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Old 27th Oct 2020, 12:03 am   #41
Graham G3ZVT
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Default Re: Heatsink compound - refresh or not?

Clearly I was sitting at the back not paying attention.
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Old 27th Oct 2020, 12:37 am   #42
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Default Re: Heatsink compound - refresh or not?

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Hi Maarten.
I was aware about the pink insulating material on magnetrons which I believe is beryllium oxide.
It's probably coloured alumina.

Alumina is naturally a very white-white. BeO has a cream/grey tinge to it. Pink is deliberate colouring for whatever reason.

Magnetrons in normal use have their anodes grounded and need no electrical insulation in their main heat path. Their filament is floated at the high negative voltage powering the device. In cookers, they are very cost-sensitive, and BeO is expensive.

The main places you find BeO is in RF power transistors (inc in cell phones) and RF power transmitting valves like the 4CX250B and its larger bretheren.

BeO insulating washers get used occasionally when someone is desperate to get the last bit of thermal conductivity short of diamond. So they're not at all common and could turn up anywhere.

The prestel terminal power supply is surprising, someone must have been very desperate. It was probably before the risks of BeO were generally known and the cost went up due to the stink about what the BeO plants were doing to their environment.

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Old 27th Oct 2020, 12:49 am   #43
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I'm keeping my fingers crossed that I never encounter a radiographer who gets barium and beryllium confused.
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Old 27th Oct 2020, 12:53 am   #44
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Default Re: Heatsink compound - refresh or not?

We used to laugh at the old advert "Relax in a Radox bath" and substitute Radon for Radox....Ah we were young and simple. Now I'm older.....
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Old 27th Oct 2020, 1:23 am   #45
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Default Re: Heatsink compound - refresh or not?

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I'm keeping my fingers crossed that I never encounter a radiographer who gets barium and beryllium confused.
Barium is pretty toxic too according to Wikipedia, Not sure if I would want to make a meal of it too often.
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Old 27th Oct 2020, 12:04 pm   #46
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Default Re: Heatsink compound - refresh or not?

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Pink is deliberate colouring for whatever reason.
I think alumina containing chrome has slightly different physical properties. I don't think chrome or whatever the material, was added just for the colour.

With regards to the Prestel, if it's inside the LM323K it would be inside all LM323K of the same brand. It could be used for mounting it, but that would surprise me a bit.

Last edited by Maarten; 27th Oct 2020 at 12:09 pm.
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Old 27th Oct 2020, 12:40 pm   #47
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Default Re: Heatsink compound - refresh or not?

I am sure that there was once a TO3 packaged regulator on sale that had either 3 or 4 pins as opposed to the usual two. It had the beryllium warning on it with plenty of safety bumph. I never bought one as they were too expensive.
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Old 27th Oct 2020, 12:56 pm   #48
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Default Re: Heatsink compound - refresh or not?

Just found this post, apologies for being late to the party.

As per the main question, I would only renew the compound if components were to be replaced or there is any sign of the old stuff going hard/powdery over time.

For the record (pun or not, make what you will) I have never had to touch the transistors or compound on an old Pioneer SA-540 amp for the length of time of owning it (more than 10 years), it has seen countless hours of use, even at high volume but appears to not want to die any time soon.

A great way of telling if the compound is getting knackered would be by the colour, white is good, yellow is on the verge of going to dust (powdery residue).
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Old 27th Oct 2020, 1:54 pm   #49
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Default Re: Heatsink compound - refresh or not?

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Originally Posted by Glowing Bits! View Post
Just found this post, apologies for being late to the party.

As per the main question, I would only renew the compound if components were to be replaced or there is any sign of the old stuff going hard/powdery over time.

For the record (pun or not, make what you will) I have never had to touch the transistors or compound on an old Pioneer SA-540 amp for the length of time of owning it (more than 10 years), it has seen countless hours of use, even at high volume but appears to not want to die any time soon.

