Choosing a BGA inspection system: a review of three types of nondestructive equipment: 2-D x-ray, endoscopic and 3-D x-ray.

While BGAs offer certain advantages over gull-wing parts, the equipment used to verify and maintain solder joint integrity in BGAs adds a step to the inspection process and increases costs. Among the three types of nondestructive BGA inspection equipment--2-D x-ray, endoscopic and 3-D x-ray--each has its own particular capabilities.

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2-D x-ray transmission systems generally show many obvious defects such as bridging, voids or missing balls. In many cases, the systems have difficulty detecting other common defects such as opens and insufficient or cold soldering. These types of defects require a high-powered system (100 to 165 kv) with oblique view at highest magnification (OVHM) capability and an experienced technician. Standard 2-D x-ray offers image resolution of 8 to 10 [micro]m, while many advanced systems can achieve a resolution of less than 1 [micro]m.

As an alternate method to x-ray, endoscopic inspection technology (a form of optical inspection) can be used to locate defects such as opens, insufficient or unreflowed solder, lifted pads and debris. It, too, has limitations when inspecting center balls of a BGA component. While these systems are capable of seeing 10 to 15 rows in, it is difficult to make observations on the entire ball, as viewing is limited to the outer edge of the ball.

3-D x-ray laminography or planar computed tomography (PCT) combines the capabilities of 2-D x-ray and endoscopic technology with added features. These systems view BGA balls and joints in a 3-D model. The system cross-sections a view, creating sliced layers that permit inspection of the inner structure of the ball. This allows a more detailed inspection of the solder joint connection and inner metallics. The disadvantages are its high cost, slow speed and excessive capability. Much of what a 3-D x-ray inspection system can do is considered over-analysis, unnecessary to detect a common BGA defect. This is evident in a production environment. And ROI may not be realized if the system is not be used to its full potential.

Choosing the optimal system depends primarily on how detailed the analysis must be. For OEMs or production facilities, a standard 2-D OVHM system provides more than enough capability to view BGAs at several angles. Most of these systems also have software for void and ball calculation, a critical inspection parameter. The speed at which many of these systems can operate permits a high throughput. Our in-house experience has shown 2-D x-ray to be an excellent analytical tool to inspect prototype and small production boards.

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An endoscopic system is much less expensive than x-ray and can prove a versatile inspection tool. By itself, an endoscope can provide clear captured images or video of a BGA process. Combined with 2-D x-ray, it can help locate defects (such as opens) commonly missed in x-ray inspection. This additional inspection tool permits technicians to further analyze what has been located under x-ray. Endoscopes are designed for sample lot inspections in a production environment and provide additional analysis in prototyping.

3-D x-ray is useful for more intricate processes and products. These systems are best-suited for failure analysis and R&D projects that require more detailed information on the intermetallic layers of the BGA ball and solder joint. 3-D systems work best when used to evaluate internal structures of the device. As a result, they operate slower and cost more than 2-D x-ray.

Software capability. BGA inspection equipment software has come a long way, with increased analyzing and programming capabilities. Many advanced 2-D x-ray systems permit measuring of voids, ball size, ball count and distance calculations (Figures 1 and 2). An option called computer numeric control (CNC) can be added to aid in automating the inspection process. This allows the user to create programs that perform sample manipulation and image processing. Additional test criteria that are programmable include presence of solder bridges, missing solder joints, deviating diameter of solder joints, insufficient wetting, tilted component, non-circular shape of solder joint and solder joint opens.

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The software used to control many endoscopic systems requires more user input and less programming. It features full-blown optical microscopy analysis on the BGA ball. This includes measuring, labeling, image processing, video recording and presentation mode. A guided failure analysis database comparing many common BGA defects is available. The software also permits importing and exporting of images, reports, spreadsheets and database information. Some of the visual controls include contrast and color changes of many levels to retrieve the best image possible.

3-D x-ray software has the capability of 2-D systems with technology for 3-D image rendering. This is created by bringing multiple 2-D images through software computation while reconstructing it to show the depth dimension (z-axis). The user can then choose the height, geometric magnification and resolution to be used for the model. The 3-D image can be moved in all directions to precisely reveal the BGA's inner structure. The programming is simplified with a software-based "wizard," which guides the user through the test process.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.