Recent Advances in X-Ray Technology
Taking x-ray images goes back over 100 years. Since then, there have been numerous advances in x-ray technology and these have been increasingly applied in helping the manufacturing of electronic components and assemblies, as well as in their failure analysis.
Most recently, this has been rapidly driven by the reduction in device and feature size and the movement to using newer, lower density materials within the structures, such as copper wire replacing gold wire as the interconnection material of choice within components. Another driver for developments is the engineering of single 3D packages with multiple chips stacked vertically one on top of the other, which results in smaller and more efficient packaging of devices.
In order to meet these challenges and those in the future, there have been a number of recent key improvements to the vital components within x-ray systems.
The choice of available technologies, however, means selecting the tube/detector combination, which is optimum for a particular electronics inspection application, is no longer so clear-cut. For example, one configuration may provide certain benefits that are applicable for one area of electronics inspection, whilst being less valid for others. This paper will review the various x-ray tube and detector types that are available and explain the implications of these choices for electronics inspection in terms of what they provide for inspection regarding image resolution, magnification, tube power, detector pixel size and the effects of detector radiation damage, amongst others.
This paper will also look in detail at the capabilities of high end CT systems to inspect wafer bumps, copper pillars and TSV’s, new designs are reducing key dimensions of all of these interconnections challenging x-ray systems to produce clear images.
Key words: x-ray, semiconductor, Image chain, PCB inspection, CT methods
The role played by x-ray systems in the inspection of electronic assemblies is well understood. Unlike machine vision and optical inspection equipment that permit line-of-sight inspection of components and circuits, x-rays penetrate material to expose the hidden solder joints and the contact side of devices, such as flip chips and BGAs.
As component densities increase, interconnects are becoming more obscure, and x-ray imaging is growing more critical in the detection of defects. For this reason, manufacturers of x-ray systems are giving particular attention to ways of improving contrast, sharpness, and real-time inspection capability.
Fortunately, x-ray systems have kept pace with the inspection requirements of smaller feature sizes (<1 µm) being mandated during packaging and assembly.
The development of microfocus and nanofocus tubes and the added benefits of technologies such as automated control over tube output intensity, oblique viewing, rotation of the image chain, and sophisticated GUI (graphical user interface) software dramatically extended the imaging capabilities of x-ray systems.
THE TUBE, THE HEART OF ANY SYSTEM
Carl H. F. Müller developed the first x-ray tube in 1896, the fundamental operating principle has remained unchanged over time. In all tubes, the electron beam emitting from the cathode enters the target and collides with particles of the target material. When an electron beam hits the target, the electrons enter the target material (interaction layer) and collide with target material particles, and are slowed and deflected in various directions. They then collide again and again with target material particles until the kinetic energy drops to practically zero. With each collision, electrons are slowed and their loss in kinetic energy translates into radiation energy. The point of the collisions is called as Focal Spot. The location and size of the Focal spot is a key factor in determining image resolution and the quality of an x-ray image.
Early x-ray systems used within the electronics industry had a “closed” or “sealed” x-ray tube.
Sealed tube technology is over 100 years old and still used in low cost systems due to its low maintenance costs. However, the image quality of those sealed tubes deteriorated throughout its life as the x-ray target (anode) could not be rotated or changed. When this type of tube exhausts its resources, it has to be replaced at considerable cost. Another limitation of directional x-ray tubes is its mechanical construction. The location of the x-ray focal spot relative to the closest position that a sample could be placed to the tube also severely limits the magnification that such an x-ray system could have. All together it meant that the Feature recognition, the ability to see small objects, was also very limited with 8 microns being very good for a new system and 20 microns typical.
The sealed transmission tube is a development of closed directional tube. It works using filament-free technology to generate the x-ray beam and the tube is still factory sealed. It allows higher magnifications to be achieved because the focal spot can be much closer to the sample like in open transmission tubes.
Due to the large x-ray cone angle (>170 degrees) angled inspection views, so important for joint inspection of BGAs, can be achieved without loss of magnification. However, the downside of closed tube remains – these tubes are unable to be serviced and require the whole tube to be replaced upon failure or wear. Therefore the power is usually reduced in order to extend the lifetime of the tube.
The “open” transmission x-ray tube was a major step forward for x-ray inspection. This technology itself is almost 55 years old, but was taken into use in the electronics industry in 1982 when the German company Feinfocus introduced the first open Microfocus tube. Most modern high technology electronics systems use Open Tubes.
Figure 3. Open transmission x-ray tube
Open Microfocus tubes – a stainless steel tube that can be opened anytime for cleaning and maintenance, and is evacuated prior to each use – are used in high-resolution applications of electronics assembly and packaging. Such tubes can provide a spatial resolution of less than 1μm, with geometrical magnifications of as much as several thousand times.
