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AOI and X-Ray Inspection Systems Combined Provide Valuable Yield Enhancement Solution

By Kevin Garcia, Nordson YESTECH

AOI and X-Ray Inspection Systems Combined Provide Valuable Yield Enhancement Solution

As board densities increase and component sizes decrease, the need for a fast, accurate and more efficient inspection process is critical. A growing number of electronics assemblers have integrated Automated Optical (AOI) and X-Ray (AXI) inspection into their quality control inspection processes. These two inspection technologies offer very distinct advantages to manual inspection. Utilizing either an automated inspection or x-ray solution on a production floor provides key advantages to electronics assemblers. The most ideal inspection solution, utilizes both technologies in combination, ultimately providing the highest-level inspection capability and best production yields.

There has been a steady progression in performance and usability since the introduction of the first machines in the early 1980s.  Initial versions were very expensive, limited in capability, difficult to program, and required many hours or days to create and maintain inspection programs. Unless running in a high volume, low mix, production line, it was difficult to justify these technologies as a viable inspection solution. Thanks to the rapid evolution of hardware and software technologies, the latest generation of AOI and AXI systems has overcome most of these limitations.  The AOI and AXI systems of today provide powerful inspection solutions, with a broad range of features. 

Automated Optical Inspection (AOI)
When comparing the speed, efficiency, and flexibility of other test and inspection methods, the benefits of AOI are clear.  A typical manufacturing line, for example, may employ between two to four inspectors to visually identify and repair component and solder defects.  In contrast, an AOI system requires only one operator to detect and repair defects as well as collect all necessary data for yield improvements.  This can reduce the per-shift requirement for labor or enable reallocation of resources to another part of the manufacturing process, ultimately reducing costs.  Because AOI systems can be placed in various in-line and off-line configurations, it is important to thoroughly analyze the factors that influence the overall yields to determine the best fit for your process. The most advanced AOI solutions offer various key features, ideally suited to meet the demands of today’s inspection requirements:

Ease of Use: One important, but often overlooked, factor to consider when implementing AOI is ease of use. There are some systems that are better suited for dedicated higher volume production lines. These systems are often programming intensive and do not lend themselves to line change over. Other types of systems are designed to provide the flexibility needed to match production throughput on multiple line configurations. The inspection system should be flexible enough to be used anywhere in the line, depending on desired inspection objectives. The programming may change somewhat depending on where the machine is used, but with a flexible system, the hardware usually stays the same. Portability of the program can be a critical factor as well, and should be considered when evaluating an AOI system.

Magnification: Pixel counts (megapixels), optical and digital magnification (zoom) are important criteria that can often determine the ultimate capabilities of an inspection system. To accurately inspect small devices, such as 01005’s, it is important that the combined optical and digital magnification be sufficient to provide the necessary amount of resolution and information to the AOI software inspection algorithms. However, higher magnification leads to a smaller field of view (FOV) and therefore, longer image acquisition times, creating more data to process. Therefore, as cycle times shorten, the AOI system must offer a balance between magnification and image acquisition speed.

Optics: For increased accuracy and repeatability, the use of tele-centric optics has become increasingly popular in AOI equipment. Traditional lenses exhibit varying magnification for objects at different distances from the lens and can show the apparent shape of objects varying with distance from the center of the field of view. Tele-centric lenses have the same magnification at all distances. An object-space tele-centric lens creates images of the same size for objects at any distance and has constant angle of view across the entire field of view.

Programmable lighting: In any machine vision application, lighting is critical to achieving the desired results. Trying to find one light source that will enable detection of all defect conditions is nearly impossible, given the ever-changing environment of electronics manufacturing. To ensure the widest defect coverage for component and PCB configurations, a dynamic light source is crucial. With the reduction in cost of light-emitting diodes, AOI equipment manufacturers are able to configure light arrays that are highly customizable within the inspection software. With multiple colors at various angles, AOI users have the flexibility to enhance the contrast of the image to easily identify a multitude of defect conditions. Even within a given component type the possibilities are endless. There are many variations of color and surface properties, along with the variations of paste composition, pad size and material, as well as PCB color and texture. Programmable lighting is an invaluable tool for ensuring the widest range of defect detection.

Positional accuracy: As component size decreases, the positional accuracy of both manufacturing and inspection equipment becomes increasingly critical. As a rule-of-thumb, the AOI system should have sub-pixel accuracy. This will ensure the system is sufficiently accurate to detect small deviations in position that can lead to a 01005 defect.

Network capable software: One important function of any AOI system is data collection and retrieval. The data can be in the form of a text output, database, image collection, or a combination of several formats. Collecting the data is a basic function of most AOI systems, however, retrieving the information is often more complex and depends on the configuration of the manufacturing line.

In a networked environment, the AOI can simultaneously inspect a PCBA while transmitting results from the previous assembly to a “downstream” review/rework station. The downstream station is not only communicating with the AOI system, but also storing inspection results and review operations into a SPC database. Inspection results can be viewed in real-time or archived for later review.

