[Elecraft] Heat sink compound

[AJSOENKE at aol.com]

Something I thought might be helpful for builders and other techy/service 
types.
 
Last week I was working with a student in one of my computer classes and we 
were involved in fixing a MB that was having a thermal shutdown problem with 
the CPU.
 
It is a late model device with an Intel P4 3GHz that is mounted on a MCI MB 
(not really pertinent)
 
The CPU was getting up to around 80C and shuting down, so we decided to lift 
the cooling fan and CPU to check out the contact area. problem was a broken 
finger on the plastic grip in one corner had snapped. (You'd think intel could 
supply slightly better hardware for a $300 CPU). There was a lot of dust, etc 
so we decided to clean of the heat sink compound and replace with fresh 
compound. We did a thorough cleaning of the compound, a type which is commonly found 
on computer CPUs, a silver based compound that has high thermal conductivity.  
Alas, when we went to the storage cabinet in the classroom, we found the 
compound we usually have on hand, a similar silver filled compound was all used 
up. So we took out the alumina thermal grease we use for transistor packages and 
applied this. We uaed to use this all the time on the older 386s and 486s. We 
removed the broken finger on the cooling fan grip and replace with a tye-wrap 
that was small enought to fit. Everything worked well and brought the 
temperature down to about 38C, which was acceptable. 
 
The next day, I decided to grab the heat sink compound I had on my home 
workbench and take it to class with me.  We were going to do an experiment, because 
I had been thinking about this and bouncing it off of 40 years experience in 
aerospace electronics and satellite assembly.  I knew the silver filled 
compound had better thermal transfer characteristics than the Alumina filled silicon 
grease compound. In assembly of most space electronics you cannot use these 
compounds and rely on the solid film tpes when needed.
 
We decided to repeat the compound replacement on the computer, using the more 
commonly used silver filled on the CPU. After replacing and reassembly we 
found that the CPU temp was slightly lower at about 32C aaginst a room ambient of 
24C.  About what we expected to see.
 
OK where am I headed??? There is a significant difference between the two 
compounds that has little to do with the thermal properties, but has a little to 
do with the slight improvement of the metal filled compound. . . it is also 
ELECTRICALLY conductive.
 
My cautionary note here relates to those of you who might opt to use the 
silver filled material on a power transistor application.  Unlike the computer 
CPU, most power transistors have power and signal voltages on the collector or 
source/drain. IF the heatsink is part of the circuit and the entire side of the 
circuit is grounded, there is probably no reason for concern. But, if it is 
isolated and is "hot electrically",  or there is a mica or other insulator 
between the Device and the heatsink/mounting surface, you could cause a short 
circuit by using any heat sink compound that is not an electrical insulator.
 
I hope this is helpful and maybe saves someone from a shocking experience.
 
73 de WA6VNN,  Al



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