New angles on X-ray inspection: X-ray systems can be used in-line to identify errors prior to drilling and electrical test and trimming. (PCB Process Control).

Until a few years ago electrical testing and visual inspection were deemed sufficient for quality control of bare-board PCB manufacturing, with x-ray inspection reserved only for inspection of assembled boards. X-ray's advantage has always been its ability to reveal what lies beneath visually opaque objects and thus its major use in PCB manufacture was traditionally for inspection of components such as BGAs whose solder joints were otherwise obscured. Complex multilayer boards, however, have brought about a new need for x-ray inspection.

X-ray systems provide the only reliable method to see into the increasingly complex internal layers of PCBs and to assess the plating of vias. They are also more effective in determining broken or shorted traces and offer the capability to determine the size and nature of shorts in a way that cannot be determined by electrical testing.

Historically considered a tool for the failure analysis laboratory, microfocus x-ray inspection is becoming a production necessity. X-ray inspection can be used at key points during manufacturing to identify critical errors prior to drilling and final electrical test and trimming. By using x-ray at these critical steps, it is possible to detect errors cost-effectively and correct process problems quickly. This reduces scrap and ultimately increases yields.

The lowest cost x-ray inspection systems use a fixed top-down view where the board is positioned between the x-ray source and an image intensifier. More than any other factor, resolution determines what can be discerned from an x-ray image, and image resolution depends on two key factors: resolution of the image detector and the focus size of the x-ray source. The best microfocus systems (with a spot size of less than 5 [micro]m) offer magnifications of in excess of 2,000 times. Even at such high magnifications the latest 1000 x 1000 pixel detectors can easily resolve the resulting x-ray images, so image resolution depends almost entirely on focal spot size of the x-ray source.

Using a simple top-down x-ray view, operators can simply detect offsets and drills can be adjusted accordingly. While laminate alignment is now generally of a very high quality, innerlayers may still move out of their determined position during manufacturing. X-ray is the only inspection method that can detect this type of fault. In addition to alignment offsets, top-down techniques can be used for innerlayer misalignment, shrinkage and distortion.

Oblique Angle Viewing

As copper layer thicknesses get thinner and layer counts increase, the current top-down x-ray techniques have reached their limits. To consider a visual metaphor, looking directly down the longest end of a pencil can give an unrecognizable image (of a circular lead center surrounded by a hexagonal wooden shape). In spite of the human brain's remarkable power to comprehend complex images from unusual angles, only by viewing the pencil from an oblique angle will its true nature be revealed (e.g., What is it? And how long is it?). In the same way, oblique angle viewing is essential to gain a full and accurate inspection of advanced PCBs, such as when inspecting via holes for proper plating through the hole. Other applications for this technology include looking for broken copper traces, contamination and shorts, drill hole positioning and onboard connector and component quality.

In a production environment, automatic defect recognition (ADR) systems can be designed to look for, recognize and create alerts for specifically known or suspected critical faults. The better software packages permit users to self-teach the software to determine or simplify the measurement of key parameters. Additional tools determine drill offsets and other quality checks that can be fed back to drilling stations or used to adjust the production process.

CT, or computed tomography, can be used to create 3D images of objects under inspection, giving unparalleled depth and complexity of interpretative information (FIGURE 3). By aggregating the data of multiple fine x-ray "slices" taken through a board, a range of 2D and 3D images can be generated, including slice images, transparent and rendered views and animation fly-bys (FIGURE 4). However, CT is relatively time consuming to create and is therefore probably best suited for development, pre-production and failure-analysis applications.

[FIGURES 3-4 OMITTED]

The advantages of CT for board inspection can also be limited by the distortion that occurs when looking through the entire thickness of a 12 x 12" board. However, as control and imaging software for x-ray inspection systems continues to develop apace, we are likely to see the introduction of segmented board analysis to section through layers of a profiled board, without the need for oblique angle views. Advanced imaging software of this type is currently under evaluation and is likely to be available this year, making x-ray inspection an even more powerful tool for PCB manufacturers.

Once considered too expensive, microfocus x-ray inspection systems have become more affordable of late, thanks to advances in x-ray tube technology and wider market acceptance. Other improvements include real-time viewing, enabling instant fault analysis.

NICK HADLAND is director of X-Tek (xtekxray.com). He can be reached at 978-649-6333.