AOI as Part of a Winning Test Strategy
By: 

The Capability of AOI Systems – More than Just the Sum of their Parts
By: Jens Kokott, Team Manager, AOI Systems at GÖPEL electronic GmbH

With the move towards finer pitches and smaller component sizes, reliable visual inspection for correct assembly has become increasingly difficult. As a result, implementation of automated optical inspection (AOI) has become widely accepted as a key process in completion of deliverable products.

AOI offers early detection of defect trends and generates critical data required to make informed process decisions. Efficient use of the data generated by AOI systems to identify manufacturing problems can not only have a positive impact on product quality, but also can dramatically enhance product yields and reduce costs.

A variety of AOI systems on the market effectively detect manufacturing defects on a printed circuit board (PCB) production line. These systems can perform comprehensive PCB inspection for paste deposits, component presence, part nomenclature and reflowed solder joints at production line speeds. However, the technology and capability offered by these systems is promoted in such various ways that it may tend to confound potential users at times.

Prevention vs. Detection

The two guiding philosophies for implementation of AOI are prevention and detection.

• Prevention places the priority on process control and elimination of defects by implementing corrective action.

• Detection, on the other hand, focuses inspection efforts on ensuring that no defective products escape from the factory floor.

The first step in effectively implementing AOI requires a clear understanding of inspection philosophy and goals so that use of the equipment and data collection can be performed effectively. Different inspection goals dictate the need for different process control information and at varying levels of detail. Having a well understood set of goals helps ensure that the AOI system can be used to maximum benefit.

Inspection Goals

First Article Inspection. The use of AOI at during first article inspection reflects the need to rapidly and effectively validate assembly setup prior to production runs. It is effective in confirming that all feeders are set up properly, nozzles have not worn out, and the placement machine is placing parts properly. The focus at this stage is inspection for part presence or absence, wrong part, or wrong orientation or polarity. This step is equally critical in higher-mix environments, where a variable part supply and setup errors tend to be the leading cause of defects, and in higher-volume environments where setup errors could have a significant negative impact on the production run.

Process Tracking. The AOI system is used with the goal of producing information to monitor the production process. Typically, this includes detailed defect classification and component placement offset information. Manufacturers tend to prioritize this inspection goal when product reliability is important and specific process problems are targeted in higher-volume/lower-mix environments.

Characterization. For inspection of high-reliability products where characterization often is the inspection goal, AOI is used to find all possible anomalies. The resulting inspection data reflects an almost nonexistent escape rate, but more false calls and a longer inspection time. Solder joint inspection can be an important criterion if the AOI system is used post-refow.

Product Quality. Manufacturers focusing primarily on product quality are interested in the product’s status at the end of the production process. Manufacturers typically prioritize this as an inspection goal when production issues are relatively well known and product mix is high. A wide range of general process data can be collected at this inspection stage because the AOI system is placed at the end of the line.

First-pass Yield at ICT.  Implementing AOI can improve yields at in-circuit test (ICT) by capturing and routing defective boards before ICT. The goal is to minimize the potential bottleneck that ICT creates with large production runs. Process control data available through implementing AOI at this stage is broader in scope because inspection generally is performed at the end of the line.

The best location for AOI on the production line depends on equipment capability and what production problems must be solved. If the primary objective is to monitor the process and promote corrective action to prevent future defects, then pre-reflow inspection is arguably the best use of AOI. For pre-reflow inspection, the system can be placed right after paste printing, after the chipshooter, or after the fine-pitch placement machine. If the primary concern is detection and hence, quality of the end product, the best location is at the end of the line. For most production lines, this means post-reflow inspection. Although most, if not all, process defects are apparent and can be monitored at post-reflow inspection, use of the information may be drastically different than that collected in a pre-reflow location.

Equipment Selection

Once inspection philosophy and goals are determined, the equipment selection process requires a good understanding of AOI capabilities to meet these goals under various operating environments. Important considerations include system accuracy, repeatability, speed, false fails and false accepts, ease of setup, and CAD compatibility.

Other key equipment selection criteria include reporting and repair tools, the important link between inspection, repair, yield enhancement and cost reduction. After all, the true benefit realized from implementation of AOI comes from its ability to rapidly identify and report the location and nature of faults, thus providing the operator with data required for effective process control and/or repair.
Real-time data transmission and statistical process control (SPC) software monitors defect trends, and automated alarms help improve early warning capability, pointing manufacturing personnel toward the source of process excursions. Integrated repair solutions also are available in various configurations for either in-line or off-line use, and pinpoint the exact defect location. Color images also help operators to verify defects to be analyzed and repaired.

