Basics Cooling Reviews Thermal grease and pads

Myths and facts about thermally conductive pastes: roughness of the contact surfaces and required layer thicknesses, including re-evaluation of the pastes

After testing over 100 different thermally conductive pastes, of which almost 80 are already available in my online database, I was able to gather extensive experience that calls into question some common assumptions in this area. Of particular interest was the analysis of contradictions between my generalized measurement data, the frequently stated general values for the thermal conductivity of the pastes and the results actually measured under practical conditions. These discrepancies occurred above all with very thin layer thicknesses of less than 100 µm, whereby the deviations were particularly clear in the range below 30 µm.

The thermal conductivity values given in many data sheets often suggest a linear transferability to practice, but this could not be confirmed in my tests. While certain pastes certainly exhibit high thermal conductivities in standardized laboratory measurements, they do not always achieve these values in real applications, especially with very thin layer thicknesses. This is partly due to the interaction between the surface properties of the components to be cooled and the specific viscosity and filler structure of the paste in question. Especially with layer thicknesses of less than 30 µm, the surface roughness and micro-irregularities of the contact surfaces have a considerable influence on the actual heat transfer, which is often not taken into account in the general data.

In today’s article, I will therefore discuss the most frequently occurring surfaces and their properties. I will show how different structures – from polished heatspreaders to coarser, machined surfaces – affect the effectiveness of the thermal compound. I will then turn to the tested pastes themselves, focusing on how to recognize whether a particular paste is sufficient for the intended use. Not only the nominal thermal conductivity values play a role here, but also aspects such as the processing properties, the flow behavior under pressure and the behavior at minimum layer thicknesses.

Cured paste with a minimum layer thickness that is far too high and already destroyed in the structure

I will pay particular attention to the maximum layer thickness actually required. It has been shown that many users either overestimate or underestimate the thickness of the layer, which can lead to suboptimal results. In my measurements, I was able to clearly define for which applications thinner layers are sufficient and where problems could potentially occur – be it due to insufficient wetting, air inclusions or failure of the paste with extremely thin applications. These findings should help users to select the right paste for their specific requirements and at the same time optimize application techniques to achieve the best possible thermal performance. The database will be continuously updated with new findings and further products to provide a sound and practical basis for decision-making and most importantly: I have modified it once again in the evaluation and search criteria.

As a sorting criterion, I use the effective thermal conductivity as the mean value for the selected layer thickness, as it takes into account both the thermal resistance of the paste and the contact resistance of the two contact surfaces and therefore accurately reflects reality. I decided against sorting by temperature because the differences measured at very low layer thicknesses may no longer be clear in some cases due to inaccuracies and measurement tolerances. The determination of the effective thermal resistance and the effective thermal conductivity, on the other hand, includes six independent temperature values and the consideration of a gradient and is therefore many times more accurate.

The necessary changes to the thermal compound database

After extensive tests and detailed measurements, I have redefined and restructured the three search criteria and the evaluation of the thermal pastes. The aim of this adjustment is to better reflect the real performance, especially for very thin layers of less than 30 µm, as this area is the most relevant in practice. In previous databases and manufacturer specifications, this critical area was often not sufficiently taken into account, although it plays a decisive role in determining how efficiently a thermal compound actually works in real application scenarios. The results of my research will later clearly demonstrate why this area is so important.

The first group within my new categorization (“Smooth and even surface”) therefore focuses exclusively on performance with layer thicknesses below 30 µm. Here it is particularly clear which pastes still guarantee high thermal conductivity and reliable wetting even when applied extremely thinly. In this range, there are many contradictions between nominal thermal conductivity values and real measurements, as filler distribution, viscosity and surface adaptation of the paste play a decisive role. Most manufacturers do not provide any specific data for this area, although it is precisely here that the thermal efficiency of a cooling system is significantly influenced.

The second group (“Rough and uneven surface”) refers to rather rough, curved or uneven surfaces, as they occur in particular with certain CPU and GPU variants. These include CPUs in the LGA1700 socket, which place particular demands on the thermal compound due to their characteristic curvature, as well as very large monolithic GPU chips, whose surface structure also requires an adapted assessment. This segment measures how well a paste is able to ensure consistently effective heat transfer despite such unevenness, without unwanted air pockets or excessive thickening.

Finally, the third group (“gap filler”) forms an average performance value of the effective thermal conductivity across all possible layer thicknesses of a paste up to the upper limit of 400 µm. While most applications are in a significantly lower range, this group enables a holistic evaluation of paste performance, especially for scenarios where a thicker layer is required due to greater unevenness or mechanical compression. It serves as a complementary benchmark and allows a well-founded selection to be made even for unusual applications.

