As printed circuit boards (PCBs) are developed with increasing complexity, the challenge of inspecting them quickly and efficiently increases. Not long ago, automated optical inspection (AOI) alone was enough to detect any faults. But today's PCBs are made with components that conceal solder joints and other points of possible error that require the addition of X-ray technology to thoroughly inspect them.

AOI has been around since the early 1980s, when it was first introduced to the PCB assembly industry as a welcome alternative to the naked eye. AOI systems, in their most basic configuration, consist of several video cameras, a light source and an image-processing computer. The systems can discover missing parts, determine the use of incorrect components, polarization errors, placement errors and solder defects. After the AOI system detects an error, the defective PCB is reported and reworked. By ferreting out the defects and reworking the PCBs, AOI helps manufacturers keep costs low and quality high. AOI is, however, limited to making surface level inspections, a serious drawback when dealing with the newer PCBs.

X-ray systems have been used by various industries for decades, but until the development of real time technology, the costs and delays associated with X-ray film have limited their use for PCB inspection. Currently, the demand for components that can inadvertently obscure the surface such as ball grid arrays, chip scale packages and flip chips has made the use of X-ray inspection a necessity for ensuring the quality of PCB assemblies. In addition, many engineers believe X-ray inspection is the best way to identify errors such as shorts, voids and opens as well as the solder integrity on new chip packages.

To be suitable for their task, X-ray inspection systems need a sufficiently powerful source ray to penetrate the PCB and a clear enough resolution to reveal the smallest defects. An X-ray-sensitive video camera converts the X-ray image of the PCB into a video signal that is then analyzed for defects. The video conversion is important considering the low contrast and fuzzy reception typical of X-ray images. The X-ray inspection process can be automated by combining the board handling system with programmable indexing.

For all of their benefits, X-ray systems by themselves cannot verify correct components, component orientation, polarization or a host of other such characteristics. As a result, the most effective method of PCB inspection to date is the combination of X-ray and AOI technology. Incorporating both X-ray and video inspection capabilities in one system provides a complete analysis, allowing manufacturers to inspect all of the components with hidden solder joints, while simultaneously inspecting the assembly for correct parts, position and orientation. In addition, the combination system is easy to set up, typically taking less than an hour.

The integrated inspection system uses four cameras and features an X-ray source, an X-ray-sensitive video camera, radiation shielding and interlocks. The X-ray camera takes the place of the fourth camera position on the scan head. The X-ray images are analyzed using the same software as the AOI system, with the addition of special algorithms for ball grid arrays and flip chips. The video cameras and machine vision algorithms are used to verify the presence, correctness and orientation of the components and the integrity of visible solder joints, while at the same time, the X-ray system verifies the integrity of hidden connections without interference from reflections, surface appearance or incident lighting.

An example of this technology at work is through the inspection of a ball grid array component. To the vision camera inspecting for part number, orientation and position, the image may not reveal apparent defects. However, an X-ray inspection of the same part reveals the solder balls hidden under the component and the quality of their solder connections. Fault detection at this X-ray level includes improper solder bridges, undersized solder balls, or an excessively large void. Considering the precision and effectiveness of combining both methods, it doesn't take an X-ray to see why AOI/X-ray inspection is the choice of many PCB manufacturers.

Source: PCB Inspection: X-rays Enhance AOI
Richard Amtower
Assembly, Nov. 1999
http://209.64.216.143/articles/1999/nov/1199arc3.htm