An openness of the tube might have generated some concerns about maintenance in past. These have been addressed by leading manufacturers of open tubes:
- Pre-vacuum pumps are maintenance free
- Vacuum inside the tube is higher improving feature recognition
- Filament lifetime has been extended some 4 times
- Exchange of the filament is fast as a pre-adjusted quick change unit can be clicked in place fast and easy
All in all, the benefits of the open tube technology outweigh the need for maintenance:
- No limited lifetime
- Best “as new” performance after exchange of the filament
- Higher target powers provide higher image intensity
- Availability of different targets for specific applications
- High Power Target
- High Resolution Power target
- Conical target
ADVANCES IN X-RAY TUBE TECHNOLOGY
The trend toward smaller and more densely populated electronics components, and the emergence of MEMS and MOEMS, led to the development of nanofocus x-ray technology. Nanofocus technology is defined as having a focal spot of less than 1 mm in diameter, which enables the level of detail and resolution needed for the inspection of low-density structures and ultra-small features common in today’s electronics components. The technology is an integration of tube and sophisticated software for controlling performance aspects such as short- and long-term stability, image contrast, brightness, and amount of radiation.
Modern open type x-ray tube’s technology includes the following features:
True x-ray intensity (TXI) control. Unlike technologies that attempt to measure and control the input level of the high voltage and current to the x-ray tube, TXI control ensures controlled and stable output X-ray intensity. The result is a consistent and sharp x-ray image throughout each time inspections are carried out. TXI ensures repeatability during automated analysis routines, a capability that is of particular interest in the production environment.
Figure 4. X-ray Image Quality without True X-ray Intensity (TXI) Control.
Due to the varying x-ray intensity output over the time of the inspection process, the image contrast and brightness varies considerably over a 24-h period.
Figure 5. X-ray Image Quality with True X-ray Intensity (TXI) Control.
Due to the constant long-term stability of the output X-ray intensity, the image contrast and brightness is 100% consistent, even over a long period of time.
Multifocus x-ray capability
A need exists for both microfocus and nanofocus tubes for real-time x-ray inspection. For contract manufacturers, where inspection requirements can vary from microfocus applications that demand high x-ray output to nanofocus applications that demand high-magnification and high-resolution, Multifocus x-ray tubes are ideal for this. They incorporate a high-power mode (>15W target power) for dense structures that require high intensities for inspection. One of the challenging applications is insulated-gate bipolar transistor (IGBT). Without high target power it becomes hard to get meaningful information for analysis.
Figure 6. X-ray Image of IGBT, 130 kV, 61µA
High-power mode can deliver added value also if an inspection of the end product is required. Especially if the ROI (region of interest) is hidden in the high density case. Figure 7a shows an image taken with a Multifocus tube in Microfocus mode. Some information is missing due to the lack of intensity. Figure 7b shows the same ROI but the image is taken with High-Power Mode.
Figure7a. X-ray Image of IC inside of end product, Microfocus mode, 160kV, 55 µA
Figure7b. X-ray Image of IC inside of end product, High Power mode, 160kV, 54 µA
Typical microfocus x-ray devices don’t necessarily meet the unique inspection challenges posed by complex electronic devices, such as MEMS (micro-electromechanical systems) and MOEMS (micro-opto-electromechanical systems). The emergence of MEMS and MOEMS was a driving force behind the development of nanofocus x-ray technology. With a focal spot of less than 1 µm in diameter, nanofocus x-ray inspection systems can provide the detail and resolution required to inspect ultra-small components.
Besides inspections of MEMS and MOEMS devices, nanofocus systems are used to examine sub-micron components, circuitry, and assemblies in wafer-level packaging. In these applications, nanofocus tube design and system technology is the only inspection option that provides the resolution and sharpness required to detect defects in solder bumps and interconnects.
Figure 8. X-ray Image of a solder crack in 50μm Cu pillar, Sample size: 300mm wafer
Nanofocus technology is also used to inspect packages with non-filled die attach material, including the thermal adhesive that holds microchips in place. A nanofocus tube is needed to detect the slight difference in contrast attributable to the adhesive. In addition, nanofocus tubes can be used to check the silver particle loading in electrically conductive adhesives. This ensures that the material is homogeneous and filled with sufficient particles to achieve the desired conductivity.
