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Publication Date: 08/1/2008
Archive >  August 2008 Issue >  Special Feature: Test and Measurement > 

Non-Destructive Cross-Sectioning with Quick Micro-CT
CT results of BGA with micro vias.

Computed Tomography is technology that allows a sample to be virtually cross-sectioned without harming the sample. Long familiar in the medical world, it is only in the last decade that it has been actively used in the electronics industry. As CT has been adopted for electronics, the industry has seen dramatic reductions in cost along with an increase in resolution.

Computed Tomography requires three stages: scanning the sample and collecting 2-D projections, reconstructing the projections into a 3D-volume set, and analyzing the volume set as a 3D model or as virtual cross-sectional slices. Scanning and reconstruction times typically depend on the level of resolution and detail required, and can range from 30 minutes to over five hours.
Slice of BGA balls with void.

A new CT solution drastically reduces this time to a total of less than five minutes for scanning and reconstruction while maintaining sufficient resolution and details for the majority of applications. These results are possible by using enhanced synchronization of scanning algorithms and sampling rate. Add to this a combination of a high-power Microfocus X-Ray tube and fast reconstruction algorithms supported by hardware accelerators, and we reach previously unknown speeds for CT 3D X-Ray inspection.

Electronic Inspection
Today's product trends that push for continuous miniaturization have increased the need for ever-increasing quality and product reliability, which in turn drives the need for high resolution and fast X-ray inspection solutions. The semiconductor packaging and electronic assembly industries were the first to require high resolution CT. Similar needs are now being seen in the automotive, aerospace and medical device industries as well. By extrapolating from the needs of the electronics industry, we can anticipate the future needs of these industries.

Shrinking geometries mean increased system complexity, leading to developments in packaging technology that address increasing I/O interconnects. Increasing interconnect complexity requires key packaging technology parameters to scale accordingly. New approaches, such as high density wafer-level bonding or stacking of modules in a cube-like fashion lead to a need for further 3D system integration.
Cone beam reconstruction.

Technological advances of this nature invariably lead to some significant challenges for inspection — whether it is in production testing, process monitoring, quality assurance, failure analysis or R&D.

For example, AOI is widely used to ensure I/O interconnect integrity. However, with increasing numbers of hidden interconnects — ranging from ball grid arrays (BGA) to contacts within encapsulated components (SiP), inspection techniques are needed that can visualize internal structures and component composition. High resolution X-ray inspection provides exactly what is needed here. 2D Microfocus X-ray inspection had been introduced in the early 80s and continues to be the most common X-ray system in use today. 2D inspections enable the assessment of hidden solder points including automated voiding calculations or in-depth multiple parameter pass/fail criterion for BGA testing. Through the use of oblique viewing, where an X-ray image can be taken at almost any viewing angle, advanced inspections can be performed for bonding wire integrity or BGA open contacts. But now the technology trend toward using stacked die and 3D packaging has forced the need for true 3D X-ray inspection.

When compared to 2D inspection, 3D inspection can isolate areas of interest and ignore information above and below the region of interest. An example is the ability to select a single die to be easily viewed in a stacked die component. Another application is comparing 3D models of a sample before and after highly accelerated testing to determine the effects of the test.

To ease understanding and image interpretation, 3D images can be view as cutaway models, showing just where the analysis is occurring. With precision systems, 3D inspection allows true measurements to be performed, regardless of system magnification. In addition, the dataset can be virtually sliced with a high degree of precision, such that the same plane of production samples can be repeatedly analyzed automatically.

Fast-moving advances in semiconductor packaging technology calls for increased levels of inspection accuracy, leading to a significant move from 2D to 3D X-Ray inspection, since 3D circuit constructs demand 3D X-ray inspection. Applications range from packaging analysis — with the full assessment of wire bond alignment and integrity including stacked dies — to analysis of multi-layer boards for thermal stresses and via plating and board delaminations, inspection of assembled PC boards with lead-free solder joints to final assembly of various electro-mechanical devices embedded within integrated sensor systems.

Conventional µCT
CT is the method of choice when performing 3D-analysis in R&D as well as failure analysis. Microfocus 3D µCT X-ray inspection solutions have been available for more than a decade. The methodology typically deployed is based on the cone beam reconstruction algorithm, called the Feldkamp method. Radiation sourcing from the focal spot leads to a shadow image of the object on an X-ray sensing detector. During a CT-scan a sample is rotated around 360° in the X-Ray tube's beam. The rotation of the sample is paused at predefined steps. At each step the rotation stops while the X-Rays are gathered. These images are called projections. The main limitation to widespread use of µCT is found in typical image acquisition times of 1 to 8 hours. The underlying cause of long-scan durations is to improve 2D image quality. Among many other factors, such as focal spot size, geometric magnification, sensitive detectors, etc., the two major image quality requirements that directly correlate to scan duration are reduction of noise effects and limitation of longer-term shifts in geometry or performance.
Conventional µCT (left) and QuickScan (right) of a BGA with volume views (top) and views of a slice (bottom).