While AOI offers distinct benefits, Automated X-Ray Inspection (AXI) is also a viable inspection solution, providing a myriad of advantages for a variety of applications.

Automated X-Ray Inspection (AXI) 
Automated X-Ray Inspection (AXI) is becoming increasingly popularity because, like its counterpart, Automated Optical Inspection (AOI), AXI is a non-evasive inspection solution that provides real-time process data and thus can be utilized effectively for defect detection and yield improvements. X-ray images of solder joints can be analyzed automatically to detect structural defects, such as insufficient solder, voiding shorts, opens and other defects that can account for upwards of 90% of the total defects on a complicated board. But unlike AOI, x-ray imaging is not hindered by hidden solder joints, component shields, and high density double-sided boards. 

This key advantage of inspecting hidden solder joints makes AXI the logical choice for inspection of complicated boards, especially ones with BGAs, CGAs, CSPs or components that are under RF shields. This is a critical advantage of automated x-ray inspection, considering a significantly larger number of boards fall into this category with the increasing popularity of array-style packaging. Additionally, many cell phones and wireless communication products are placing RF shields over unsolder components at pick and place, using the reflow processes to solder them to the board.   

As mentioned earlier, ensuring quality and reducing costs through automated inspection during surface-mount assembly is most valuable when used to detect defects close to the fault source, ensuring quick detection, correction and enhanced process control. Inspection cycle time is also critical for in-line AXI systems, which must be fast enough to do the job within the allowed cycle time, while accurately verifying correct assembly and identifying any errors. 

Automated x-ray inspection can be useful at many stages of the assembly process, but time and resource constraints usually limit most products to a single x-ray inspection. Therefore, it should be implemented where it will provide the maximum benefit to the process. Since automatic analysis of finished solder joints is AXI’s strength, most systems are placed after the solder process whether wave or reflow. At this point, all solder joints on the board are present and can be covered in a single test. Also, by waiting until the completion of the assembly process, any other defects such as damaged or missing components will be detected. X-ray inspection is available utilizing two different technologies:

2D or Transmission X-Ray – With this technique, x-rays are generated at a fixed point source, pass through the PCBA and form an image on an electronic detector. The image is converted into a digital image and transfers it to a computer where the analysis takes place. This technology is widely used for single sided boards in automotive and other high reliability applications. Advanced image processing software is now available to distinguish components and conduct automated inspection of solder defects. Transmission x-ray is the most common form of x-ray inspection for electronic assemblies. 

3D X-Ray – This x-ray technology provides clear images of single layers (or slices) of the board to facilitate unimpeded inspection of double sided boards in a single pass. The Laminographic 3-D technique requires the x-ray source and detector to move in a circular pattern 180 degrees out of phase. Only features in one plane are in focus and features (components and/or solder joints) not in the plane are sufficiently blurred out. The Tomosynthesis technique creates 3-D images by reconstructing multiple transmission images taken from different angles. These images are digitally combined to create slices at any depth. Both techniques are commonly used today in x-ray inspection applications for more complicated double-sided electronic assemblies.

A critical challenge for AXI systems has historically been to accurately identify defects within the allowed cycle time. To maximize throughput and defect coverage, recently available systems now permit selective 2D and/or 3D inspection on the same assembly. Specific components or regions of interest can be selected for 3D inspection without significantly impacting overall inspection time.  

AOI and X-Ray Technologies Combined Offer Superior Inspection Solution

AOI and AXI have been following the trend of most recent technologies to provide more performance for less cost. Another recent development is the portability of data between AXI and AOI systems from some suppliers. The ability to share libraries, inspection programs and SPC data between AOI and AXI systems can greatly enhance machine usage and defect coverage. In addition, programming the systems of today is fast and intuitive. Operators are now able to quickly and easily create inspection programs and manage daily runtime operations with very little intervention.  As a result, both high mix and high volume manufacturing lines can recognize immediate yield improvement automated inspection results. 

In addition to real-time process feedback, many proponents of automated inspection have praised the substantial time reduction on first article inspection and line changeovers.  By utilizing the latest network and communication methods, there are unlimited possibilities in how data is recorded and retrieved.  In-line or off-line review stations can seamlessly convert the inspection results into an efficient rework process by identifying the defective areas of the PCBA and recording the actions performed by the operator. The operator’s actions and machine data can then be analyzed via web based SPC (statistical process control) and SQC (statistical quality control) software packages to provide an instant snapshot of the process via standard Internet browsers on a desktop PC. 

The flexibility, accuracy and ease of operation of today’s AOI and AXI systems extend well beyond the role within the manufacturing process. Ultimately, the resulting price/performance ratio of these systems has become the driving factor in the ever-increasing acceptance of these technologies.  Whether high volume or high mix, both large and small companies alike can quickly realize the value in automated optical and x-ray inspection.  Developing a successful yield enhancement plan, that is able to handle the dynamic nature of the manufacturing process, can be complex.  When considering the power of automated optical and x-ray systems that can be used in multiple process locations, with unlimited data collection and detailed reporting capabilities, the benefits of AOI and AXI inspection for the electronics industry is clear.