Recent discussion has centered around a closed-loop arrangement between each machine in the process and the AOI system(s). In this arrangement, defects are detected with AOI and automatically traced to the contributing machine. Theoretically, the AOI system could be used for process control by changing the variables to correct defects (or trigger operators to do so) on a real-time basis.
Finally, equipment cost must be considered with any AOI selection. System prices vary from a little more than $30,000 for off-line units to more than $200,000 for high-end systems. Initial acquisition costs for AOI can be significant in any return on investment (ROI) calculation and certainly could determine whether AOI will be implemented in multiple locations on the line. In any case, recent advancements in commercially available technology are pushing the price-performance curve in a favorable direction for AOI users. Considering the speeds at which camera and computing options are changing, it is important not to overpay for yesterday’s AOI solution.

Conclusion

Electronics manufacturers have long  been driven to identify and control defects to improve quality and enhance yields. Implementation of AOI has proven to be a successful way to achieve these goals. By assessing AOI inspection goals and methods that best meet specific needs, manufacturers can realize measurable gains in both quality and overall yields while dramatically reducing costs.

 Jens Kokott, Team Manager, AOI Systems at GÖPEL electronic GmbH

There are a multitude of vendors selling Automated Optical Inspection systems for assembled PCBs. Selecting the “right” supplier is an extremely challenging task with possibly far reaching consequences. The biggest difficulty is to keep a view in the apparently impenetrable jungle of AOI system vendors. Most muddle results from the “Babylonian confusion of tongues” because different vendors use different technical terms and advertise with so called “special features.” The result is a difficult situation for potential users, in particular in terms of facts that are really important for the future application of an AOI system.

The most important question is: “Which system fits the bill for my production situation?” Some basic parameters can simply be estimated: whether the AOI system should be integrated in the production process (in-line) or used as so called “AOI solution” (stand-alone), or if test time is within the range of production time. These questions might be answered without specifically testing some systems. More critical questions concern system flexibility in respect to test program creation, fault recognition or false call rate. They can only be answered by firsthand equipment checks. When test running an AOI system, you should definitely pay attention to the following:

• Test program creation for a typical PCB

• Check of the debug process

• Tests of fault-free and faulty PCBs from one’s own production

Of course, even after basic elimination of systems a lot of work lies ahead so one needs to narrow the field further and therefore, it is necessary to consider additional criteria in the pre-selection. such as an analysis of the system configuration. Great importance should be attached to the applied camera module as this is one of the critical parts of any AOI system’s capability.

Fish-eye-view or high precision photo?

Over the last few years it has become well-known that for high-performance IC and solder joint inspection, camera systems rather than scanner based systems have become the accepted standard. Some of the reasons for this are the higher achievable resolution and lower distortion brought about by telecentric lens as well as flexible illumination and configuration possibilities.

To meet the speed requirements of a modern production a large field of view (FOV) in one image is essential but not at the cost of image quality. Industrial-grade Four-Mega-Pixel cameras allow high-resolution image capturing of nearly 20 cm² in one image. The new AOI platform of the GÖPEL electronic OptiCon series enables the user to select between a One- and Four-Mega-Pixel camera, whereby quality is identical and the only difference is FOV. Users that go the One-Mega-Pixel route have the option to upgrade to Four-Mega-Pixels at a later date should volumes rise.

By default image recording in systems of the OptiCon series is carried out via a telecentric lens. The extremely high quality and dramatically reduced distortion found in the vertical view at each position of the inspected area results in a significantly reduced debug effort during the test program creation, but also a reduced false call rate in production tests. In particular for higher components but also multi-pin, fine-pitch ICs, this system characteristic proves to be a decisive benefit.

It must be emphasized, that a telecentric lens is a real innovation, especially for a Four-Mega-Pixel camera. In terms of the OptiCon 4M series systems, an aligned optic chain was developed which delivers a telecentric image of excellent quality in an inspection area of 42mm X 42mm. On the one hand, this is the basis for a defined scale reference to the components’ desired position – independent from their height extensions. On the other hand, it is also critical for the performance of the applied test functions.  