Nanofocus systems can also check:
- Package delamination that can go “unseen” by microfocus x-ray systems;
- Sub-micron cracks and flaws in silicon packaging and fine bonding wires (under 25 µm);
- Orientation of the fibers in polymer materials
Figure 9. X-ray Image of a polymer material, voids and orientation of fibers in different layers are visible
Active cooling of the tube and target
The quality of CT applications depends, among many other factors, on the stability of the tube performance. As less than 2% of the energy of electrons appears in the form of x-rays. The remaining energy is mostly heat that should be effectively controlled by the system. For high-end CT applications with a long scan time it is critical to use active cooling of the target and the coils inside of the x-ray tube. The benefits of active cooling are:
- Quick temperature balancing allows higher productivity
- Minimized drift of the focal spot guarantees sharp images and the highest resolution
Figure 10. X-ray image of a 3-5μm crack in 500μm MMLC assembly. Sample size: 0.5mm without cutting
New types of targets
Transmissive targets, used in both open and sealed tubes, require that the x-rays, once produced at the focal spot, must pass through the thickness of the target to exit the tube and irradiate the sample (Figure 3.). Tungsten is the most commonly used target material. As the focal spot is reduced in size, the energy density at the target rapidly increases. For example, if a tube produces 1W of power into a 1-micron spot then to achieve the same energy density with a 20-micron spot requires 400W. Such large energy densities at small focal spot sizes produce heat that should be balanced by increasing the focal spot size. Otherwise we may destroy the target and the tube becomes non-operational. As a consequence of a bigger focal spot size we lose the system resolution. This is due to an effect called geometrical unsharpess or penumbra Figure 11.
Figure 11. Influence of Focal Spot Size on Image Quality/Resolution
Due to the geometrical unsharpness it will be difficult to inspect high dense chip components where resolution below 1 micron is required.
Open tube design allows the use of dedicated, Diamond-based targets, developed specifically for demanding applications. 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:
- High Power target – offers a resolution at a high-power factor 2-3x higher compared to standard targets (Figure 12)
- High Resolution Power target – offers a resolution at a nanofocus factor 2x higher compared to standard targets until 8 W
Figure 12. High power target keeping focus (left) and Conventional target with de-focusing (right)
Traditionally, analog detectors have been used, and are still popular today for applications where the resolution is satisfactory for the level of inspection required. An analog detector consists of an x-ray image intensifier (amplifier) coupled with a high resolution CCD (charge-coupled device) camera, which senses the x-rays and converts them into an analog signal that is then fed to a computer. Processing by the analog-to-digital converter in the computer results in some of the signal being lost to inherent noise, and the image that reaches the monitor is not capable of displaying the full range of grey tones. The best image intensifiers have a 8-bit image chain. Thus, despite having an x-ray tube with a microfocus or nanofocus spot size, the resolution of the image at the monitor is degraded to the point where subtle changes in material density cannot be detected, the low-density regions bleeding into regions of higher density.
Due to these known limitations of analog detectors, Digital Flat Panel detectors have become an industry standard. The Digital Flat Panel was a huge leap forwards, from 0.3 MPixels to 1 MPixels.
From a schematic standpoint, digital detectors are somewhat similar to analog detectors. The digital detector differs to the extent that the analog signal is converted to digital within the detector, as opposed to within the computer. x-ray photons striking the phosphor are converted to visible light, which, in turn, is converted into an electric charge by photodiodes. The photons create an electric charge at each pixel in the array proportional to the intensity of the light.
Digital detectors are available in various configurations, the most common being an amorphous-silicon imaging array with a cesium iodide (CsI) scintillator deposited on the imaging array.
ADVANCES OF DIGITAL DETECTORS
Image quality and the ability to inspect for defects is put to the test in situations where the density varies little between the solder ball and pad or within an interconnect that has a defect. Under such conditions, where the absorption of x-rays provides almost indistinguishable edge detection, smaller feature size requirements can exacerbate the condition, and viewing can be difficult, even with a nanofocus tube.
Therefore, while the target power and the size of the focal spot of the x-ray tube is the primary determinant of image quality, spatial resolution, feature recognition, contrast, and sharpness are also dependent on the quality of the digital detector. It plays an important role as it is responsible for processing x-ray waves into an image of visible light that can be seen and examined by the human eye, or by automated vision systems.
A modern amorphous-silicon flat panel detector (FPD) features a 16-bit format, more than 65,000 shades of gray, and some million pixels. These detectors also deliver selectable frame rates from 1 to 60 fps (Figure 13). The frame rate, which is the speed at which the detector can acquire images, enables the amorphous-silicon detector to provide image data in real-time, which means the results are displayed on the monitor as a live image, as opposed to being “frozen” for delayed analysis.
Figure 13. Image quality in movement. Real time image 127μm pixel at 30 fps (left). Standard mode 127μm at 10 fps (right).
