Burn-in measured: Honeywell PTM7950 Phase Transition Material examinated and how to recognise a copycat

1 - Introduction and necessary definition of terms 2 - Measurement of the Honeywell PTM7950 in detail 3 - Comparison with a another pad (OEM) and conclusion

So-called phase change pads are currently marketed as PCM (Phase Change Materials) or, more correctly, as PTM (Phase Transition Material). They are important components in thermal management technology, which have now also arrived in the PC sector for home users and are also enjoying a certain amount of hype. These materials improve heat transfer between electronic components and heat sinks by changing their aggregate state at certain temperatures. Reason enough to take a closer look, because the differences in detail are significant. First of all, we need to explain the term, because even manufacturers are a little too generous with the term PCM!

The difference between Phase Change Material (PCM) and Phase Transition Material (PTM) lies mainly in their use and their physical properties. Both terms refer to materials that undergo phase transitions, but in different contexts and with different application goals. And I can already spoil the fact that what we like to use recently is a PTM and not a PCM. So I’ll start by getting the terminology straight and then measure the burn-in process using the Honeywell PTM7950 and the resulting changes in layer thickness and changes in interface resistance at different temperatures.

Phase Change Material (PCM) or Phase Transition Material (PTM)?

PhaseChange Materials (PC M) are materials that can change their physical phase (e.g. solid, liquid, gaseous) at a certain temperature, thereby storing or releasing large amounts of thermal energy. They are mainly used for temperature control and energy storage, e.g. to prevent overheating in electronic devices and batteries. During the phase transition (e.g. from solid to liquid), the PCM absorbs or releases a considerable amount of latent heat without the temperature of the material changing significantly. For those who are not yet familiar with this, here’s a practical example: there are these great pocket warmers that can be bent in the winder in the solid state so that they then release the energy previously stored during heating from the liquid to the solid state. It is therefore a kind of latent heat accumulator. It is sometimes incorrectly stated that PCM is better as a heat conductor because it can initially store the heat and use it for the phase transition, but this is completely wrong.

Phase transition materials (PTMs), on the other hand, are materials that undergo phase transitions between different structural or electronic states. The phase transition in PTMs leads to a change in the physical or chemical structure of the material, which influences its properties in a targeted manner. But this is exactly what we actually need as a heat conducting pad! In PCMs we find the classic phase transition between classic aggregate states (solid, liquid), in PTMs the phase transition between different structural states. Or, to put it in a nutshell: The PCM is used to absorb or release latent heat, the PTM uses the change in physical properties without significant heat aspects.

So I have to correct myself a little in my (lousy) use of language, because what we use for cooling are in fact pure phase transition materials, i.e. PTM and not PCM! And now we come to the problem for us Germans, namely the translation. Both terms translate to phase change material, which has resulted in the common term and which in turn leads to phase change pad. And now? In order not to cause any more confusion, I will continue to use phase change pad for the general description, but instead of PCM I will use PTM as an abbreviation, as long as it is not part of a (actually incorrect) product name. But you can also see that even manufacturers confuse the whole thing and have a problem with the two terms.

The basics of burn-in

PTMs are thermal interface materials that soften or melt at a defined temperature. This property enables them to fill microscopic irregularities on the surfaces of components and heat sinks, which minimizes thermal contact resistance and maximizes heat transfer. Typical PTMs consist of a polymer matrix with thermally conductive fillers and exhibit thermal conductivities of 1 to 8 W/mK, depending on thickness and design.

The “mysterious” burn-in process of PTM involves repeated heating and cooling of the material to achieve an optimal thermal bond. During burn-in, the pad reaches its phase change temperature, wetting the surfaces and achieving a minimum bondline thickness (BLT). This process is often carried out through a controlled temperature exposure over several hours or days to ensure the long-term reliability and performance of the material. I repeated this exact point a total of 10 times until I achieved a consistently (almost) constant temperature behavior and the thermal resistance remained approximately the same for identical temperatures.

Changing the interface resistors

A key aspect of the burn-in process is the change in the thermal interface resistances. At the beginning of the burn-in (i.e. before the first phase change), the resistances are typically significantly higher, as the PTM is not yet (completely) melted and wetted. During the burn-in process, the interface resistances decrease significantly as the material fills the microscopic gaps and unevenness of the contact surfaces and thus reduces the thermal barrier.

This is exactly why I pointed out in the individual test of the Honeywell PTM7950 that a burn-in is essential for optimum performance. Yes, this pad will certainly work better than simpler, cheaper thermal pastes due to the particles it contains, even without (complete) burn-in, but you won’t get anywhere near the values of a good paste. I invested almost 2 days in this test this time and have to correct myself slightly with regard to the only partial phase change of the PTM7950. I have a separate paragraph on this. In terms of the phase change, the PTM7950 is a kind of evolution with slightly different characteristics. These in turn will help us to distinguish the original from non-identical imitators, even if the color is correct. You always learn something new, even about the electricity bill for such tests.

After the burn-in process, PTMs offer improved thermal performance and reliability. The materials remain firmly in place without a pump-out effect, where the material is forced out of the interface. This is particularly important for constant pressure applications such as those found in many electronic devices. But we’ll see that in a moment.

Further links and basics