Cheers

Phil Hobbs


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Dr Philip C D Hobbs
Principal Consultant
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