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The_Document
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PostPosted: Sat Mar 31, 2018 6:27 am    Post subject: Swapped thermal grease on laptop cpu Reply with quote

Was wondering if I applied a rather thick coat, would the excess just be squished out thus not hurting overal thermal conductivity severely? Idling at ~35°C is this temp good?

Used arctic silver 5 wanted liquid metal but I read the stuff drips and eventually leaks away also causing short circuits on the mobo.
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bunder
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PostPosted: Sat Mar 31, 2018 7:15 am    Post subject: Reply with quote

It was tested that overdoing thermal paste usually accounts for 3-5c worth of temperature. An idle temp of 35c might be fine depending on what cpu you have.
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PostPosted: Sat Mar 31, 2018 7:42 am    Post subject: Reply with quote

It actually idles at ~32°C on low cpu clocks and I pressed down about 2.5mm on the syringe on a 3.5 gram tube of grease. I think it CAN be better if I redo it, but it seems fine and redoing it would really be a waste. Im sure the OEM grease was way worse anyways because there was a lot of it all around the CPU die, seems like they used too much and the thermal conductivity is probably way lower also.
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PostPosted: Sat Mar 31, 2018 9:25 am    Post subject: Reply with quote

The_Document,

Thermal grease is a very poor thermal conductor but its a lot better that the air that would otherwise be in the gaps.

Here's the physics.
Two planes (the top of the CPU and bottom of the heat sink) can only ever touch at three points.
Consider how a three legged stool is unconditionally stable and a four legged chair will rock between several stable states.
Further, a 'point of contact' has a vanishing small surface area, so very little heat transfer can take place.

With two clean practical surfaces in contact, there is effectively a very small air gap.
Lets ignore the flatness for now except to say that minimising the air gap by improving flatness helps. You can't beat the three points of contact.

Thermal paste aims to replace the air in the gap with a material with better thermal properties.
When its applied as a 'sandwich filling', the original three point contact is lost and one poor thermal conductor is replaced by another.
It won't squeeze out enough, or it would run out over its intended useful life.

If you think you have put too much in, you probably did.
The right amount is no more than the size of a grain of rice.
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PostPosted: Sat Mar 31, 2018 1:56 pm    Post subject: Reply with quote

NeddySeagoon wrote:
The_Document,

Thermal grease is a very poor thermal conductor but its a lot better that the air that would otherwise be in the gaps.

Here's the physics.
Two planes (the top of the CPU and bottom of the heat sink) can only ever touch at three points.
Consider how a three legged stool is unconditionally stable and a four legged chair will rock between several stable states.
Further, a 'point of contact' has a vanishing small surface area, so very little heat transfer can take place.

With two clean practical surfaces in contact, there is effectively a very small air gap.
Lets ignore the flatness for now except to say that minimising the air gap by improving flatness helps. You can't beat the three points of contact.

Thermal paste aims to replace the air in the gap with a material with better thermal properties.
When its applied as a 'sandwich filling', the original three point contact is lost and one poor thermal conductor is replaced by another.
It won't squeeze out enough, or it would run out over its intended useful life.

If you think you have put too much in, you probably did.
The right amount is no more than the size of a grain of rice.


Great explanation but I read ppl say pea sized amount which is right? Im afraid of detaching heatsink might damage to processor processor can it happen? Is it common?

Just bought Thermal Grizzly Kryonaut will do it over anyways. Might as well much better than AS-5 too.
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PostPosted: Sat Mar 31, 2018 2:12 pm    Post subject: Reply with quote

The_Document wrote:
but I read ppl say pea sized amount which is right?

From the horse's mouth, so to speak:

https://www.intel.com/content/dam/support/us/en/images/processors/sb/img/cs030329_2.jpg

https://www.intel.co.uk/content/www/uk/en/support/articles/000005576/processors.html
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PostPosted: Sat Mar 31, 2018 3:52 pm    Post subject: Re: Swapped thermal grease on laptop cpu Reply with quote

The_Document wrote:
Used arctic silver 5 wanted liquid metal but I read the stuff drips and eventually leaks away also causing short circuits on the mobo.

IIRC Arctic Silver lasts very long, it is still good after 5 years so not bad choice at all.
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PostPosted: Sat Mar 31, 2018 4:15 pm    Post subject: Reply with quote

NeddySeagoon wrote:
The_Document,

Thermal grease is a very poor thermal conductor but its a lot better that the air that would otherwise be in the gaps.

Here's the physics.
Two planes (the top of the CPU and bottom of the heat sink) can only ever touch at three points.
Consider how a three legged stool is unconditionally stable and a four legged chair will rock between several stable states.
Further, a 'point of contact' has a vanishing small surface area, so very little heat transfer can take place.

With two clean practical surfaces in contact, there is effectively a very small air gap.
Lets ignore the flatness for now except to say that minimising the air gap by improving flatness helps. You can't beat the three points of contact.

Thermal paste aims to replace the air in the gap with a material with better thermal properties.
When its applied as a 'sandwich filling', the original three point contact is lost and one poor thermal conductor is replaced by another.
It won't squeeze out enough, or it would run out over its intended useful life.

If you think you have put too much in, you probably did.
The right amount is no more than the size of a grain of rice.
and on the flipside... it is a VERY good thermal insulator if you use too much...
When I give a damn I will take a razor blade to both faces to smooth them of any additional bumps, "pea size blob" in the middle and then smooth that out evenly with the razor. Now... I just do the pea blob.

* note I still do the razor method but for 100kVA inverters I am hand-building.
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PostPosted: Sat Mar 31, 2018 5:39 pm    Post subject: Reply with quote

P.Kosunen wrote:
The_Document wrote:
Used arctic silver 5 wanted liquid metal but I read the stuff drips and eventually leaks away also causing short circuits on the mobo.

