-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Enhancing the Imaging Chain in X-ray Inspection
December 31, 1969 |Estimated reading time: 7 minutes
To meet the challenge of inspecting smaller components, tougher-to-view interconnects and geometries measured in microns and sub-microns, X-ray image quality is becoming paramount in PCB assembly. As a result, digital detectors are supplanting analog systems for improved resolution and grey-scale sensitivity. What is the next step? An optimized image chain with automated noise reduction and digital output in real-time.
By Thorsten Rother
The ring of a cell phone could be heard over the buzz of restaurant conversation, and a dozen people reach to see who is calling. At another table, two people intently check their schedules on their PDAs; while in a nearby booth, an office worker waits for her meal, hunched over her wireless laptop.
Welcome to the world of telecommunications, a world in which “smaller,” “faster” and the incorporation of multiple features in electronics products are mandating the packing of more miniaturized devices onto substrates with decreasing real estate. As a result, each step in the assembly of PCBs and flex circuits on the SMT production line - from the dispensing of solder paste to inspection and test - is being stretched to maintain yields without sacrificing quality.
As component densities mount, interconnects and traces are becoming more obscure and X-ray imaging is becoming more critical in detecting defects. For this reason, manufacturers of X-ray systems are paying attention to ways of improving contrast, sharpness and real-time inspection capabilities. While switching to a digital detector is a step in the right direction, enhanced results can be achieved through selected features built into the image chain.
Difficulties in SMT X-ray Inspection
The role of X-ray systems in the inspection of electronics assemblies is well understood. Unlike machine vision and optical inspection equipment that permit line-of-sight inspection of components and circuits but cannot view “hidden” parts and interconnects, X-rays penetrate materials to expose hidden solder joints and the contact side of devices, such as flip chips and BGAs. Fortunately, X-ray systems have kept pace with the inspection requirements of smaller feature sizes (<1 µm) being mandated for packaging and assembly. The development of microfocus and nanofocus tubes, as well as the added benefits of technologies such as automated control over tube-output intensity, oblique viewing, rotation of the image chain and sophisticated graphical user interface (GUI) software have extended the imaging capabilities of X-ray systems dramatically.
Figure 1. X-ray image with poor contrast.
Image quality and the ability to inspect for defects, on the other hand, is put to the test in situations where density varies little between 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 exasperate the condition, and viewing can be difficult, even with a nanofocus tube. In such situations, contrast, sharpness, geometric magnification and minimization of noise effects are paramount concerns. Figure 1 shows an X-ray image with insufficient contrast, while Figure 2 shows the negative effects of noise on image quality.
Figure 2. X-ray image degraded by noise effects.
While the X-ray tube is the primary determinant of image quality, spatial resolution, contrast and sharpness also depend on the image chain in an X-ray system. The detector, in particular, 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 factor that contributes to both contrast and sharpness is the number of pixels per frame. The larger the pixels due to fewer pixels filling the frame, the poorer the image, in other words, the lower the resolution despite focal spot size of the tube. Where the number of pixels is too few, pixelization occurs, which is most noticeable on lines and curves.
The effects of noise represent a significant problem that can be solved partially through dose rate and filtering. The relation of absorption signal to the grey-value noise level (frequently referred to as the signal-to-noise ratio) increases the intensity significantly. Thus, the objective in X-ray inspection is to achieve the highest possible intensity, which at times, can be at odds with magnification requirements.
A recursion filter is also an efficient and helpful tool for reducing noise level without losing the real-time impression of the image. Such a filter creates an output image that is the weighted sum of prior images in a time sequence. During imaging, grey values of the individual captured images are increased per pixel and averaged. Images that were captured earlier will be less weighted than those captured more recently. Thus, considerable noise reduction can be achieved for a stationary sample. Ultimately, when imaging components with ceramics or high copper heat sinks, inherent sources of noise and image clarity depend on the flexible technical features of the detector.
Analog vs. Digital Detectors
Within the image chain of an X-ray system, without question, the detector plays the primary role in determining image quality. 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 charge-coupled device (CCD) camera, which senses X-ray images and converts them into an analog signal that is fed to a computer. This signal may be influenced by external electric noise or bad contacts between the camera and computer. Due to this inherent noise, important information within the signal is lost, and the image displayed on the monitor is not capable of being inspected at its most optimized quality. Despite having an X-ray tube with a microfocus or nanofocus spot size, the image resolution at the monitor is degraded to the point where subtle changes in material density cannot be detected and low-density regions bleed into regions of higher density.
From a schematic standpoint, digital detectors are similar to analog detectors. The digital detector differs to the extent that the analog signal is converted to digital within the sensor, as opposed to within the computer. Direct digital detectors (DDDs) 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. X-ray photons striking the phosphor are converted to visible light, which is converted into an electric charge. The photons create an electric charge at each pixel in the array proportional to the intensity of the light.
Of the various types of detectors, the amorphous-silicon, flat-panel, DDD offers film-equivalent quality performance. More advanced amorphous-silicon digital detectors feature 16-bit format, more than 65,000 shades of grey and millions of pixels. They also deliver selectable frame rates from 1 to 30 fps. One feature of a developed imaging technology*, allows the user to activate an automatic background-subtraction process, visibly optimizing image contrast and facilitating interpretation of the images, which may improve clarity (Figure 3).
Figure 3. Automated subtraction of image background.
null
Enhancing the Image Chain
The image chain consists of more than the digital detector, as the electronic interfaces and wire connection, frame-grabber, computer and monitor all play a role in image quality and the ability to view images in real-time. In the past, operators adjusted individual components of the image chain. Due to this individual processing, resulting X-ray images could not be optimized in real-time.
A more advanced development in X-ray imaging technology* occurs during the image stream and allows all images viewed on the monitor to be optimized permanently. This distinction can be significant. In viewing a crack in a solder ball, for example, instead of selecting images that appear to best show the crack, then processing them, viewing of an enhanced image of the crack can occur in real-time, while the position of the sensor is changed to provide multiple viewing angles. Also relevant in terms of the image chain is the dynamic range of the monitor. More advanced monitors provide a way to select the viewing area, meaning the dynamic range of the monitor can be correlated with the dynamic range of the DDD. As a result, pixelization can be avoided and image quality is optimized.
Conclusion
In the SMT production line, X-ray inspection of high-density PCBs and flex circuits is mandating improved image quality and real-time viewing in defect detection. In meeting the challenges of smaller feature-size requirements and X-ray viewing of components where edge detection is difficult, contrast, sharpness, geometric magnification and noise minimization are paramount. While digital detectors have improved the viewing of components and circuits, DDDs that substitute sensors for a camera are obtaining optimum results through advancements in the entire image chain. The result is film-like quality with every digital image.
* HDX-ray technology, FEINFOCUS.
Thorsten Rother, product specialist at FEINFOCUS, may be contacted at +49-5131-70980; e-mail: t.rother@feinfocus.com.