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3D X-Ray Inspection Provides Quality Check of Double-sided PCBs
2D x-ray: Superimposed solder joints on PCB top and bottom sides makes evaluation of a single side impossible (image: Goepel electronic GmbH)

Today's electronics are more compact, complex and faster than earlier versions of the same products. Smaller products demand reduced power usage along with lower costs. Space saving techniques used to accomplish these goals includes newer component packages with hidden leads under the body. Combining these packages with double-sided PCBs allows for high performance assemblies with a smaller footprint. This trend presents new challenges for test and inspection technologies.

Solder joints hidden beneath components such as BGAs, can only be evaluated by "looking through" a PCB. X-ray inspection technology takes over where conventional AOI systems reach their limits. 2D x-ray systems are limited when used to inspect double-sided PCBs, providing an image of one side superimposed on the other.
3D X-ray: Separated solder joints on PCB bottom side permits evaluation of solder joints (image: Goepel electronic GmbH)

This means that obtaining a detailed, process-reliable evaluation becomes impossible. 2.5D inspection provides a better fit yet does not yield an ideal view. An angled PCB X-ray depends on radiation angle and board thickness providing a separation of the solder joints on the top and bottom side assemblies. Dense component layouts pose a challenge to a simple angle X-ray that produces insufficient results. Similar to the 2D technology, superimposition of the top and bottom sides prevents satisfactory evaluation of the solder joints.

Digital Tomosynthesis
A 3D X-ray inspection system using digital tomosynthesis captures slices of the top and bottom side of a PCB from different angles. Captured 2D images are translated into 3D data via algorithmic reconstruction. The result is an image that is free of superimpositions showing a distinct separation of top- and bottom-side solder joints regardless of component density.
2D, 2.5D and 3D X-ray inspection use (courtesy Goepel electronic GmBH)

Superimposed and electrically connected assemblies such as package-on-package (POP) can now be successfully inspected. The 3D reconstruction layer-by-layer enables suitable visualization of top- and bottom-side components to provide a successful evaluation of solder joints. 3D X-ray is an imperative for double-sided production lines.

Component Library
Using a true geometric reconstruction of the PCB from the CAD program allows for the use of a standard component library. Users are able to generate a test program with the CAD data and component entries in the library quickly.

Compensating for PCB Warp
A successful solder joint analysis requires an understanding of the topology of a PCB. Board warp, sag, wavy surfaces or skewed components adversely affect the ability to provide a valid analysis. Failure to compensate for these spatial anomalies properly may lead to false errors, incorrect readings or rejection. A 3D X-ray inspection (AXI) system must detect and compensate for board warp, waved surfaces or skewed components that may exist on the PCB. A combination of laser triangulation and a software-based algorithm can compensate for PCB sag in the recorded images.

Using a true reconstruction of the solder layer height yields a high-quality evaluation of solder joints.

Contact: GOEPEL electronic LLC, 9737 Great Hills Trail, Ste 170, Austin TX 78759 512-782-2500 fax: 734-471-1444 Web:

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