IIRC Arctic Silver lasts very long, it is still good after 5 years so not bad choice at all.


Does the paste I just bought have shorter life?

Naib wrote:
for 100kVA inverters I am hand-building.


Any sot-227 in there? Send pics I like big IC packages.
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PostPosted: Sat Mar 31, 2018 6:42 pm    Post subject: Reply with quote

The_Document,

Too little is safer than too much thermal compound and CPUs have got bigger heat spreaders too.

Thermal compounds come in two sorts.
Those that set over the first few hours of use and those that don't.

It can be very difficult to separate the CPU and heatsink if you use the first sort.
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PostPosted: Sat Mar 31, 2018 6:43 pm    Post subject: Reply with quote

The_Document wrote:


Naib wrote:
for 100kVA inverters I am hand-building.


Any sot-227 in there? Send pics I like big IC packages.
Surprisingly not. Got a couple of these though
https://www.infineon.com/export/sites/default/media/products/Power_Semiconductors/High_Power_Semiconductors/Product_Page_Pictures/Product_image_EconoPACK_4.jpg_599402819.jpg
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PostPosted: Sat Mar 31, 2018 7:01 pm    Post subject: Reply with quote

Naib wrote:
Got a couple of these though


Is that an IGBT? Have you seen these SiC JFETs?
https://www.mouser.com/ProductDetail/USCi/UJN1205K?qs=sGAEpiMZZMv4z0HnGdrLjjzLSQIXpgmQvbMLWeB%252b14c3VAj%252b6yrg6Q%3d%3d
only 35mills of on resistance, might come in handy.
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PostPosted: Sat Mar 31, 2018 7:03 pm    Post subject: Reply with quote

yup that is an IGBT module. Coincidently I am designing an inverter right now using SiC MOSFETS (TO247 package)
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PostPosted: Sat Mar 31, 2018 7:05 pm    Post subject: Reply with quote

Naib wrote:
SiC MOSFETS

Mentioned JFETs are probly better because higher voltage handling at lower resistance than MOSFETs.
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PostPosted: Sat Mar 31, 2018 7:39 pm    Post subject: Reply with quote

The problem with JFET's is their "always on" default state. In the event of control powerloss OR failure of the gate PSU you are entering into shoot through territory. SiC MOSFETs are down in this ohmage as well so it is all good :)
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PostPosted: Sat Mar 31, 2018 7:47 pm    Post subject: Reply with quote

Naib wrote:
The problem with JFET's is their "always on" default state. In the event of control powerloss OR failure of the gate PSU you are entering into shoot through territory. SiC MOSFETs are down in this ohmage as well so it is all good :)


I PMed you.
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PostPosted: Sun Apr 01, 2018 10:18 am    Post subject: Off-topic physics Reply with quote

NeddySeagoon wrote:
...
Here's the physics.
Two planes (the top of the CPU and bottom of the heat sink) can only ever touch at three points.
Consider how a three legged stool is unconditionally stable and a four legged chair will rock between several stable states.
...

All four legs on a square chair with the leg-ends in a plane can be arranged to simultaneously touch a continuous surface by rotating the chair by no more than 90 degrees. (I'm not sure of the exact proof, but effectively it's that if three legs touch, rotating by 90 degrees interchanges the legs, so at some point they must change from being in contact to not, and vice-versa, so by the definition of continuous there will be a point where all 4 will touch.)

This probably means that rotating the heat sink may find a closer fit to the chip below it. Note to chip makers: can we have circular chips and heat sinks.
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PostPosted: Sun Apr 01, 2018 11:08 am    Post subject: Re: Off-topic physics Reply with quote

Goverp wrote:
NeddySeagoon wrote:
...
Here's the physics.
Two planes (the top of the CPU and bottom of the heat sink) can only ever touch at three points.
Consider how a three legged stool is unconditionally stable and a four legged chair will rock between several stable states.
...

All four legs on a square chair with the leg-ends in a plane can be arranged to simultaneously touch a continuous surface by rotating the chair by no more than 90 degrees. (I'm not sure of the exact proof, but effectively it's that if three legs touch, rotating by 90 degrees interchanges the legs, so at some point they must change from being in contact to not, and vice-versa, so by the definition of continuous there will be a point where all 4 will touch.)

This probably means that rotating the heat sink may find a closer fit to the chip below it. Note to chip makers: can we have circular chips and heat sinks.


That only hold true for a perfectly flat plane and perfectly equal length vectors/legs. This isn't the case in reality. A 3-legged stool is always steady, but a 4-legged stool can be wobbly.
http://mathforum.org/library/drmath/view/53267.html

Now take two complete planes NOT points because we are dealing with a heatsink and a heatplane NOT a barstool.... However smooth you think these are they are not. If it was economically viable we would be seeing cold welding being used (nano-coating is making this viable)


The two surfaces are essentially random and thus three point will make contact which is in no way good enough to minimise the thermal resistance of this interface THUS some form of TIM is used. Paste is best as it will uniformly fill in the gaps (but relies on a control process as too little is bad, too much is bad), phase-change material is the next best as it will essentially fill in the gaps (consistent results). So now you will still have 3 points making contact BUT then additional thermal conductive path via the TIM

https://myheatsinks.com/wp-content/uploads/2013/12/gap_and_tim.png
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PostPosted: Wed Apr 04, 2018 11:01 pm    Post subject: Reply with quote

Re did the thermal paste idling at 29C without X and 31C with KDE. I actually made it worse with Arctic Silver 5 because the temps were much higher a bit after applying idling at 41C, meaning I put too much, I put less than a size of a pea ammount of Thermal Grizzly Kryonaut paste, spreading it was very difficult. Works great.
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