[GreenKeys] Fwd: [OT] Watch That (Thermal) Grease!

[Duncan Brown]

Beware!  Thermal conductivity grease designed for computers is not 
usable with high voltages:


-------- Forwarded Message --------
Subject: 	[Wireless-History] Watch That Grease!
Date: 	Wed, 29 May 2024 19:52:09 -0700
From: 	Joe Stoltz K2AEI <wb2iid at frontiernet.net>
Reply-To: 	Wireless-History at groups.io
To: 	Wireless-History at groups.io



Recently I helped a gentleman troubleshoot his AWA Battery Eliminator 
board. He had completed assembly of the supply and was experiencing some 
problems with it. On powering up the board there were audible crackling 
sounds emanating from the supply as well as resistors overheating.

To cut to the chase, the problem was with the heat sink grease or 
thermal compound used to mount the high voltage regulator transistors. 
(I call it grease because when I started in electronics in the early 
1970s plain, transparent silicon grease was used on the thermal 
interface to heat sinks.)

At low AC input voltages everything was fine but as I approached 100V 
input the crackling began. I was also able to make the crackling sound 
come and go by wiggling the heat sinks. There were no cold solder 
joints, broken leads or any other mechanical defects.

Dismounting the series pass transistors, wiping away all traces of the 
original silvery grey thermal compound, then remounting with white 
Wakefield compound cured the problem.

The only plausible explanation is that the thermal compound broke down 
above a certain voltage level.

Doing some research, I found a number of "CPU Cooler" thermal compounds 
available on the web. Apparently many of these can have a silver content 
to aid in thermal conductivity. Volume resistivity specs look very good 
but dielectric strength is not mentioned.

The "good old" white paste was merely zinc oxide suspended in silicone 
grease. The builder did not know what brand or type compound he used.

Lesson gained is - if building projects where heat sinking of active 
devices is required, or repairing existing solid state equipment - 
choose your thermal compound carefully. For applications where all 
mating surfaces are at ground potential and no insulators are needed, 
the new CPU Cooler compounds may be fine.

For power supply applications where TO-220 series pass transistors are 
mounted to heat sinks (the AWA Battery Eliminator, Capacitor Checker and 
HV Power Supply boards come to mind) make sure you use the right (white) 
grease.
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