[Elecraft] Heat Sink Compound Question

[Ray]

Chuck……. Great information for the K3 Radio’s  
Ray WA6VAB   K3 


From: Charles K0MV
Sent: Tuesday, March 29, 2022 6:30 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Heat Sink Compound Question

Hi Tony,

I posted this reply last November.

This topic appeared on the list previously from last October "sticky 
thermal pads for K3 LPA Mosfets".

One of my LPA Mosfets went bad in my K3s.  I saw a comment on the 
reflector about these devices failing and I'm guessing the heat sink 
design is subject to production variation.  I used to design switching 
power supplies.  Reliability is a very strong function of device 
temperature so keeping cool is important.  I hate to take time to fix 
stuff, so try to fix it so it never fails again.  I replaced both of 
them but also improved the thermal design.  As Ray WA6VAB mentioned, I 
got mine from RF Parts.

The epitome of outstanding thermal design is the Intel I7 processor heat 
sinking.  The processor and the heat sink are polished, shiny, and very 
flat.  The best thermal compound is a silver based paste included with 
the heat sink.  It does 100 watts over a couple of square inches so very 
low thermal resistance (remarkable!). Any gaps compromise the performance.

There is a very good tutorial on this here: 
https://forum.digikey.com/t/thermal-interface-materials

I examined the bottom cover and found the surface of the metal to be 
uneven and this inhibits heat transfer (raises the thermal resistance). 
I also found the bottom cover may not be perfectly flat. Moreover, the 
typical flexible thermal pads are not as good as alternatives.

I polished the K3s bottom cover where the heat sink attachments are with 
a Dremel tool and polishing pad and automotive rubbing compound, a fine 
abrasive.  You can see it becomes more shiny and I verified that with a 
microscope.

I got a larger diameter flat washer for placing under the screw head on 
the outside of the K3s.  This places compression over a larger area to 
help heat transfer.  Digikey part number H734-ND.

Since Elecraft has different board revisions you should check that the 
tabs of the MOSFETs can be grounded.  If so, then there is a type of 
thermal pad that is very good.  From the data sheet: High-PERFORMANCE, 
cost effective thermal interface material Used where electrical 
isolation is not required, Tgon TM 800 is ideal for where electrical 
contact and thermal transfer are desired. High thermal conductivity of 5 
W/mK in Z axis and 240 W/mK in the X-Y axis.  This type not only has low 
thermal resistance across it, but also a low thermal resistance in the 
plane of the pad helping to spread the heat out.  Digikey part number 
926-1471-ND manf part number A15037-112.  Use just a very small amount 
of the silver based thermal compound.

I run FT8 so the duty cycle is 50%.  It's been several months and no 
problems (1000s of QSOs)--at least yet!

Good luck and 73,

Chuck K0MV
On 3/29/22 15:24, Tony wrote:
> All:
>
> It seems that thermal compounds are not all the same and I was 
> wondering which type is best to use for transceiver applications? I 
> understand Elecraft uses thermal pads these days, but what type of 
> compound was used prior to the switch?
>
> Just purchased a produced called MX-5 from Arctic which is commonly 
> used for CPU applications and was wondering if it can be used for HF 
> transceivers, kits etc.
>
> See: 
> https://www.amazon.com/gp/product/B08T64M68V/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&th=1
>
> Thanks,
>
> Tony
>
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-- 
Charles K0MV
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