Test equipment for material tests on thermally conductive pastes: microscopy and laser-induced breakdown spectroscopy (LIBS)

1 - Thest methods and equipment 2 - Microscopy and material analysis

I have already written in detail about the determination of thermal resistance and thermal conductivity, but of course I also have to explain the second part of the measurements transparently, because I am not doing these measurements for their own sake, but for a good reason. The microscopy, material testing and measurement of the pastes and pads is carried out by my Keyence VHX 7100 together with the EA-300 as the analysis unit. This enables both precise measurements and quite accurate 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 is a huge time saver. 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 distinct advantages.

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. Of course, the energy supplier is happy.

• 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 (microscopy)
• Check alignment of LIBS optics and standard objective, 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. For the very curious, here is the product description: