Linus Tech Tips PTM7950 Review – Original, OEM Product or Fake?

1 - Introduction and overview 2 - Microscopy, pad thickness and homogeneity 3 - Microscopy, particle analysis and chemical composition 4 - Burn-in and performance measurements 5 - Summary and conclusion

The maximum BLT (Bond Line Thickness)

The measurements of the cross-sections shown make it very clear that the actual layer thickness of the material supplied does not correspond to the nominal thickness of 0.25 millimetres stated on the packaging. Instead, the measured values range from around 112 to just under 230 micrometers, depending on the position.

This range is well below the nominal 250 micrometers and corresponds more to a typical 0.20-millimeter pad, as is common with many OEM supplies from Honeywell or its assembly partners. The visible structure of the material with the fine-grained, evenly distributed filler matrix clearly indicates a genuine phase change material at OEM level. However, the uneven progression of the layer thickness is striking in this combination because PTM pads are normally supplied with relatively tight manufacturing tolerances. Either LTT remnants have been sold or it is from a rejected batch. This is not particularly bad, but it is a fact that the surface is not homogeneous either. The following pictures were taken with different light angles and resolutions:

The reasons for such deviations can only be meaningfully classified if the typical production method is taken into account. Most PTM pads are produced as large cast or coated sheets, which are then calendered to the desired thickness while still solid. The material is compressed between two rollers to its target thickness before the sheets are laminated onto carrier film and later cut to size. This process inevitably creates edge zones in which the pressure drops slightly or the material deforms laterally. This explains the measurement images in which the thickness drops significantly towards the edge or increases again slightly at certain points, depending on how the web has behaved during the coating and rolling process.

Another possible cause is the way the pads are made up. If individual pieces are punched out of a larger web, sections of edge areas with their greater tolerances can inadvertently end up in the finished product. In the case of retail packaging that is tailored to end users, it can happen that pieces of web that are not perfectly centered are used. This is not necessarily a quality defect, but often a question of material utilization, as long as the function is not impaired by the subsequent burn-in. PTM materials in particular compensate for some of their initial tolerances themselves during heat treatment because they liquefy briefly and distribute themselves homogeneously under pressure.

The fact that the average values of the measured thicknesses tend to be around 0.20 millimeters indicates that the underlying raw material actually corresponds to the usual 0.2 mm variant. The specification of 0.25 millimeters on the packaging can have various background reasons. It could be a case of rounding, where the manufacturer’s specification has been deliberately rounded up in order to communicate a wider tolerance. It is also conceivable that the material was intended to be slightly thicker during processing, but the final degree of rolling produced a slightly thinner result. Of course, this cannot be verified without looking at the production batches.

The 3D measurements of the cross-sections also show local compaction and zones in which the material was visibly more compressed. This suggests that the web had a certain degree of micro-roughness or thickness variation in its initial state and that this was not completely homogenized during subsequent cutting. Although the difference between around 110 and over 220 micrometres is significant, it is still within the range of what can occur with large-area, unlabeled PTM pads. Only during installation, when the material melts at around 45 to 60 degrees Celsius and spreads under contact pressure, does the layer level out significantly. For this reason, the BLT after burn-in is the much more significant parameter for the PTM type, while the raw tolerance ex works hardly plays a physical role.

The measured values and the visible structures therefore fit very well with a genuine OEM PTM on a 0.20 mm basis, which was presumably specified to 0.25 millimetres during the manufacturing process, but does not achieve the nominal thickness in practice. Whether this is a deliberate internal compromise or simply part of the material tolerance cannot be clearly determined. However, the subsequent thermal behavior will show whether the burn-in curve corresponds to the known Honeywell characteristic.