Heat sink compound

[AJSOENKE]

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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[Curt Milton]

All has been well up until now, including setting the
bias per instructions.  Without PA's my bias current
was 330 mA, so I successfully set it for 730 mA with
the PA's.  

I had adjusted R26 for 5w and got the VSWR sensor
zeroed.  During my high power test (into a matched
antenna on a clear channel into a good antenna) my
VSWR is reading 1.2 on the K2 and similar on my
external meter.  When I transmit the PA devices do get
very hot (too hot to touch after about 10 seconds) but
they quickly cool (maybe around 5 seconds or less)
after transmit ceases.  

Here is the behavior i am getting.  First column is K2
power setting (on 20m), second is what the K2 reads on
transmit, and third is my meter.  My meter is not
state of the art but it is reasonable with my other
rig.  

K2 Setting        K2 Indicated       Meter

15                15                 20
20                20                 28
25                25                 30
30                23                 28
35                22                 30
40                22                 35
50                23                 32

Thinking that maybe R26 was way off, i set the K2 for
40 watts and tried to adjust R26 to read 40 watts - no
this got me no where.  

I tried to operate on different bands with two
different antennas.  Same impact on all bands, except
it has even lower output on 10m.  

Troubleshooting suggestions appreciated!  

73, Curt
   


       
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