I present this revised tripartite division with a corresponding overview at the beginning of the following pages of my analysis so that I can refer to it directly in the subsequent investigations into surface properties. This restructuring not only ensures a more differentiated evaluation, but also makes it possible to work out contradictions between theoretical manufacturer specifications and practical measurement results. Particularly in the relevant area of thin layer thicknesses, this makes it clear which pastes actually deliver what they promise and which merely impress with high nominal thermal conductivity values on paper but do not deliver the expected performance in practice.

I must also mention a mistake that we took the opportunity to correct. Very viscous pastes, such as the Thermalright TFX, whose minimum BLT is 35 µm, are now displayed correctly in the diagrams and no longer appear in the 25 µm charts once the allocation problems have been resolved. This means that the classification and evaluation of this paste is correct again. I apologize for this.

Why I look at the curvature separately

CPUs and GPUs usually have less curvature when installed than when unmounted due to several mechanical and thermal factors. The most important reason is the contact pressure exerted on the processor by the CPU cooler and the bracket. Modern socket mechanisms, such as the LGA1851 for Intel or the AM5 socket for AMD, are designed in such a way that they ensure an even distribution of force on the CPU’s heat spreader. This contact pressure means that slight deformations, which are visible in the unmounted state, are compensated for by the mechanical tension. Intel’s LGA1700 socket was an inglorious exception, but there are workarounds and remedies for this.

Another factor is the stress distribution in the material of the heatspreader and the substrate. Internal stresses can arise during the manufacturing process, which are partially relieved after installation due to the pressure of the cooler and the fixation in the socket. The cooler not only exerts a vertical force, but can also cause a certain “planarization” through its installation by maximizing the contact surface between the heatspreader and the cooler. This leads to a reduction in curvature and better thermal coupling.

Heat development during operation also plays a role. As the CPU heats up, the various materials (such as copper, nickel and the silicon substrate) expand differently. This thermal expansion can help the heatspreader to adhere better to the cooler, especially if the materials are selected so that their expansion coefficients are matched. The operating temperature range of modern CPUs has generally already been taken into account during the development phase so that thermal deformation is minimized when installed.

Finally, the distribution of the thermal paste between the CPU and the cooler also influences the curvature. The paste fills microscopic irregularities and its viscosity can help to compensate for minor deformations. This not only improves heat transfer, but also contributes to a more even contact surface. The combination of mechanical contact pressure, stress distribution, thermal expansion and the use of thermal pastes therefore results in a reduction in curvature when installed, which is crucial for optimizing heat transfer between the CPU and cooler. If you consider the curvature factor to be important for your specific design, you should use the search and selection criterion “Rough and uneven surface” to be on the safe side. The rest simply ignore this factor.

Test setup, measurement methods and basics

Our database is based on real laboratory values that we have elaborately determined according to industry standards. However, many of these results contradict the manufacturers’ marketing claims and ruthlessly expose contradictions and lies, but they are all well-founded, reproducible and legally sound. These measurements not only reflect the general performance values of the pastes, but also enable an assessment of the suitability for a specific area of application (layer thicknesses, surfaces) as well as the suitability taking into account the individual capabilities of the respective user. In addition, the material analysis including digital microscopy is suitable for making your own assessment of the possible durability of a paste, even if I do not want to and cannot assume any guarantee for this. Unfortunately, measuring more than 3000 cycles per paste is not feasible. Statements about the matrix and the particles used, including their size, are also important. Please refer to my other articles and all individual tests on pastes that have shown certain abnormalities.

Test setup and methods Material analysis and microscopy Basic knowledge
Here you can find out why effective thermal conductivity and bulk thermal conductivity can be completely different in practice, what role the contact resistance between the surfaces and the paste plays and how thermal paste can be measured accurately. There is also a detailed description of the equipment, the methodology and the error tolerances. You will learn how laser-induced plasma spectroscopy works and the advantages and limitations of the measurements. There is also high-resolution digital microscopy and analysis of particle sizes. This information is also used to estimate the long-term stability of a paste. Anyone who has always wanted to know what is or is not in a paste and how these pastes are produced will find what they are looking for here. The basic article provides a better understanding of what is often sold for far too much money and sometimes with adventurous promises.

