CT brings clarity, precision to PCB inspection

Originating in medical and industrial applications, computed tomography (CT) has made inroads into printed-circuit-board (PCB) failure analysis. At first, the technology was used primarily in R&D laboratories, failure-analysis labs, and on new product production lines where operators need very high levels of resolution. More recently, CT has become necessary in mainstream production lines as 3-D stacking techniques and increases in board complexity have made it difficult for engineers to differentiate between layers in an x-ray. “You can see quite a lot with 2-D x-ray, but sometimes fine detail gets obscured by the layer above it,” said Paul Walter, managing director for x-ray systems at Dage Precision Industries.

Production machines like the XD7500NT are beginning to take advantage of CT technology. Courtesy of Dage Precision Industries.

Whether CT is required on an electronics production line depends on PCB complexity and the type of faults that are found in production. For example, much of the demand for CT originated in the manufacture of extremely dense cellphone boards. The cameras in mobile phones are so complex that 2-D x-ray isn't capable of inspecting their faults.

By developing inspection systems that combine CT and x-ray capabilities, manufacturers are making it easier for their customers to uncover and evaluate faults in PCBs in a variety of environments. Dage itself offers two such systems—the Dage Precision XD7500NT and XD7600NT—that provide CT scan for 3-D modeling via optional modules. The company also offers new software called QuickView that speeds up the use of CT in both production and failure-analysis tools.

“Traditionally, to discover 3-D faults you might have created a microsection slice that would have been observed by a scanning electron microscope,” said Walter. “But you have to make the slice in exactly the right position, and the method is both time-consuming and destructive. With CT, you can clear out the clutter above and below the fault and see it clearly,”

Using CT, operators can create a 3-D model, place it on the viewer, and visually slice through faults like a virtual microsection. Dage's CT option for its x-ray systems allows the creation of volumetric slices through solder joints, fast reconstruction of CT images, and rapid creation of custom slices. Reconstruction resolution is up to 1024x1024x1024 volume pixels (or 1024³ voxels).

CT imaging can help uncover faults on complex ICs, such as ball-grid arrays.  Courtesy of Dage Precision Industries.

Despite the advantages CT offers for failure analysis in PCB inspection, the technology does have disadvantages. Higher-resolution inspections take longer because more images must be taken to create the CT model. And taking many images over a long period of time increases the chance that errors such as those caused by heat variances will occur. This is what prompted Dage to develop the QuickView software, which is based on an algorithm that speeds up image creation and processing.

“With QuickView, we tried to increase speed without sacrificing too much resolution by achieving a 3X to 4X speed improvement and retaining about 75% of resolution, compared to standard CT operation,” said Walter. Operators can create many slices quickly, within less than 5 min each, while saving the slower, high-resolution slices that require 15 to 20 min each for the most critical areas.