Gigabyte’s so-called “Server-Grade Thermal Conductive Gel” and a degrading thermal paste on the Radeon RX 9070 Gaming OC

Random find: thermal paste with weak points

However, the thermal paste could be or become significantly more problematic than the putty in the broad mass of units sold, where many factors have to interact. The two images below clearly show the imprint marks of a viscous thermal paste after three months of operation on a GPU with a large die and a flat, polished cooler base. The visual evaluation of these contact images not only provides information on the current distribution and wetting, but also on the structural integrity of the material with regard to ageing, flow behavior and thermomechanical load capacity.

If you start with the imprint on the cooler base, the first thing you notice is the clearly structured edge bead. This indicates a strong lateral deflection of the paste during assembly pressure. This is not unusual with viscous materials, but the extent of the flow indicates that the paste has a relatively low shear strength and no longer develops any internal resilience under static pressure. However, the central zone is particularly critical: there are crack-like formations in the form of fine networks that spread from the center to the edges. These fractures (especially the lighter-colored areas) indicate the onset of dehydration and mechanical shrinkage, accompanied by the beginning dewetting of the carrier medium. The initially homogeneous matrix obviously begins to decompose during operation. The resulting shrinkage stresses lead to material cracks and the formation of micropores, i.e. air pockets, within the active heat sink. Binder that has already emerged (shiny liquid) can also be clearly seen at the edges.

On the GPU die itself, you can also see an almost identical, mirror-inverted pattern of a wavy structured surface with a crack-like, glassy-looking extraction of the paste. The central plateau, where the paste has partially detached from the substrate, is particularly striking. The detached flap in the upper third is a classic example of delamination: the material neither adheres reliably to the substrate nor could it withstand the shrinkage tensile forces. The distribution structure along the edges is also inconsistent, possibly due to initial bubble formation or uneven surface tension when the cooler was placed. The strongly crystalline-looking streaks along the die also indicate oil migration or segregation – presumably the low-molecular silicone oil, which is used to adjust the applicability, was displaced to the interfaces over time and deposited there.

From a thermal point of view, this development is problematic. The original area-wide coupling is interrupted, creating local thermal islands with greatly increased interface resistance. The dissolution of the homogeneous contact surface into a structurally interrupted pattern leads over time to increased contact thermals and, in the worst case, to an increase in the core temperature of individual subcomponents in the die (hotspot areas). In this constellation, the thermal equilibrium point shifts further and further outwards, which in turn accelerates the ageing of the material. This is therefore a self-reinforcing degradation process.

In the long term, this behavior leads to a continuous decline in thermal performance. While sufficient performance can still be guaranteed at the beginning, the effectiveness decreases with every thermal load. This becomes particularly problematic in a vertical installation position, as previously discussed, as gravity also favors material migration and the edge zones become the drain line of the paste. The inhomogeneous removal over the months is a clear indication of this. The viscous paste used here shows all the typical characteristics of limited long-term stability. The cracking, delamination and oil separation indicate insufficient structural cross-linking of the carrier matrix, combined with a high proportion of low-molecular components. The thermomechanical load, especially with high load cycles, leads to an irreversible loss of thermal coupling. The observed contact patterns are therefore not only an optical indication, but also microstructural evidence of incipient degradation – and a clear indication that this material is not a stable solution in the long term without maintenance or reapplication.

Microscopy and consistency

Let us now turn to the microscopic analysis of this viscous thermal paste, which was examined with regard to particle distribution, tear-off behavior and mechanical structure. The chronological sequence allows an in-depth assessment of the material consistency and potential weak points in terms of application, ageing and heat transfer. The first image shows an overview at medium magnification, in which the macroscopic edge transition between the ceramic surface (right) and the applied thermal conductive film (left) is clearly visible. The contour of the paste edge appears blurred, which indicates a relatively soft matrix with moderate cohesion. The paste flows into the finest recesses of the polished surface, but increasingly loses its dimensional stability. This behavior is typical for materials with a high oil content or insufficient network bonding between fillers and the carrier matrix. The homogeneity of the color distribution indicates a relatively even primary distribution of the fillers, although there are already localized thinning in the outer areas. Such partial detachments indicate weak adhesion to the metal surface and incipient interface destabilization.

In the second image, at higher magnification, the microstructure of the particle matrix becomes clearer. The paste shows a dense, finely dispersed grain structure with a high filler density. The particle sizes still appear quite uniform at this level, with few inclusions or pores. The typical characteristics of a silicone-based filler paste with spherical to slightly irregular particles are clearly recognizable. The surface in this area is still largely closed and intact, without any significant air inclusions or bubble formation. However, this closed structure is highly dependent on the applied compressive load and tends to be locally over- or underloaded if the compression is uneven.

In the third image, the particle size distribution is analyzed at the submicron level. This shows that the original assumption of a finely dispersed structure is only partially correct. The particle sizes scatter significantly between around 5 and 10 µm. This is acceptable for a viscous heat conducting material. However, the distribution is rather anisotropic, with sometimes dense agglomeration and clearly visible microballoons. Such structures favor so-called mechanical shadow zones during application, in which thermal coupling is poorer due to incomplete displacement. In addition, the tear-off structure is irregular when the contact surfaces are separated, which indicates an inhomogeneous bond within the matrix.

Finally, the fourth image shows a targeted laser ablation of the uppermost matrix layer in order to make the underlying, coarser filler particles visible. This method allows a differentiated view of the distribution of secondary particles that are normally covered by the viscous silicone matrix. In this exposed zone, significantly larger particles with diameters between around 10 and over 16 µm are now visible. The particles appear irregular in shape and distribution, suggesting incomplete dispersion in the mixing process. Some of these particles also have sharp-edged contours, which can lead to fracturing of the surrounding matrix under stress. These local stress peaks favor microscopic cracking and promote mechanical disintegration during thermal cycling. Good thermal conductivity will certainly be achieved, but unfortunately not for long.

From all these observations it can be deduced that the viscous paste under investigation enables an initially homogeneous application, but on closer inspection shows clear deficits in structural stability and long-term homogeneity. The combination of a fine basic structure, isolated agglomerates and larger sharp-edged particles leads to an overall fragile system that tends to break off and interfacial instabilities under mechanical stress.

In addition, there is an increased risk of sedimentation or segregation due to the lack of particle retention in the polymer network. This can lead to material migration, particularly if the modules are aligned vertically, which in the long term leads to uneven thermal coupling and ultimately to degradation of the heat transfer. The study shown here therefore underlines the need for precise formulation and quality control of viscous interface materials.

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