New possibilities for opening electronic components

Significance and challenges of single event effects (SEE)

Single event effects (SEE) are playing an increasingly important role in the investigation of radiation effects in electronic components. SEEs occur when particles of cosmic, ionizing radiation from space strike electronic components. This can cause both temporary malfunctions and permanent damage that renders an electronic device irreparably inoperable.

The investigation of SEEs caused by heavy ions is one of the most challenging areas of radiation effects research. On the one hand, SEEs are very diverse and require extensive test environments. Secondly, heavy ion beams are required to test the components. Alternatively, the effects of heavy ions can also be simulated using short pulsed lasers. These require direct optical access to the sensitive areas of the components.

Methods for opening enclosures for SEE tests

Whichever method you use, a direct access to the die of the component is required, without the casing being in the way. For this, the casing must first be opened. Especially COTS (commercial off-the-shelf) components, which are mass-produced for commercial use, pose significant resistance. They are typically fully encapsulated in an epoxy resin casing.

Fraunhofer INT already has extensive experience in opening such casings using hot acid. This method has the advantage that the components can be opened very quickly (often in less than a minute) and large quantities of components can be opened in a short time. However, there are some serious disadvantages. The acid not only harms the packaging material, but also other components. The bonding wires in particular are at high risk as they are only a few micrometers thick. Gold bonding wires are normally resistant to sulphuric and nitric acid. In the COTS sector, however, there has been a trend towards other materials such as copper or silver for years. Such bonding wires can only withstand brief contact with cold acid and even then, are already partially corroded, which can impair their electrical properties. The interesting components from the automotive industry in particular have very resistant housings that cannot be opened with acid without rendering the die unusable.

Plasma process as an alternative to acid opening

These disadvantages can be avoided by dissolving the plastic casing using a plasma process. In 2023, Fraunhofer INT therefore procured a new device that works with a patented, oxygen-only process that is fundamentally incapable of damaging the die or the bond wires. An example is shown in Figure 1. The disadvantage of this process is its slowness (several hours), so that acid methods are still used for large quantities.

In order to assess whether the component opening was successful, precise optical images are required that can resolve structures down to the micrometer range. A recently acquired digital microscope has capabilities even beyond this. The software is also able to obtain three-dimensional information by superimposing images with a slightly different focus. In this way, for example, the thickness of material residues on the die can be determined. Figure 2 shows an image of a die surface after an etching process on which material residues with a height of up to 60 micrometers were measured. Material of this thickness can significantly distort the results of a SEE-test. 

Challenges and technologies for precise SEE tests

SEE-tests on a die opened from the top increasingly lead to problems. The metallization layers are becoming thicker and thicker and thus slow down the ions considerably. With a SEE-laser-system, it is even impossible to penetrate through metallic layers. Additionally, components built using the flip-chip method cannot be opened from the front at all. In this case, the backside of the component must be exposed, the heat sinks need to be removed and the silicon substrate must be uniformly thinned down to a few micrometers. For this purpose, a special precision milling machine is required.

Thinning the substrate to a few micrometers is a major challenge in practice, as the dies are often thermally stressed on the housing, i. e. they are not flat but curved. If the substrate is milled flat, a substrate layer with thickness differences of up to several tens of micrometers remains at the end. These can lead to dramatic differences in the irradiation tests and therefore to incorrect results. 

To take this into account, the new milling machine at Fraunhofer INT carries out a mechanical 9-point curvature measurement of the surface before milling and uses this to create a map of the curvature, which the milling head then follows. Figure 3 shows such a map.

Once the chip has been roughly thinned and polished, the milling machine can perform an interferometric thickness measurement of the remaining substrate. The display of the spectrometer can be seen in Figure 4. Here, a residual thickness of 29 micrometers was determined. This measurement is then carried out over the entire chip and the curvature map is corrected for precise further processing.

Use of X-ray technology 

Opening components often reveals unpleasant surprises about their inner structure. This leads to a number of components being destroyed before a non-destructive method can be found. It is therefore important to know the inner structure of the component before the first opening attempt. For this reason, an X-ray machine was also procured, which can X-ray components with micrometer accuracy. The images can be imported into the software of the milling machine and placed as an overlay over the camera image. If the component is rotated, the X-ray machine can also be used to reconstruct details of the component using computer tomography.

Thanks to the new equipment for component opening, Fraunhofer INT is now able to successfully perform SEE-examinations on components that are difficult to open and has multiplied its options in this respect.