Choosing an Automated Inspection Strategy -- AOI and AXI are viable and often complementary inspection technologies to use in PCB assembly.

Automated inspection has become a key component of the printed circuit board (PCB) assembly process, thanks to increasing product complexity, component miniaturization and density, and market volume and pricing pressures. The two key technologies that have been adopted for the automated inspection of PCB assemblies are automated optical inspection (AOI) and automated x-ray inspection (AXI). The characteristics of these systems require some criteria for choosing the best technology or combination to maximize inspection effectiveness and product quality.

The Need for Automated Inspection

The lack of effectiveness and repeatability of manual visual inspection has been well known for years. Studies in the 1980s suggested that, when two humans inspected the same board four times, they agreed with each other less than 28 percent of the time, and they only agreed with themselves about 44 percent of the time. Despite this finding, manual visual inspection continued to be relied upon because no effective automated inspection solutions existed.

However, in the late 1980s and 1990s, several factors led to the development of effective automated inspection solutions:

- increased use of surface-mount technology and the accelerated trend toward component miniaturization

- increasing production automation coupled with faster cycle times

- the development of computing technology that made automated inspection faster, easier and more cost effective.

The need for automated inspection solutions has continued to develop as the relative cost and difficulty of electrical test methods have increased significantly. The decision as to whether AOI or AXI is most appropriate requires an understanding of the key characteristics and capabilities of the technologies as well as an understanding of the key causes of process defects.

Assembly Process Defects

Obviously, the differences in design, assembly processes, equipment, materials and the people involved in the process lead to different results in terms of defects produced. Analyzing these defects and their sources is an important first step in determining the optimal inspection strategy. While appropriate automated inspection equipment exists for each process, a trade-off must be made in terms of capital investment.

Assembly-related defects for the surface-mount process can be divided into solder joint-related defects, originating either at the paste printing process or during the soldering process itself, and placement-related defects. In practice, most defects are usually related to solder joints.

AOI Characteristics

Many AOI systems are available today, and close scrutiny of their capabilities is necessary. Expect the following characteristics:

- available for the key process points of post-paste inspection, post-placement inspection and post-soldering inspection

- finds placement defects such as missing and skewed components and incorrect polarity and part marking

- measures the accuracy of component placement

- finds many visible solder joint defects

- offers in-line speeds for most lines

- provides specific process inspection and feedback

- offers very fast return-on-investment (ROI) in most cases

- offers programming times of typically less than one day

- offers lower capital investment than AXI systems.

AXI Characteristics

While most x-ray systems used in PCB assembly today are off-line sampling-based systems, some significant gains in coverage and inspection effectiveness can be achieved by using AXI. Ideal characteristics include:

- finds solder joint defects that often cause latent field failures

- measures solder thickness

- finds solder joint defects not visible to AOI or human inspectors, including many defects in area array packages

- finds most placement defects

- offers in-line speeds for some lines

- offers very fast ROI in most cases, especially for dense or complex boards.

However, AXI systems may need a programming time of two to three days and require a higher capital investment than AOI.

Decision Criteria

Both AOI and AXI have individual and complementary strengths in their capabilities. Determining which one is the most appropriate to use must also take into account some key end product and situation-specific circumstances. Consider the following issues:

What is the key focus: monitoring the process, defect detection or both? AOI and AXI both provide important process-related data. AOI systems can provide more flexibility for process feedback for solder paste printing and component placement.

How complex are the products to be inspected? The product's complexity is a key criterion in the decision-making process. As the PCB size, solder joint count and component density increase, so does the difficulty in providing high product turn-on rates. In many cases, electrical and functional tests are limited by available PCB real estate for test points and the time required to troubleshoot problems. AXI is attractive in these situations because solder joint-related defects are responsible for most defects. When the end product's quality requirements are particularly stringent, a strong argument exists for combining AOI after paste printing and component placement with AXI after soldering.