The spatial resolution of detectors gets better. Although the size of the focal spot of the x-ray tube is the primary determinant of image quality it cannot be used for the benchmarking of different x-ray systems since there is no reliable/unified method for measuring focal spot size. The only reliable and independent method of compare the performance of x-ray systems is to measure the spatial resolution of the detector. The most widely used test gauge for this purpose is a JIMA mask – 3 line pairs: 0.4 – 15μm. The smallest visible gap between pairs shows the spatial resolution of the system (Japan Inspection Instruments Manufacturers’ Association; www.jima.jp/english)
Figure 14. Jima mask
It is important to mention that spatial resolution depends also on many other factors like geometrical magnification of the system and signal to noise ratio (SNR) of the detector. This subject will not be extended in this paper, but it should be noted that for better spatial resolution the higher value for both characteristics is better.
Another critical characteristic of the detector is contrast resolution AKA feature recognition. It indicates the smallest material difference, which can be resolved in the image (Figure 15). Some manufacturers of x-ray systems have developed their own test gauges for feature recognition, made of high contrast material. However, there is no independent gauge for benchmarking. Contrast sensitivity „works“ in the x-ray beam direction (z-axis) and is directly dependent on the amount of the shades of grey levels of the detector. The industry standard is 16 bits, given approximately 65000 shades of grey.
Depending on the type and manufacturer of the test gauge feature recognition down to 100 nm can be achieved.
Figure 15. Etched logo on the homogeneous ceramics substrate, thickness 0,8 mm
Approximate ratio between spatial resolution, feature recognition and focal spot size is as follows – the spatial resolution of an x-ray tube is approximately one half the focal spot size. Feature recognition for the tube is approximately one half of the spatial resolution. Thus, for single features in the 125 nm range, the x-ray tube must have a focal spot size of 500 nm (0.5 µm).
ENHANCING THE IMAGE CHAIN
The image chain consists of more than the digital detector. The computer processing the image data, and the monitor itself both play a role in image quality and the ability to view images in real-time.
Where the frame rate is sufficient (30 to 60 fps), images can been viewed and manipulated on a monitor. Typically, in instances where greater quality is desired for a particular image, the frame is captured and processed by the computer to produce the desired results.
A more advanced development of the image chain allows processing images during the image stream, and as a result, all the images viewed on the monitor are enhanced. The distinction can be significant. When viewing a crack in a solder ball (Figure 16), for instance, instead of selecting images that appear to best show the crack and then processing those images, the viewing of an enhanced image of the crack can occur in real-time, while the position of the sensor (or the specimen) is changed to provide multiple viewing angles.
Figure 16. Oblique view of BGA with 16bit DFP
Another high value tool for live image analysis is μHDR (Figure 17) which is averaging the exposure of the overall image and showing in glance the structure of the sample object.
Figure 17. μHDR live filter
ADVANCES OF COMPUTED TOMOGRAPHY
With the increasing demand for more functionality and the smaller size of electronic devices (such as cell phones, controllers, etc) the performance and size of print circuit boards and individual electronic components (such as different 3D packages) have become critical.
These complex assemblies drive the need to virtually cross-section a sample to simplify inspection. Computed tomography (CT) is the preferred way for 3-dimensional analysis of complex electronic assemblies.
This paper will not address the basics of CT theory as this has been covered in several papers therefore the focus will be on the various technologies available – Quality Scan CT, Quick Scan CT, laminography AKA Inclined/Oblique CT, limited angle CT.
QUALITY SCAN VERSUS QUICK SCAN CT
Microfocus x-ray technology has been used for µ3D applications for more than a decade. This methodology is based on the cone beam reconstruction algorithm, called the Feldkamp method. In the case of a cone beam CT, an entire volume is generated using one single 360 degree scan on an array detector.
Despite some limitations, described later, this method delivers the best possible CT scan quality. Depending on the system configuration (the type of tube, detector and manipulation) resolution below 2 microns is achievable.
Figure 18. the slice of micro bumps (60µm diameter)
This method allows fast data acquisition as the data required for multiple slices can be acquired in one rotation. However the scan/reconstruction time can still be from 30 minutes to several hours.
In order to pave the way towards even faster µCT inspection the following goals had to be fulfilled:
- Achieve high x‑ray intensity while keeping the 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 the implementation of fast µCT
Advanced x-ray systems, equipped with a Multifocus tube with High power target and TXI technology, 16-bit real time flat panel detector, and advanced reconstruction software can deliver QuickScan CT result within a couple of minutes.
A comparison on Figure 19 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 illustrated on the left 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.
