Measurement according to ASTM D 5470-17
All pastes tested today were tested under identical laboratory conditions. I determine the comparative test data in a uniform and standardized manner, whereby all interfering factors (such as die distortions or non-coplanar contact surfaces) can be excluded. For me, ASTM D 5470-17 is the primary test method for determining thermal conductivity and thermal resistance, whereby I only use ASTM D 5470-17 and not the complementary electrical methods according to JESD 51-14. The values in a TIM provider’s data sheet should be developed using ASTM D 5470-17 to produce comparable values to be determined under the following conditions:
– Controlled surface conditions
– Unidirectional heat flow conditions
– Parallel contact surfaces
– Precisely known clamping forces
And this is exactly where my TIMA5 comes into its own. It is a fairly compact all-in-one tabletop device that combines the measurement setup and the required PC in one device. It is therefore a self-sufficient and, above all, automated measurement setup that I can also run in parallel to other tasks in the background. All data is stored directly on the NAS via the network, so that’s safe. The device is calibrated and has already passed the first plausibility test. I measure the pastes at an average paste temperature of 60 degrees.
As this all looks a bit complex to outsiders, I have placed the individual assemblies against the function diagram so that you know where and how the measurements already explained take place. I have already explained in detail what happens in the background and how the whole thing works in the linked basic article. I don’t need to repeat all that again.
I will illustrate this once again in the already familiar diagram so that you can better visualize the meaning of these values to be determined. We can see that the effective thermal resistance affects both the material and the two contact surfaces. Yes, there are very sophisticated methods, including pulsed lasers, which can also evaluate the pure bulk value very accurately, but in practice we ALWAYS have contact surfaces. I use reference bodies with a standardized (low) roughness for the measurements so that I can also draw conclusions from these in practice. In the end, I have two values, the effective thermal conductivity and a value averaged over all measuring points of the different layer thicknesses BLT minus the extrapolated contact resistance.
External cooling is provided by a laboratory chiller from IKA, which can maintain the water temperature almost to the decimal place and which not only cools but can also reheat if necessary so that the required 20 °C water temperature can always be maintained. The hoses were connected using Festo couplings and special hoses.
Test equipment for material tests, accuracy and test preparation
My Keyence VHX 7000 and EA-300 are responsible for material testing and measuring the pastes and pads, enabling both exact measurements and fairly precise mass determinations of the chemical elements. But how does it actually work? The laser-induced breakdown spectroscopy (LIBS) I used for this article is a type of atomic emission spectroscopy in which a pulsed laser is directed at a sample in order to vaporize a small part of it and thus generate a plasma.
The emitted radiation from this plasma is then analyzed to determine the elemental composition of the sample. LIBS has many advantages over other analytical techniques. Since only a tiny amount of the sample is needed for analysis, the damage to the sample is minimal. This relatively new laser technique generally requires no special preparation of samples for material analysis. Even solids, liquids and gases can be analyzed directly.
LIBS can detect multiple elements simultaneously in a sample and can be used for a variety of samples, including biological, metallic, mineral and other materials. And you get true real-time analysis, which saves a tremendous amount of time. As LIBS generally requires no consumables or hazardous reagents, it is also a relatively safe technique that does not require a vacuum as with SEM EDX. As with any analytical technique, there are of course certain limitations and challenges with LIBS, but in many of my applications, especially where speed, versatility and minimally invasive sampling are an advantage, it offers significant benefits.
I would first like to point out that the results of the percentages in the overviews and tables have been intentionally rounded to full percentages (wt%, i.e. weight percent), as it happens often enough that production variations can occur even within what is presumably the same material. Analyses in the parts-per-thousand range are nice, but today they are not useful when it comes to reliable evaluation and not trace elements. However, every day in the laboratory starts with the same procedure, because when I start, I work through a checklist that I have drawn up. This takes up to 30 minutes each time, although I have to wait for the laser to warm up and the room to reach the right temperature anyway.
- Mechanical calibration of the X/Y table and the camera alignment (e.g. for stitching)
- White balance of the camera for all lighting fixtures used
- Check alignment of LIBS optics and standard lens, calibrate alignment of laser to own optics (x300)
- Test standard samples of the materials to be measured and correct the curve if necessary (see image above)
I will leave the theory at that, because we are waiting for the measurements.
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