Replacing Heat Sink Compound?

[aquaman8]

Hi,

I was wondering what the thoughts are regarding the need to replace old (original, 30 years old) heat sink paste on power transistors.

I have some socket mounted TO-3 case transistors mounted on heat sinks in my integrated (Kenwood KA-8006) and was wondering if the paste degrades enough over time to a point where it might insulate more than conduct heat?

What is conventional wisdom on the need/desire to remove the old paste, clean up the transistor and the heat sink and apply new paste (sparingly of course) and remount the transistor?

Thanks in advance for your opinions.

Mitch

 

[whsh93a]

Check the exposed heatsink compound (if any) around the outside of the transistor. If it is still gooey, then do not replace the original stuff. If it is cakey and flakes off, then replace it.

Some of the Kenwoods used a different type - sort of like a glue - that bonded the transistor very effectively to the heatsink. If you remove the transistor, breaking that bond, then you must remove all of the old bonding agent and either use a greaseless new insulator or add "regular" heatsink compound.

DH

 

[cademan]

Also make sure that there is enough paste so you don't short the collector side of the transistor to the metal chassis. If you can, I would also suggest using the mica spacers with the compound.

 

[EchoWars]

The mica insulator is mandatory, not optional.

 

[cademan]

Thank you EchoWars, I only mentioned that because I have come across many cheaply built receivers which didn't even use the mica insulator, but the heatsink compound only! :thumbsdn:

What a CROCK! :yes:

 

[marty59]

You'll see much more insulator compound used than really needed, and it does a fine job of collecting dirt/gunk etc. A little bit goes a long way. It's job is to aid in transfering heat from one place to another..that's all!
And yes insulators are mandatory but if the heatsink is at the same potential as the mounted component you may not see one but it never hurts to use one if you want!

 

[avionic]

Thank you EchoWars, I only mentioned that because I have come across many cheaply built receivers which didn't even use the mica insulator, but the heatsink compound only! :thumbsdn:

What a CROCK! :yes:

thermal compound is not a substitute for and insulator. If you find a transistor with just compound you will find that the heatsink is isolated from ground therefore a mica insulator is not needed.Thermal compound is strickly to provide a smooth, complete contact area for the transistor to the mica and mica to the heatsink.The more contact area the more efficiently the heatsink will draw the heat away from the transistor.

Rod does a pretty good job of explaining the proper heatsink mounting techniques in this article.
http://www.sound.westhost.com/heatsinks.htm#9

 

[tarior]

Thank you EchoWars, I only mentioned that because I have come across many cheaply built receivers which didn't even use the mica insulator, but the heatsink compound only! :thumbsdn:

What a CROCK! :yes:

Worked on a couple of McIntosh amps with no mica insulators. Like Avionic said, the heatsinks themselves are isolated from the chassis.

 

[TerryS]

Sometimes the heat sinks are hard anodized. The anodization is non-conductive, so the transistors can be mounted directly to the heat sink without an insulator. This has the advantage of keeping the transistor cooler than if an insulator is used, because the thermal resistance of the insulator plus the extra interface is higher than without it.
I worked for a small audio amp company (small company, not small amps). That's the way we did it. Not because it was cheaper to not use the insulator, in fact it cost more to get the heat sinks hard anodized than the cost of the insulators, but it helped keep the transistors cooler. Very important for transistor life and bias current tracking.

Terry

Terry

 

[2chanman]

TerryS:
The problem with relying on the anodizing for insulation is that it makes the mounting of the transistors critical because of the risk of breaking through the anodize and having a short to the heat sink occur. I wouldn't risk it unless the heat sink was fully insulated from any other potential.

 

[cademan]

In my experience, I had never seen an amplifier to not use the mica spacer, so I'll give that one to y'all! :thmbsp:

 

[aquaman8]

Rather than heat sink compound, any opinions on using "thermapad" type part? If I understand correctly, it replaces the mica electrical insulator and acts as a good thermal coupler all at the same time. I realize that there have to be compromises with such a "too good to be true" item, but was curious as to the downside of using thermapads to mount my TO-3 transistors to the heat sink.

Any thoughts?

Mitch

 

[TerryS]

TerryS:
The problem with relying on the anodizing for insulation is that it makes the mounting of the transistors critical because of the risk of breaking through the anodize and having a short to the heat sink occur. I wouldn't risk it unless the heat sink was fully insulated from any other potential.

Yep, It's gotta be done right. But the big advantage of lower thermal resistance is worth the extra effort for some designs. The biggest issue is usually the screws shorting against the sides of the holes in the heat sinks. The sharp edge of the screws will easily scratch through the anodization. We used heat shrink sleeves over the screw threads to prevent this from happening. You still had to be carefull when assembling or you could end up with a short.

My point is don't assume that because the manufacturer chose not to use mica insulators that he was motivated by cheapness. Leaving the mica off is actually more expensive in the long run, but gives better performance.

Terry

 

[pustelniakr]

Therma-pads work, but have more thermal resistance than mica/paste.

In answer to the OP, I replace the paste, as a matter of course, in all gear I restore. This is in addition to reflowing circuit board solder joints, and replacement of old 'lytic caps...

Enjoy,
Rich P