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www.us-tech.com
September, 2013
Technologies Automate X-Ray Inspection A
By Frank Silva, Vice-President of Sales, MatriX-FocalSpot, Inc., San Diego, CA
utomated x-ray inspection (AXI) is an effective means of evaluating solder-joint integri-
ty in support of surface-mount-tech- nology (SMT) process monitoring and defect detection prior to in-circuit or functional testing of printed-circuit boards (PCBs) and other electronic products. Although automated-opti- cal-inspection (AOI) systems and sol- der-paste-inspection equipment (SPI) are used throughout SMT manufac- turing, neither approach is as effec- tive as x-ray inspection at finding defects following the reflow stage of manufacturing. In addition, neither approach is effective at dealing with bottom terminated connections (BTCs) on a PCB. Fortunately, MatriX Tech nology GmbH (Munich, Germany) has evolved AXI technology not only in terms of image acquisition techniques through software automa- tion, advanced three-dimensional (3D) reconstruction technology, defect detection algorithms, and factory floor traceability solutions. The firm’s tech- nology and equipment is used through out Tier 1 electronics compa- nies and ia also used for high-reliabil- ity, high-value electronics assembly around the world. Two of the key assembly consid-
erations for SMT designs are imag- ing of circuit boards with solder joints and bottom terminated compo- nents and dealing with double-sided assemblies (DSAs). In the first case, two-dimensional (2D) transmission imaging provides high resolution and fast imaging combined with easy pro- gramming and setup. Trans mission imaging is limited on some boards, however, such as dense double-sided circuit boards where certain types of defects, such as head-in-pillow (HIP) or pin-in-paste solder joints, must be
In the second case, inspecting
DSAs with an x-ray system requires special consideration, since solder joints on sides A and B of a circuit assembly can overlap, causing “x-ray shadowing” and resulting in a lack of solder joint x-ray test access. Effective software technology is needed to deal with a DSA board where many areas overlap, so that
would be a more appropriate descrip- tive term.
Reconstruction Techniques A horizontal x-ray cross-section
can be obtained from two or more images projected from different directions. A number of reconstruc- tion techniques are available to process these images, originating
Enter Laminography Laminography is a technique
used in the 5DX Series of AXI sys- tems from MatriX Technologies. The technology was developed by a for- mer provider of AXI equipment, Agilent Technologies, a firm which has since exited the market. The technique uses geometric focusing by synchronizing the circular motion of the x-ray source and detector. Unfortunately, this analog image reconstruction technique produces poor image quality with a high degree of unwanted artifacts. For each slice under study, the board must be repositioned up or down within the slice plane, motions which also create image quality issues. Some AXI systems employ digital tomosynthesis (DT), something of an improvement on laminography. DT relies on a set of off-axis or oblique images stored and synthesized through computational processing. To separate top and bottom compo- nents, these angled projections are digitally shifted from each other. The result is improved image quality with fewer shadowing artifacts. Unlike laminography, DT allows multiple two-dimensional (2D) cross- sectional slices to be extracted from the original projections.
The latest system from MatriX Technology, the X3, offers a
variety of technologies that can be used together or separately for optimum visual inspection.
different components can be separat- ed and treated independently, as if they were on side A without connec- tion to side B or vice versa. To inspect DSAs, imaging
reconstruction techniques had to be developed to separate top-sided from
Transmission imaging is limited on some boards, such as dense double-sided circuit boards where certain types of defects, such as head-in-pillow or pin-in-paste solder joints, must be detected.
detected. By working closely with customers and their requirements, MatriX developed algorithms for off- axis transmission imaging in two- and-one-half dimensions (2.5D), identifying specific types of measure- ments to solve detection issues relat- ed to head-in-pillow defect forma- tions and pin-in-paste barrel fill sol- dering.
bottom-sided solder joints. These techniques were developed by math- ematicians, to deal with medical imaging problems, where multiple shapes and structures overlap and interfere with each other. The term three-dimensional (3D) AXI, while widely used in the industry, is a con- fusing and misleading term. For AXI use, cross-sectional x-ray imaging
from medical imaging technologies, to separate overlapping “objects” such as solder joints. These tech- niques include laminography, digi- tal laminography, tomosynthesis, algebraic reconstruction technique (ART), simultaneous ART (SART), and planar CT (pCT), all developed to separate overlapping objects to treat them independently of each other. The imaging separation process employs off-axis image pro- jections and SART enables these objects (such as solder joints) to be “sliced” into horizontal cross-sections at different height locations from the surface of the circuit board. Unfortunately, all imaging recon- struction techniques degrade image quality and contrast compared to a standard transmission image. Some approaches leave fewer artifacts after reconstruction than others.
Optimum Separation For optimum visual separation
of top and bottom PB components, MatriX Technologies has implement- ed two different types of reconstruc-
By implementing 3D-SART with a graphical process- ing unit, MatriX has
achieved fast image recon- struction and enhanced processing speeds.
tion techniques into its test equip- ment. The firm has developed a patent-pending image-processing technique called Slice Filter Tech - nology (SFT) to address DSA setup in a single reflow line configuration. In a single reflow line, AXI can be used to quickly and easily inspect side A of a board in transmission mode. Overlapping solder joints pre- sented by leaded components on side B over the top of Side A create a loss
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