Figure 19: Conventional µCT (left) and QuickScan (right) of a BGA with volume views (top) and views of a slice (bottom)
Further examples for high resolution QuickScan applications are depicted. Volume views allow detailed inspections of BGAs and bonding wires. The slice through the illustrated 3×3 BGA segment show solder interconnects micro-vias and voiding. The slice through a single BGA ball even shows the plating and filling of the micro-via below.
Figure 20: QuickScan – volume view and virtual cross-sections of micro-BGA with micro-vias, wedge bonding
LAMINOGRAPHY aka INCLINED OR OBLIQUE CT
While the cone beam method can deliver the best CT quality with the fastest time, it is limited by the width of the sample. Quite often the region of interest (ROI) is cut out from the bigger board. Otherwise the maximum magnification and therefore the resolution will not be sufficient for effective failure analysis on the board or wafer tray.
As the destruction of the sample is not acceptable in many situations, the laminography method is the second best alternative and could be called the true non-destructive test method.
It is most beneficial for the inspection of multilayer boards or even 3D packages where overlaying structures do not allow allocating the defects like voids, cracks etc.
Figure 21 shows a standard double-sided PCB where void calculation is not possible in 2D mode and a 3D scan is impossible due to the size of the sample.
The laminography method allows separating the horizontal slices of the sample by using a complex and synchronized movement of the tube, detector and sample tray. Contrary to the traditional cone beam scan the sample will not be rotated horizontally. Due to its purpose this method can be also called Micro3D Slicing.
Figure 22: Micro3D Slice, the horizontal layers of Figure 21 are separated and void calculation can be done
This method becomes especially useful when the automated inspection of complex boards and samples is required. Micro3D Slice can be integrated into a FNC sequence where we generate a 3D volume of ROI area and define the focus layer what will be used for void or BGA analysis later.
Figure 23: The Multi Area Void Calculation algorithm is used for defined slice in Micro3D Slice scan
CHALLENGES OF MICROELECTRONICS
All the latest advances of x-ray technology, described earlier, address the need to verify the quality of smaller and smaller 3D packages.
3D packaging is the general term that encompasses stacked components, 3D IC’s, Package-on-Package, System-in-Package, and many others. The primary driver of 3D Packaging is that the technology saves space by combining separate chips in a single package. The general expectation is that stacked packages must be able to maintain or lower the Z-height of a package, requiring thinned die, low-level interconnection techniques such as Thru-Silicon Via (TSV), copper pillars, microbumps. These technologies promise increased system integration at lower cost and with a reduced footprint.
All stacked packages have all the normal reliability concerns such as TSV void, bump uniformity, flip-chip solder reliability (see figure 8), package warpage and thermal stress.
Although most of these defects are difficult to detect due to the sub-micron sizes of defects or due to thinner and low density materials, modern x-ray systems make it possible by combining 2D, µ3D scan or Micro3D Slicing methods.
A recommended x-ray system for microelectronics applications would have a Nanofocus x-ray tube with TXI feature and at least 16-bit digital flat panel detector with balanced spatial and contrast resolutions.
Additionally, the manipulation of the detector, the tube and the sample tray is important. If the tube and detector can be moved independently it allows the operator to find the best possible signal-to-noise ratio at specific target power and magnification (Figure 24) and reduce the risk of potential damage to sensitive devices.
Figure 24: increasing intensity without changing tube parameters, the tube and detector are moved synchronously together or apart
The importance of these components becomes clear when inspecting the quality of wafers and single dies (Figure 8, 25, 26 and 27).
Figure 25: A 1 nm crack in 10μm TSV, Sample size: 300mm wafer
Figure 26: Micro void in 30μm TSV, Sample size: 50mm part of wafer
Figure 27: The slice of the copper pillar (30µm diameter)
With current and future advances in electronics and semiconductor packaging technology, a significant move from 2D to 3D X-Ray inspection has to be expected to account for continued miniaturization and expansion into the third dimension.
The future-proof x-ray system should address certain characteristics:
- Multifunctional x-Ray tube for versatile applications starting from 3D packages to IGBT-type of components
- True X‑ray Intensity (TXI) control for maximum x‑ray performance stability and hence consistent image quality
- High-Power Target – achieving small focal spot sizes for high resolutions at high x‑ray intensity
- High speed digital flat panel detectors for fast image capture supported by dedicated solutions for fast reconstruction
- Real-time image processing what allows fast and effortless detection of faults
- Versatile CT tools for NDT inspection of every sample
Friedhelm W. Maur – YXLON, Manager Asia Sales Electronics
Keith Bryant – SMT Solutions
by Ragnar Vaga