Historically, for microfocus X-ray sources to have small focal spot sizes, the X-ray intensity was also low. Low intensity meant that the X-ray image could be significantly affected by background radiation, called X-ray "noise". Microfocus X-ray systems used image integration for noise averaging where 10 to 100 images would be averaged together, and stray noise would be filtered out. These integrations led to even longer scan times. In addition, since the intensity was so low, slight shifts in geometry or performance over time (focal spot stability, X-ray intensity, thermal expansion etc.) would have a considerable effect on the repeatability of X-ray output. Other factors also have an impact on µCT quality and resolution, such as optical distortion in image intensifiers and low sensitivity or acquisition speed of digital panels.

Technological Advances
The goal of fast µCT inspection required three new developments:

  • Achieve high X-ray intensity for small focal spot size to reduce noise levels and averaging.
  • Develop techniques for maximum stability in X-ray intensity and image quality.
  • Employ advanced detector and reconstruction solutions for implementation of fast µCT.

High-Power Target
One problem is the thermal stress that builds in the within the minute focal spot area. This happens because up to 98 percent of the electrons' kinetic energy is transformed into heat within the focal spot area. This thermal stress can lead to target damage. The majority of µCT systems use a directional target X-ray source to achieve high X-ray intensity with minimum target damage. Unfortunately, directional target technology cannot achieve the very small focal spots required to resolve advanced interconnect applications. While a transmission target can achieve the small focal spots, typically there is significantly lower intensity. When higher X-ray intensity is desired, as in faster µCT, limitations in heat conductance require broadening the electron beam which defocuses the focus spot and degrades image resolution because of the larger focal spot size.
QuickScan volume view and virtual cross-sections of µBGA with micro-vias, wedge bonding.

This shortfall has been addressed by the development of a "High-Power" target. A 10-fold increase in thermal conductivity has been achieved compared to conventional transmission targets. Hence high energy electron beams can be kept in focus to maintain small focal spot size for high image resolution. Using a JIMA mask, a test pattern of 2µm can be clearly resolved even for a target power over 20 Watts.
True Intensity Control
In Microfocus X-ray tubes, electrons emitted at the filament are accelerated towards a transmission target while focusing the electron beam to a small focus spot. While in conventional Microfocus X-ray tubes only the emission current at the filament and the acceleration voltage are controlled, an alternative target technology allows assessment of the true current reaching the target. Based on a continuous feedback, the TXI technology adjusts emission currents to ensure maximum X-ray performance stability and hence consistent image quality. This leads to less stringent µCT averaging requirements for the acquisition of a single projection and, even more vital, to more stable projection quality over the 360° sample rotation.
Reconstruction Technology
New developments in digital X-ray detector technology have shown that advanced sensor arrays can deliver the high dynamics and resolution needed for a fast µCT. It was decided to use a high speed X-ray detector with a pixel size well below 150µm and a dynamic range better than 2000:1, with contrast resolution better than 0.5 percent. The detector has sufficient contrast resolution to acquire the high intensity X-ray images without integration or averaging, and the pixel size of the detector is smaller than the geometric lack of sharpness that most µCT applications produce. Reconstruction time varies greatly with the number of projections and required µCT resolution. Usually the reconstruction of a cube with 512 x 512 x 512 volume elements (voxels) can take 15 to 30 minutes with standard reconstruction software. It was decided to use a reconstruction solution with dedicated hardware accelerator boards (the equivalent of 16 coprocessors) achieving a 5123 voxel reconstruction within 2 minutes.
Ultra-Fast µCT
The resulting CT (QuickScan) achieves complete µCT, from initiation of the scanning to inspection of virtual cross-sections in the reconstructed volume model within a couple of minutes. A comparison for a BGA shows slight variations in the details between a Quality Scan (conventional CT and a Quick Scan (Fast CT). For the conventional µCT 1024 projections were acquired and 880 for the QuickScan. Volume views show that both scans enable an in-depth inspection of the solder balls and interconnecting surfaces. Minute differences can be seen in surface smoothness. Slices through the BGA show that even small voids can be visualized equally well in the significantly faster QuickScan.
Continuous growth in electric and electro-mechanical system complexity with increasing exploration of all three dimensions drive the need for 3D Microfocus computed tomography.
We have seen that µCT inspections can provide: true X-ray Intensity (TXI) control for maximum X-ray performance stability and hence consistent image quality; high-power target that achieves small focal spot sizes for high resolution at high X-ray intensity; high-speed digital flat panel detectors for fast image capture supported by dedicated solutions for fast reconstruction.
By reducing µCT inspection times from hours down to a couple of minutes, the innovative QuickScan solution achieves: significantly improved inspection throughput; reduction in inspection cost; higher volume µCT applications; lower sensitivity to shifts in geometry or performance; increased confidence in terms of product integrity, quality and reliability.
YXLON now offers Y.QuickScan, the ultra fast µCT solution on its Feinfocus X-ray inspection systems.
Contact: YXLON International, 3400 Gilchrist Rd., Akron, OH 44260 330-798-4800 fax: 330-784-9854, E-mail: Web:

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