A great way of telling if the compound is getting knackered would be by the colour, white is good, yellow is on the verge of going to dust (powdery residue).
This was my whole point at the very beginning of the thread, lots of people say that, but is it actually true? What evidence is there that when the heatsink compound goes hard that it is no longer as efficient as when it was applied? In fact, people on here are saying that it's the solid particles of the compound that provide the heat transfer. If that's the case then it being hard (solid) should not be a problem so long as the mechanical joint is still sound ie tight enough, and that the compound is still in place and has not literally fallen out of the joint - hard to imagine.

On the subject of examples, I've got high power receivers where the heatsink compound has not been changed in 45 years and they work perfectly well, or at least, the power transistors are not going pop.
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Old 27th Oct 2020, 2:22 pm   #50
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Default Re: Heatsink compound - refresh or not?

The oil content in the paste is there to stop it blowing away.
Once the paste is compressed into the joint the mineral content fills the joint and is pressed into place by clamping the component down.
If the excess compound around the edge of the joint turns to dust and blows away that will not effect the mineral that is clamped into the joint.
Once the component has been moved relative to the heat sink dried out compound will fall away so will need to be replaced. If the oily content is still there and no dust has got stuck to it it may all be it a bit risky be ok to re-spread the compound.
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Old 27th Oct 2020, 2:37 pm   #51
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Default Re: Heatsink compound - refresh or not?

The conversation seem to have returned to the points I made in post 18. To what extent is the overall thermal conductivity of the paste due to (i) the fluid and (ii) the mineral oxides? Is it even possible that once the device is tightly mounted in position, loss of the fluid makes no difference or actually improves things? Perhaps it could vary from one paste to another?

The contributors to the thread seem really divided. Maybe we should ask a reputable maker of HSC? At the moment, we have opinions, but as is often the case, data seems lacking!

B
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Old 27th Oct 2020, 3:07 pm   #52
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Default Re: Heatsink compound - refresh or not?

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The conversation seem to have returned to the points I made in post 18. To what extent is the overall thermal conductivity of the paste due to (i) the fluid and (ii) the mineral oxides? Is it even possible that once the device is tightly mounted in position, loss of the fluid makes no difference or actually improves things? Perhaps it could vary from one paste to another?

The contributors to the thread seem really divided. Maybe we should ask a reputable maker of HSC? At the moment, we have opinions, but as is often the case, data seems lacking!

B
Exactly Baz! lots of opinions but not the facts that I would like to see.

Here's where I am at on this:

As I said at the beginning, I wouldn't replace heatsink compound with new stuff unless I was replacing the component, in which case you have to anyway in order for the compound to do its job properly ie mate with the unique surface of the new component. Otherwise, based on the belief by some on here, I'm thinking that 'old', 'dry' heatsink compound is perfectly good, as good as the same quality fresh stuff if you were to replace it.

Another issue is, has heatsink compound formulation improved over the years? is the stuff being sold now more conductive of heat than that used in the 70s and 80s?
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Old 27th Oct 2020, 3:09 pm   #53
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Default Re: Heatsink compound - refresh or not?

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The oil content in the paste is there to stop it blowing away.
Once the paste is compressed into the joint the mineral content fills the joint and is pressed into place by clamping the component down.
If the excess compound around the edge of the joint turns to dust and blows away that will not effect the mineral that is clamped into the joint.
Once the component has been moved relative to the heat sink dried out compound will fall away so will need to be replaced. If the oily content is still there and no dust has got stuck to it it may all be it a bit risky be ok to re-spread the compound.
I agree. In fact I'd have 'liked' given the opportunity.
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Old 27th Oct 2020, 3:47 pm   #54
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Default Re: Heatsink compound - refresh or not?

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Quote:
Originally Posted by Glowing Bits! View Post
Just found this post, apologies for being late to the party.

As per the main question, I would only renew the compound if components were to be replaced or there is any sign of the old stuff going hard/powdery over time.

