What’s inside my USB drive?

New packages and new packaging technologies need corresponding high-resolution non-destructive testing methods. But which testing methods are able to image nano packaging defects? It is necessary to develop evaluation techniques to find out the possibilities and limitations of different inspection technologies. Some ways are the use of so-called reference objects and defect samples. A third way to evaluate non-destructive testing methods is the use of electronic systems with a limited volume and complexity like a USB memory device.

An interesting example to evaluate NDT methods was a micro USB memory device with 2 GB memory size. These investigations have been done at Dresden University of Technology, Germany. The electronic part inside of this memory device was designed as System-in-Package (SiP) housed in a metal cage with polymer cover.  To evaluate the inside structure X-ray radiography and X-ray CT have been used (180 kV Phoenix Nanome|x and Nanotom systems). X-ray radiography results showed the inside components but even in oblique view inspection only poor information about the 3rd dimension.

The PCB, some passive components (capacitors and resistors), the bonding wires of the controller die and the memories (material Au), the crystal oscillator and the activities LED were visible. More information about the inside structure and the location of the components showed the CT results revealing the bonding area of four stacked memory dies. This was state of the art in packaging technology for relatively inexpensive goods - the price of this USB memory device was approx. 23 US$ in 2008.

During a discussion between colleagues of Dresden University of Technology and Waygate Technologies an idea was born to compare the investigation results from 2008 with USB memory devices from 2012. The questions were: 

• What technologies have been used to produce current memory devices? 
• Are the statements of technology roadmaps valid for this market?
• What details are visible with newest X-ray inspection technologies?

To do this evaluation a 32 GB USB memory device was chosen and different CT scans with different resolutions have been performed at Waygate Technologies' facility in Wunstorf (Germany). The visualizations of the CT results have been done by Volume Graphics in Heidelberg (Germany).

Detail views and virtual cross-sections led to more information about the construction of the device and of the used technologies. The PCB, a small number of passives, the controller die and two stacked memory dies are visible in the CT images.  The analysis of the CT results showed that the packaging technologies are nearly the same since 2008. The progress in development is affected by the semiconductor industry – in this case by the memory producer. But one detail was very surprising:

During the analysis of the CT results, especially of the 2D virtual cross-sections, structures on the controller die’s surface have been found. A more detailed evaluation of the pictures showed metallization layers and the wire bonding connections on the die, these were later identified by EDX analysis as tungsten traces with an aluminum coating.

These surprising results show the fast development of X-ray detectors in the last years. The CT recording has been done with Phoenix’s 14-bit grayscale temperature stabilized DXR X-ray detector with CsI-scintillator. After finding these results, an additional CT scan with a 12-bit X-ray detector with Gd2O3-scintillator on the same USB memory device has been performed for comparison reasons. This CT showed structures on die’s surface only poorly – nearly the noise in the pictures.

The non-destructive X-ray techniques (especially CT) are very important and essential for the evaluation and characterization of miniaturized electronic packages. To get excellent results with these methods a good material contrast of the sample and a high resolution of the X-ray system are necessary. The use of samples with known inside structures is a useful way to evaluate non-destructive testing methods.   

The inspection of different USB memory devices at different technological ages shows the dominance of the semiconductor development. Electronics packaging evolution runs more in waves – as semiconductors it demands.

A further result of these investigations is the evaluation of latest 14-bit temperature stabilized X-ray detectors. These detectors are able to deliver X-ray pictures with a very high contrast and low noise. Very thin structures on silicon dies are detectable. 

To watch a fascinating high-resolution 3D evaluation movie based on the voxel data of the CT scans described in this article follow the link below!