You are welcome to leave suggestions and comments in the forum, via PN or e-mail. If you would also like to contribute to the project and send me samples of thermal paste that have not yet been entered in the database, please contact me by e-mail. The e-mail address can be found in the imprint. Of course, this also applies to the manufacturers whose products we would like to test, regardless of which continent the product comes from. The scope of the database is de facto unlimited and as the methods and equipment are always the same, it can be expanded over the years and still remain comparable.

Kommentar

Lade neue Kommentare

e
eastcoast_pete

Urgestein

2,201 Kommentare 1,405 Likes

@Igor Wallossek : Danke für eine weitere Vertiefung der Materie!

Allerdings bin auch etwas verwirrt, wie man (ich) ohne entsprechendes Messgerät verifizieren kann, daß die Schichtdicke der Paste einigermaßen homogen bei ungefähr 10 - 20 um liegt? Gibt's dazu Tipps?

Antwort 1 Like

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

Dafür habe ich doch die Beispiele vermessen

Antwort 1 Like

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

So, jetzt sind auch die richtigen Temperaturen drin und zwar in direkter Korrelation zum Rth :)

Antwort Gefällt mir

e
eastcoast_pete

Urgestein

2,201 Kommentare 1,405 Likes

Die Beispiele sind sehr gut. Vielleicht habe ich nicht direkt genug gefragt: wie trägst Du die Paste so auf, daß sie auf dem Heatspreader eine einigermaßen homogene Dicke von 10 - 20 um aufweist? Ist es am besten, die Paste so zu verteilen, daß der gesamte Heatspreader gerade eben so damit bedeckt ist, also so dünn wie irgend möglich? Und dabei keine Pasten verwenden die man gar nicht so dünn homogen auftragen kann? Oder liege ich da falsch?

Antwort Gefällt mir

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

Ich nehme nur noch Klecks bzw. bei länglichen Flächen eine kurze Linie, damit außen genügen Luft ist, um alles rauszupressen.
Ist alles schon zugeschmiert, wird man nicht mehr sauber verpressen können oder beschädigt die Molekülstruktur. Man sieht auf YT teilweise echt Stuss, auch bei den sogenannten Etablierten.

Immr vorausgesetzt, es ist kein extremer Buckel. Konkav ist mittlerweile kaum noch zu finden. Entweder plan oder leicht konvex.

Antwort 1 Like

e
eastcoast_pete

Urgestein

2,201 Kommentare 1,405 Likes

Danke, das hilft!
Und wenn die Schichtdicke am Ende um die 20 um dick ist (und sein soll), ist auch klar, warum gröbere Aluminium Oxid Partikel in den Pasten richtig schlecht sind - denn die sind ja oft schon alleine 20 um oder größer.

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z
zwerg05

Mitglied

75 Kommentare 8 Likes

Klappt das auch bei Am5 und der duronout der klex in der Mitte und wie groß soll der Klecks am Ende sein😂

Antwort Gefällt mir

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

Linsengroß, aber das kannst du ja vorher einmal testen. In der Tube ist genug. Fange kleiner an und drücke den Kühler drauf. Das hat man schnell raus und mach vor dem Andrücken ein Foto. Fürs nächste Mal ;)

Antwort 1 Like

LEIV

Urgestein

1,607 Kommentare 680 Likes

Und vielleicht auch hier zum teilen, damit der nächste auch sieht wie groß ungefähr

Antwort 1 Like

Steffdeff

Urgestein

936 Kommentare 887 Likes

Tja, leider hinterlässt der Applikator von X-Apply hier eine Lücke die er nie geschlossen hat.😕
Zum auftragen der WLP war die Folie schon sehr hilfreich, auch wenn es natürlich auch mit „Klecks“ oder „Wurst“ funktioniert.
Auf die GPU kommt mir nur noch ein PTM Pad und keine Paste mehr. Das 125er hat sich ja, gerade auf AMD GPU‘s, bestens bewährt!
Auf der CPU bleibt bei mir eine gute WLP das Mittel der Wahl!
@Igor Wallossek
Danke für die umfangreiche Datenbank👍!

Antwort 1 Like

Xaphyr

Mitglied

78 Kommentare 73 Likes
RedF

Urgestein

5,270 Kommentare 3,067 Likes

Wirklich klasse Arbeit, das war echt nötig das aufzuklären. (y)

Was nützt die beste Paste, wenn sie nicht auf der passenden Oberfläche performt.