For the record (pun or not, make what you will) I have never had to touch the transistors or compound on an old Pioneer SA-540 amp for the length of time of owning it (more than 10 years), it has seen countless hours of use, even at high volume but appears to not want to die any time soon.

A great way of telling if the compound is getting knackered would be by the colour, white is good, yellow is on the verge of going to dust (powdery residue).
This was my whole point at the very beginning of the thread, lots of people say that, but is it actually true? What evidence is there that when the heatsink compound goes hard that it is no longer as efficient as when it was applied? In fact, people on here are saying that it's the solid particles of the compound that provide the heat transfer. If that's the case then it being hard (solid) should not be a problem so long as the mechanical joint is still sound ie tight enough, and that the compound is still in place and has not literally fallen out of the joint - hard to imagine.

On the subject of examples, I've got high power receivers where the heatsink compound has not been changed in 45 years and they work perfectly well, or at least, the power transistors are not going pop.
If transistors don't have an insulator, they could be attached direct to the heatsink without any compound, so long as the heatsink doesn't tarnish.
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Old 27th Oct 2020, 4:06 pm   #55
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Default Re: Heatsink compound - refresh or not?

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We used to laugh at the old advert "Relax in a Radox bath" and substitute Radon for Radox....Ah we were young and simple. Now I'm older.....
Ha Ha, and there's plenty of that in Cornwall.

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Old 27th Oct 2020, 4:36 pm   #56
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I was looking at this circuit a couple of days ago, a mid 1980s ITT Prestel decoder, you can see my hand-written annotation I made on the training course.

The big distinction of course, it referred to the BeO inside the device package.
Hi.

This is interesting. In the power supply of that ITT Prestel decoder, I note the arrow is not pointing directly at the LM323K and maybe to the heatsink area instead? So it may have been referring to the TO-3 heat transfer pad.
When I said that my pink pad had come from an ITT CVC40 then I may have been mistaken although I seem to remember those pads used in that chassis.
I had an ITT Prestel decoder back in the 1980s which was eventually dismantled and I wonder if the pad really came from there instead. Perhaps the pad is made from BeO after all. I probably still have the LM323K as well. If so then knowing the manufacturer might shed some light on the matter.

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Old 27th Oct 2020, 6:48 pm   #57
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Default Re: Heatsink compound - refresh or not?

I spent a little time searching for technical papers on this subject and got nowhere. Taking a look at the technical spec for an Electrolube product (see attached), they limit the shelf-life of their stuff to mere 36 months, but make no comment at all on in-service lifetime (are we all checking the dates on our HSC purchases?)

Finally, descending the ladder of dependability, I looked at a Youtube vid where a guy replaced the paste on his CPU after 7 years, and deemed that any change in the before and after operating temperatures was small and within measurement error. Again on Youtube, you see guys getting excited that one paste lowers the CPU temperature by a degree of two compared with another paste, but isn't semiconductor lifetime related to absolute temperature, if so, one or two degrees doesn't look that big?

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Old 27th Oct 2020, 10:42 pm   #58
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Default Re: Heatsink compound - refresh or not?

Download Core Temp from here https://www.alcpu.com/CoreTemp/ . It reports the temperature of each of the processor cores. In my case, just checking, it reports in the range 37C to 45C depending on what the processor is busy doing. That is well within the maximum permissable temperature of 105C.

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Old 27th Oct 2020, 11:03 pm   #59
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Default Re: Heatsink compound - refresh or not?

The other thing you get told is that it's an absolute must to use nothing less that 99% pure IPA when cleaning off the old compound.
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Old 27th Oct 2020, 11:10 pm   #60
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Default Re: Heatsink compound - refresh or not?

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Quote:
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Pink is deliberate colouring for whatever reason.
I think alumina containing chrome has slightly different physical properties. I don't think chrome or whatever the material, was added just for the colour.

With regards to the Prestel, if it's inside the LM323K it would be inside all LM323K of the same brand. It could be used for mounting it, but that would surprise me a bit.
I'm sure related to catastrophic failure of the TO-3 device, exposing the die.
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