X-ray systems sharpen images

SIDEBAR: 2-D systems remain alive and well READ OTHER OCTOBER ARTICLES:  Table of contents, Oct. 2005 OCTOBER FEATURES: Design meets test IEEE 1149 expands differentially X-ray systems sharpen images Chamber verifies quiet designs

Two well-known makers of PCB test equipment also produce 3-D x-ray inspection systems for the electronics industry. At present, Agilent Technologies' 5DX series dominates the market, but Teradyne's ClearVue system is emerging as a competitor.

Figure 1.  By continuing to refine the imaging and software capabilities of its 5DX family, Agilent Technologies has dominated the 3-D x-ray inspection market for many years. Courtesy of Agilent Technologies.

The system's laminography technology relies on a steerable x-ray beam that moves in synchronization with a rotating image detector (Figure 2). Laminography refers to the 5DX instrument's capability to image a PCB in 3-mil-thick "layers" that let test engineers view solder joints clearly on either side of a PCB.

By mathematically combining many images acquired during a scan—a complete rotation of the x-ray beam and the detector—the system obtains an image of components free from interference from other components in the x-ray beam. In this way, a 5DX system can quickly acquire images and analyze solder-joint characteristics. Test engineers can view solder joints on the top side of the board independent of the parts on the bottom, and vice versa.

Figure 2.  A controlled x-ray beam and a rotating detector acquire several images of an area of interest on a PCB. Software combines the images to show solder joints on only one side of a PCB.

According to Sarah Park, business-development manager for x-ray products at Agilent Technologies, the 5DX systems can handle 0201 SMT components and can resolve solder joints spaced about 8 mils (0.2 mm) apart. When it comes to voids within solder balls or joints, the 5DX systems can detect those, too.

Software passes or fails a solder joint based on a comparison of information extracted from images and characteristics preset by test engineers. Software improvements have cut programming time considerably, and test engineers find the 5DX system easy to use, emphasized Park.

In addition to inspecting small and hidden solder joints, an x-ray inspection system provides an analytical and diagnostic tool as PCB manufacturers make the switch to lead-free solders. Greg McIntosh, product manager for the 5DX x-ray systems at Agilent, noted customers must determine how lead-free solders will affect their production processes. "Whenever you change a process, you tend to see more defects, at least early in the changeover," said McIntosh.

"When you make a major process change, you expect certain things to happen, and they usually do," explained Leinbach. "Then, the unexpected things happen and most of them are bad. The 5DX systems help you get processes back in control and back to where you had them before the change." As they switch to lead-free solders, manufacturers will use x-ray systems to help reduce reject rates and reduce the number of products that require warranty service.

Figure 3.  A wide-angle beam lets the ClearVue x-ray technology take off-axis images of sections of a PCB.

Figure 4.  The Teradyne XStation MX inspection equipment provides x-ray inspection capabilities built around the ClearVue technology. Courtesy of Teradyne.

To understand how the ClearVue technology works, imagine a PCB as a matrix of squares, or regions (Figure 5). Assume the x-ray system positions the PCB to place section G between the x-ray source and the detector. The system then can obtain off-axis, or angular, images of the surrounding eight sections, A, B, C, F, H, K, L, and M. When the system moves section H into the x-ray beam, it obtains off-axis images of sections B, C, D, G, I, L, M, and N. Note the system images sections B, C, L, and M a second time, but from different angles. Repositioning the PCB lets the x-ray system build up an array of eight off-axis images for each section (Figure 6). Then, a computer mathematically combines the off-axis tomographic information to construct an image of only the components in a selected plane.

Figure 5.  With PCB section G under the x-ray camera, the system acquires off-axis images of the red and purple sections. With PCB section H under the camera, the system acquires off-axis images of the blue areas and different off-axis images of the purple sections.

Figure 6.  The eight off-axis x-ray images for a PCB section include all information for one section. Algorithms combine the information to let engineers selectively view components, solder joints, and other features. (Color added for emphasis.)

At the 6-in.²/s scan rate, the x-ray detector can resolve 1 mil (0.025 mm) per pixel, which lets an XStation MX system adequately view solder joints on 0201 SMT components. Because the system can adjust the angle of the x-ray beam, it can vary the field of view to examine packages such as flip chips and BGAs.

Unfortunately, Agilent Technologies' policies prevent its employees from commenting on competitive products, so Agilent's x-ray experts must remain mute about the ClearVue introduction. Still, Agilent's McIntosh noted, "The technology in this [Teradyne] introduction is new, so we know of no benchmark. Customers should ask for comparable data about throughput and accuracy. We haven't seen one of these systems or heard of a customer who has tested boards on it."

Test engineers will surely put any x-ray inspection equipment to good use. "Today, about 35 to 40% of all joints coming off a production line connect a BGA to a board," said Teradyne's Groome. "We have seen research that claims by 2007, an area-array package such as a BGA will hide about 50% of all joints produced. Many companies are already way above that percentage."