August_EDFA_Digital

edfas.org 19 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 3 absorption image because of the low x-ray absorption of these components. With shrinking devices coupled with 3D heteroge- neous material integration in next generation advanced packages, it has become apparent that many classes of failures may no longer be easily detected even with the best absorption contrast based XRM. This is mainly attributed to limits of resolution of the technique and the inherent lack of contrast. The best spatial resolution currently claimed by a commercial XRM is no better than 0.5 µm. But to collect data with this resolution takes several hours and may only be feasible with very small and thin packages. What is more challenging is the inher- ent lack of contrast of conventional x-ray imaging and its inability to reveal defects in low Z organic materials, especiallywhen these defects are in the presence of highly absorbingmaterials, for example, a sub-micron size crack within a solder ball, or a void in the underfill in the neigh- borhood of a solder ball (Fig.1). These light-element components are typically inspected using alternative techniques. Ultrasonic tools such as C-SAM can reveal voids if they are at least a few microns in size and if the delamination is in the X -plane (lateral) within the encapsulant. However, C-SAM gener- ally requires soaking of the package in water which may not be ideal for electronic products that normally need to be kept from moisture. Furthermore, fault isolation with C-SAMs is often made complicated with multi-layer and 3D chip architecture. Some other failure mechanisms that currently cannot be successfully inspectedwith C-SAMor XRM include side- wall (vertical) delaminationof chip,micro cracks in copper bumps, micro voids or small cracks within underfill and RDL, and epoxy voids and die cracks within silicon die. MODIFIED TALBOT-LAU X-RAY INTERFEROMETRY FOR PHASE CONTRAST AND DARK FIELD IMAGING This article describes a laboratory x-ray imaging technique that may resolve most of the aforementioned failure mechanisms, which currently cannot be detected nondestructively. Many of these issues are related to resolution and lack of contrast for low Z materials. These limitations are typical of semiconductor advanced pack- ages and other types of electronic packages, ranging from medical devices, LEDs, and battery components. The same requirements are also applicable to industrial applications including ceramics components, polymers, additive manufacturing, biotechnology, tissue engineer- ing, pharmaceuticals, agriculture, and food science. To overcome the limitation in resolution and contrast, an XRM that has spatial resolution of 0.5 micron compa- rable to the best-in-class commercial XRMwas developed, but with the additional features of a modified Talbot-Lau interferometry system. In such a system, three x-ray radio- graphs are acquired simultaneously: absorption contrast, phase contrast, and scattering (dark field) contrast. This article will focus primarily on the advantages of these new developments to solve PFA problems in advanced packaging and materials research. Radiographs of a polymer shower hose demonstrat- ing these three contrast mechanisms are shown in Fig. 2. The Talbot interferometry technique has been demon- strated successfully in synchrotron beamlines with high spatial coherence. While proof of concept of this technique has also been demonstrated in laboratory systems, so far there is limited commercial adoption because of low throughput, limited field of view, and poor resolution. [3-6] Absorption contrast is the conventional mechanism that current x-ray imaging techniques (real time x-ray and XRM) are based on. Phase contrast and dark field/scattering contrast hold potential to significantly advance the field of x-ray imaging, ranging frombiomedi- cal imaging to industrial applications, which include imaging of semiconductor advanced packaging. For example, x-ray phase contrast is three orders ofmagnitude (> 1000X)more sensitive than absorption contrast at x-ray energies greater than 20 keV. Phase contrast is gener- ated by the refraction of x-rays that occurs at boundaries of materials with different Fig. 1 On the left is an optical image of a cross section showing void inunderfill next to the solder ball. The illustration on the right shows lateral and sidewall delamination in underfill and cracks in microbumps that are difficult to detect.