Look carefully for the right optical equipment
07 May 2006
There is a multitude of vendors of Automated Optical Inspection systems for assembled
PCBs. Selecting the “right” supplier is an extremely challenging task with possibly far
reaching consequences. Jens Kokott and Steve Hooper discuss the considerations
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 probable 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, the following activities should definitely be executed:
• 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; for instance 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 have become the accepted standard rather than scanner based systems. Reasons are the higher achievable resolution and lower distortion brought about by the telecentric lens as well as flexible illumination and configuration possibilities. To meet the speed requirements of 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. There are AOI systems that enable the user to select between a One- and Four-Mega-Pixel camera, whereby quality is identical and the difference being only FOV. Users that go the One-Mega- Pixel route may have the option to upgrade to Four-Mega-Pixels at a later date should volumes rise.
Default image recording may be 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 (figure 3).
It must be emphasised, that telecentric lens is a real innovation especially for a Four-Mega- Pixel camera. An aligned optic chain was developed which delivers a telecentric image of excellent quality in a widest possible inspection area. 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
As important as the quality of the optic-chain is the illumination control; 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
• Useful application for checking component presence and solder meniscus recognition
• 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:
• Appropriate for solder-short checks, reduces false calls
• 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:
• Enables high-contrast display of coloured polarity marks as well as distinguishing components from the PCB background
• 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 easier administration and the AOI software must make this as simple as possible. An example would be where there is a special illumination design that allows a safe solder- short check at minimum pitches despite possible contaminations by fluxes or solder masks (figure 4). 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 various AOI systems do not feature the resolution or inspection area of the Four-Mega-Pixel camera. Moreover, they are barely usable for certain colour inspections.
For example, GOEPEL 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.
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. AOI systems should provide the following additional components as options:
• Camera for THT and colour inspection with a depth of field of 35mm for safe inspection of high components (e.g. encased-electrolytic capacitor)
• Laser height measurement system allows co-planarity inspection with micrometer range accuracy (e.g. for BGA components)
• Camera with angled view enables inspection of critical components (e.g. shorts and solder joint inspection at PLCC and SOJ components) (figure 5)
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. Free updates or service support for AOI systems for life are surely welcome. Additionally, a price-ratio-performance that guarantees optimal return-on-investment should be taken into consideration.
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.
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