The brighter the better

The illumination control is every bit as important as the quality of the optic-chain.   Appropriate illumination selection is the basis for a high-rate of fault detection on PCBs. Furthermore, flexible illumination is necessary to cover specifications for the huge range of electronic components and their fault variants. Because of their inherent long-term stability and light power, LEDs in various configurations and colours have become widely accepted as the norm in recent years. Illumination can be arranged or configured in a wide variety. Examples for illumination arrangements:

• Ring illumination vertically from above, mostly arranged around the lens

o        Useful application for checking component presence and solder meniscus recognition

o        Disadvantage:  In case of shorts between IC pins false calls may occur because of fluxes or solder masks

• Angled illumination from various directions and different incidence angles:

o        Appropriate for solder-short checks, reduces false calls

o        Enables high-contrast display of laser labelling, a precondition for the effective utilisation of true-OCR functions as well as polarity check

• Illumination with different colours:

o        Enables high-contrast display of coloured polarity marks as well as distinguishing components from the PCB background

o        Causes reduction of fault slip and falls call rate

Additionally, for different board materials or component colours a brightness control for certain test tasks are necessary. Because of the huge number of permutations for illumination settings there are numerous parameters which must be provided for a faster and less complicated administration and the AOI software must make this as simple as possible.

The new OptiCon system series unites all possible variations listed above, and provides a flexibility not seen before now.

For example, a special illumination design was developed that allows a safe solder-short check at minimum pitches despite possible contaminations by fluxes or solder masks.  The user can select between different colour varieties that provide a safe detection of incorrect polarity, i.e. coloured marks.

In comparison, colour cameras used in other AOI systems do not feature the resolution or inspection area of the Four-Mega-Pixel camera deployed in the OptiCon. Moreover, they are barely usable for certain colour inspections.

For example, GÖPEL electronic’s OptiCon systems provide the user with required varieties and respective parameters – predefined in library entries. Despite the huge variety, test programs can be created and optimised in the shortest time; making for rapid deployment to production.

All inspection parameters can be changed on four different levels: for a single component on the PCB; a model type in the current test program; in all new test programs or in all new and existing test programs. The OptiCon concept is based on the important goal to obtain a stable, reliable test program within shortest time, and to provide flexible adaptation possibilities to allow for specific quality requirements and supplier conditions.

Flexibility should not be simply a buzz-word

A flexible system configuration is not just about the system software and its parameter management but includes the possibility to integrate additional modules. Because of the high variety of electronic assemblies likely to be seen in a production environment, add-on modules can be very helpful to increase fault coverage. The OptiCon systems provide the following additional components as options:

o        Camera for THT and colour inspection with a depth of field of 35mm for safe inspection of high components (e.g. encased-electrolytic capacitor)

o        Laser height measurement system allows co-planarity inspection with micrometer range accuracy (e.g. for BGA components)

o        Camera with angled view enables inspection of critical components (e.g. shorts and solder joint inspection at PLCC and SOJ components)

In the end

All the points mentioned up to now characterise the performance of AOI systems. But factors such as service and support should be considered as well since they play an important role in terms of operator convenience and, especially, overall customer satisfaction. Very often expensive maintenance contracts and software update programs are offered which layer additional costs on the purchase price. GÖPEL electronic provides free updates or service support for its OptiCon series for life. Consequently, the user of such an AOI system does benefit from future developments. Additionally, the OptiCon system’s extraordinary price-ratio-performance guarantees optimal return-on-investment.

Conclusion

The selection of an AOI system means a lot of work because many different criteria must be considered. In addition to specific demands for an individual production environment and product range, system setup and components used may highlight special features or performance requirements. Special attention should be placed to the image recording module and illumination unit since they are critical for high-flexibility with a high mix of PCB types. An effective and user-friendly library administration allows time-saving handling of newly taught PCBs, which is necessary for a high variant diversity.

Of course, the user has to test the respective system before purchase decision. After a short-list selection, as described above, test program creation and test runs should be executed on an established production PCB.

• If your friend forwarded this newsletter to you, please register as a member and receive The BestTest Newsletter -- absolutely free!
• If you wish to update your news preferences or cancel the subscription, please unsubscribe.
• If you have any questions, please email experts@BestTest.com

Get the widest selection of test related books and software at the BestTest Online Store.