Antwort 2 Likes

Igor Wallossek

1

11,672 Kommentare 22,608 Likes
Soda

Mitglied

23 Kommentare 7 Likes

Hi, Danke erstmal für den Test.... ich hab gesehen das unter "Vorauswahl nach Schichtstärke" --> Rough and uneven Surface der Sockel LGA1700 als beispiel erwähnt wird, daher könnte es nützlich sein auch andere Sockel als Beispiel unter "Smooth and even Surface und " Gap Filler" zu erwähnen. Falls du die eigenschaften auch älterer Sockel kennst (jetzt nicht irgenwelche vorsintflutlichen Sockel), diese dann mit einzutragen , dann hätte man einen zusätzlichen überblick über das was man nutzen sollte, das ließe sich mit zukünftigen Sockeln weiter führen.

Antwort 1 Like

s
stch

Mitglied

46 Kommentare 14 Likes

Wahrscheinlich war es naiv anzunehmen, dass es für die Hersteller bereits Standard ist, Wärmeleitmaterialien passend zu den Oberflächen auszuwählen. Aber vermutlich spart man sich den Aufwand, funktioniert ja für eine Weile auch so.
Aus werkstoffwissenschaftlicher Sicht könnte man drauf kommen, dass die Zusammenhänge genau so ausschauen.

Antwort Gefällt mir

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

Ich finde es schick, dass endlich Bewegung in die Branche kommt und ich aktiv daran teilhaben darf. :D

Am Freitag gibt es den Test der Duronaut, die sich als normal verfügbare Paste locker in die Top 10 schiebt. Und dann gibts demnächst noch eine Apex 2.0 von Alphacool, die fast exakt auf dem gleichen Level spielt. Ohne zu viel zu spoilern, aber trennt Euch einfach mal vom Irrglauben, die Noctua NT-H2 & Co wären tolle Pasten. Außer dem Brand-Image bekommt man nämlich nichts mehr fürs Geld. Es wird in abesehbarer Zeit noch mehr brauchbare Alternativen geben, die allesamt tauglich sein werden, denn auch die OEMs kommen auf den Geschmack. Dann muss man auch keine Importe mehr tätigen, denn die Unterschiede zum echten High-End werden kleiner.

Ich kann mittlerweile selbst keine Pasten mehr sehen, denn ich teste ja deutlich mehr, als man in der Datenbank sieht. Nicht alles taugt was oder ist für die Öffentlichkeit bestimmt, aber es ist schön, dass immer mehr Anfragen kommen, ob ich nicht mal eine Auge drauf schmeißen könnte, bevor man was in den Handel brettert. :D

Antwort 9 Likes

Itihasa

Mitglied

22 Kommentare 12 Likes

Übrigens, warum haben Sie nicht das PTM Honeywell 7950 Thermopad und separat die PTM Honeywell 7950 Wärmeleitpaste und separat die PTM Honeywell 7958-SP Wärmeleitpaste auf die Liste der getesteten Pasten gesetzt, damit Sie direkte Vergleiche mit anderen Wärmeleitpasten anstellen können?

Ihr würde der Liste auch andere Phasenübergangsthermopads (Thermopasts) hinzufügen, da sich herausstellt, dass Thermal Grizzly PhaseSheet PTM überhaupt nicht mit den Funktionen von Honeywell 7950 PTM übereinstimmt und es sich um ein sehr schlechtes Produkt handelt, was durch zahlreiche Kommentare von bestätigt wird Personen, die dieses Produkt zuvor gekauft haben, was die Schwäche dieses Produkts durch einen professionellen Tester aus Asien weiter bestätigt...

Antwort Gefällt mir

Igor Wallossek

1

11,672 Kommentare 22,608 Likes

Phase Transition Material passt nicht in diese Datenbank, weil man es anders testen muss. Allerdings habe ich bereits einen Test mit 5 solcher Pads online.

Antwort Gefällt mir

Itihasa

Mitglied

22 Kommentare 12 Likes

Dann warten wir auf eine separate Datenbank mit Materialien wie Honeywell PTM 7950, damit die Community weiß, welche Thermopads mit Phasenübergang relativ schwache Klone sind und welche Aufmerksamkeit verdienen. Als erstes im Test empfehle ich das Thermal Grizzly Phase Sheet, da es ein sehr schwacher Imitator ist und vorgibt, ein Champion zu sein, was durch seinen Preis vermuten lässt.

Antwort 1 Like

Danke für die Spende



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About the author

Igor Wallossek

Editor-in-chief and name-giver of igor'sLAB as the content successor of Tom's Hardware Germany, whose license was returned in June 2019 in order to better meet the qualitative demands of web content and challenges of new media such as YouTube with its own channel.

Computer nerd since 1983, audio freak since 1979 and pretty much open to anything with a plug or battery for over 50 years.

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