How critical are line speed/cycle time and the impact of in-line inspection equipment? For most applications, users do not want automated inspection as the bottleneck in their lines. If this case does exist, though, then the cycle time of the inspection systems can be increased by decreasing coverage rates.

What process defects are most critical or most common: defects related to the placement process or solder joint-specific defects? Understanding the key process defects is essential. Figure 1 depicts the best inspection solution for numerous defects.

How important is the inspection of solder defects on area array packages? The inspection capability for area array packages is a key advantage for x-ray systems.

Does the product have radio frequency (RF) shields applied before soldering that would affect post-solder visual access? Many products, including many cellular phones, require metal shields for the RF circuitry. Many RF shields are applied before reflow soldering, which limits post-reflow visual inspection. AOI inspection before reflow can be very effective in this situation. If solder joint defects are of concern, then AXI is the best choice.

What is the available capital budget, considering that both technologies provide excellent ROI? Although AXI typically provides very high coverage levels when compared to AOI, the capital investment can be significantly higher. The coverage differences are related to the total number of defects that can be inspected by each technology and also to the fact that three-dimensional (3-D) AXI systems can inspect both sides of the PCB in the same inspection pass.

How critical is the end product quality in terms of potential field failures? The quality requirement of the end product is important in the choice of inspection technology. Analysis of failed products and warranty data often indicates the key areas on which to focus. In many cases, failure is caused by solder joint defects that were either not detected in the initial assembly process or were detected but escaped from the process. In this case, AXI is needed.

Complementary Testing and Inspection

In most cases, AOI and AXI technologies do not completely replace the existing electrical test technologies such as in-circuit test and functional test. But, as the accessibility for electrical testing becomes more limited, the need for complementary inspection steps becomes more obvious. This complementary strategy will continue to develop as increasingly no single test or inspection solution provides the coverage necessary to ensure acceptable quality levels.

When choosing the appropriate test and inspection strategies, keep this fact in mind. Also, look for vendors who can provide the necessary hardware and develop solutions to cohesively manage these diverse technologies. In some respects, this capability is still a work in progress, but it is a key capability if users are to realize the maximum return from their test and inspection investments.

Summary

Both AOI and AXI are viable and in many respects complementary technologies in the modern PCB assembly process. The choice of which one to implement can be complex, but users should look beyond the technologies' capabilities to find the best strategy for a particular situation. The need for a complementary test and inspection strategy is likely to increase, so the decision must focus on best-in-class equipment and on the ability to support and manage these diverse technologies.

Tom Molamphy is a senior inspection applications engineer with Agilent Technologies, Loveland, CO; e-mail: [e-mail].

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Confused About AXI Technology?

While AOI systems in general are typically well understood, a slight aura of mystery still surrounds AXI and the technology required to make it a viable solution. What are the key variables to remember when choosing AXI?

Automated x-ray inspection can be accomplished by using either two-dimensional (2-D) transmission-based systems or systems based on three-dimensional (3-D) technology such as scanned beam laminography. Transmission-based x-ray systems provide a complete cross-sectional x-ray image of the assembly, and the top and bottom sides of the board are imaged simultaneously. Two-dimensional AXI systems provide some benefit in throughput and cost as compared to 3-D systems. But, because no capability exists to "segment" the image between the top and bottom of the board, large areas of overlap occur between joints on both sides of the board, thereby reducing effective coverage. While this overlap might not be a problem on single-sided assemblies, it quickly reduces effective coverage on double-sided assemblies.

The benefit of 3-D AXI technology is the capability to produce x-ray images of each side of the board individually and remove the items in the image that are not of interest in making the automated defect determination. The most common technologies used to produce these images are scanned beam laminography and digital tomosynthesis. Laminography-based systems represent the vast majority of installed 3-D AXI systems primarily because this proven technology provides the best balance of the speed, cost and resolution tradeoff that is inherent in these systems. Digital tomosynthesis provides similar image segmentation capability. By combining numerous 2-D transmission images, which is computationally intensive, it builds a three-dimensional representation.

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