The solution is provided by high-resolution inspection systems capable of penetrating materials to expose hidden connections in even the smallest devices.

In 1985, Operations Technology Inc. began using Metronics Inc.’s metrology software and a digital readout with the ability to discern the edges of features in the magnified view of the feature being measured—at the time a tremendous breakthrough. By positioning to a given feature (for example, a circle), the system could analyze the X/Y coordinates at the feature’s periphery and from them calculate the size and location of the circle.

As feature size continued to decrease, circuit fabricators began looking for an alternative to optical technology, because the heat produced by the high intensity light necessary to illuminate the smaller field of view was causing distortion. A newly evolving video technology, such as used by the Metronics Quadra-Chek 4000, saved the day with its ability to work with very little light intensity. The video approach achieved such high magnifications that optical units were no longer competitive.

Miniaturization eventually called for new capabilities, most notably the ability to inspect individual layer positions in multilayer circuit boards.

How do you measure what you can’t see?
Most of today’s circuit boards have been designed with a computer program that routed the conductor traces so that artwork could be applied to the panel to make the appropriate interconnections. In some instances, a thru-hole connection was necessary so the electrons could get to their destinations. Today’s high-density interconnects (HDI), such as integrated circuits and ball-grid arrays, mandate the use of multilayer circuitry—a number of very thin circuit boards stacked together with insulators between the layers. The stack is subjected to high vacuum, heat and pressure during the lamination process. Finally, precisely positioned holes are drilled through the layers, providinga path for the plating that electrically interconnects the circuits.

The layers must be registered as accurately as possible or the proper interconnects won’t be made. To accomplish this task, an X-ray camera, source and image processor were needed to supplant the video components so that the system could literally see through the panel. This wasn’t an easy task, as the panel at that point in the production process is laminated top and bottom with solid sheets of copper. The inspection system must ignore these outer layers, as well as the fiberglass laminate, and discern the position of features within the sandwich.

Before they acquired X-ray vision, PC board fabricators were forced to use a variety of crude quality control methods. None of these methods involved the actual monitoring of the size, shape and location of the various layers, nor did they produce the corrective dimensional information that permitted the process to be optimized. The trial and error approach was common. Companies would complete the fabrication and frequently discover that the panel was inoperable and unsalvageable, including all the production that preceded the testing. Clearly, there was a need for precise dimensional measurement of the inner layers and the ability to use the data and a process control parameter.

An X-ray–based system, such as OPTEK InnerVision, does just that. The X-ray counterpart to the video-based OPTEK VideoMic, the InnerVision measures out-of-registration, scaling errors and other distortions that may affect the locations of internal details.

Making X-ray measurement practical
To make use of this measurement information, a series of coupons are positioned on the border of the panel—the part of the PC board that’s cut away and discarded during the finishing operation. A circular pad for each layer is CAD-positioned at a number of locations around the edge. Using Metronics Quadra-Chek software, the OPTEK InnerVision precisely positions to the center of each of these nominal locations, fires the video-circle tool, and captures hundreds of points from the circles periphery. The software provides the OPTEK InnerVision System with image processing and calculating capabilities that make possible the task of measuring inner layers, in some cases well over 60 inner layers. Using OTI’s proprietary (get meaning form author)MRS, the center points of the circles are used to derive the location, stretch, shrink and twist of each layer in relation to the other. This information enables the operator to adjust drilling and artwork functions to optimize the process.

The illustration shows an OPTEK coupon on the edge of a circuit board. The locations of these dots are measured, precisely locating the positions and distortions of the various layers of a multilayer circuit board. This information can then be used to rescale the artwork or to apply a drill offset to potentially salvage a circuit board.

Additional applications are envisioned. The X-ray image will be further enhanced through image-processing software, so that solder bump analyses and wafer inspection will be possible.

In the InnerVision system, intuitive displays combine a part-view window surrounded by toolbars that perform specific measurement procedures. For feedback, the QC5200 interface uses sound and color cues with next-step prompts to guide the user through the process.

Integrated profile measurement enables users to import a CAD file and analyze the form of the measured part with no assistance from third-party software. With a "what-you-see-is-what-you-get" graphic interface, software allows the system to print custom reports in full color with a graphic-designer look. Such flexibility makes it easy to customize workstations to the operation. At the same time, the system allows for control and system security by providing supervisor locks that preserve important settings.

Electrical connectors, fasteners and other hidden features are likely future targets for this evolving technology.

The combination of modern X-ray concept and machine design with powerful metrology software has made true X-ray vision a valuable reality for the printed circuit board fabrication industry.