Roundup with 5 phase change pads: Honeywell PTM7950, PTM7950SP, OEM PCM5000, PCM8500 and Thermalright Heilos Review

1 - Introduction an product overview 2 - Test methods and equipment 3 - Thermal resistance, mimal bondline thickness 4 - Thermal conductivity, CPU and GPU simulations 5 - Microscopy and laser-induced breakdown spectroscopy 6 - Summary and conclusion

Interactive charts

Once again, I refer to these interactive charts as harbingers of the new database. The products in the curve charts can be shown or hidden by clicking on the legend; moving the mouse pointer over the nodes of a curve shows the real measured values stored there. In general, nothing is interpolated. Some bar charts can also be switched for predefined layer thicknesses so that, for example, the thermal conductivity of the relevant products can be clearly displayed with the best stored database entries. However, I would like to take this opportunity to point out that all results are subject to copyright and that I generally require permission to use them for my own purposes.

Control curves in the TIMA analysis

In the data interface, you can check the determined values in advance, test them for plausibility and deselect the deviating values for the determination. In this way, you can also recognize the slight anomalies that may exist at certain layer thicknesses, where the thermal resistance no longer correlates particularly linearly with the layer thickness. But this is exactly what you have to question, because there must be reasons for this. For details please click for a full screen:

Honeywell PTM7950

Honeywell PTM7950-SP

Thermalright Heilos

PCM5000

PCM8500

Important preliminary remark

I generally test at 60 °C and after around 15 minutes of complete heating, so burn-in has already taken place in any case. However, I deliberately don’t just measure the minimum BLT here, but start at the specified pad thickness in its raw state and then work my way down in 25 µm increments. You will see why this is important later on from the curves and the actual minimum achievable BLT. But even the entry point is not optimal for every pad, so you will see from the curve diagonals which gap thicknesses you can actually compensate for. In addition, the Honeywell PTM7950SP is an exception, so that I was also able to test the BLT of 400 µm here.

The thermal resistances _Rth

Let’s start with the most important aspect, the thermal resistance _Rth. The most important property of _Rth is that it correlates nicely linearly with the layer thickness, while the thermal conductivity describes a completely different curve and remains anything but linear. But the experienced reader already knows all this, of course. We are interested in layer thicknesses of 200 µm and less on the CPU, and usually even 100 µm and significantly less on the GPU, depending on the bending.

With the PCM5000, we can see that, despite the 0.2 mm initial thickness, only 175 µm and below is guaranteed to function at all, and the Thermalright Heilos would also like to have at least 175 µm. All other pads are of a higher quality and function stably from the first contact. To be on the safe side, I measured all the pads beforehand and the layer thickness was always just above the manufacturer’s specifications.

I have now compared the relevant layer thicknesses from 25 to 400 µm as a bar chart for _Rth. But beware, not all PTM pads reach the upper limit here! The PCM5000 is also rather bitchy below 35 µm. You will have to use the tabs selectively:

Minimum possible layer thickness

But at least I wanted to know how far you can go with a little pressure and how much a paste can still be compressed. I use the usual 60 Psi (41 N) on the measuring surface of 1 cm², which is completely sufficient and is more than what a GPU cooler, for example, achieves.

Interface Resistance

What also seems interesting is the contact resistance, in our case the interface resistance. Here you can see how well the surface of the material “clings” to the contact surfaces (IHS, heatsink). These values are also easy to compare and meaningful, as they are always the same calibrated reference blocks. Coarser degrees of grinding or a less favorable microstructure can be just as negative a factor, which then influences the effective thermal resistance and thus also the conductivity, as too low temperatures and too high a viscosity. But pads have their own rules, as the values are sometimes higher than for more liquid pastes.