AOI: Agile Optical Inspection

Soundcraft Studer, part of Harman International, manufactures advanced audio mixing consoles on its SMT production line and also produces PCBs for other Harman companies. Production is low volume, high mix and constantly changing so its inspection needed to be fats and agile, responding to rapidly changing production needs.

Over the past 60 years, Potters Bar based Soundcraft and Studer have steadily grown their dedicated audio mixing console design and manufacturing businesses, with global sales and support networks. Its mixers are designed for live sound, recording, post-production, and TV and radio production.
This production facility is not currently used for a regular, high volume throughput of a stable, longterm product mix. Instead, its role is to produce prototype and low production quantities of a wide range of products as a service to Soundcraft and Studer as well as other companies within the Group, with volume production handled elsewhere. Typically, the facility processes between two and eight new products every month, for example, in-car microphones are currently being prototyped for Harman AKG in Austria. PCBs for these products include components practically invisible to the naked eye.

This high and constantly changing product mix creates complex issues, especially when some of the products are for sister factories with their own part numbering systems. Speed and flexibility are needed on the production lines to facilitate rapid product changes. To avoid bottlenecks, any test and inspection facility has to be equally as fast and versatile. When Soundcraft Studer’s x-ray inspection system’s lack of these attributes became an issue, automated optical inspection, AOI, looked promising and so the company invested in a Nordson YesTech BX benchtop AOI system.



Adopting AOI
AOI is fast, delivering cost-effective quality control compared to in-circuit testing. Once a system is programmed with the component, layout, orientation and soldering information, its strategy of inspecting by comparison to a library of parts and packages allows rapid throughput of boards during production. Equally essential to Soundcraft Studer, however, was the BX’s ability to be quickly reconfigured and even reprogrammed fast enough to keep pace with different area’s demands for inspection resource.

As the company adopted the BX system, it found that the time needed for programming new boards was heavily influenced by the availability of component data. Lead times for new programs were reduced as their component library developed and grew. Developing this component library called for careful management of the part numbering for these components, as up to three numbers related to each: the original manufacturer’s part number; the part number assigned by Soundcraft Studer and if applicable, the part number assigned by the relevant sister company.

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As for any AOI system, other parameters apply to each part too: board reference; its X and Y co-ordinates; orientation and package code. These, however, are available from the CAD files used by the pick&place machines. Accordingly the company built a spreadsheet database that integrated CAD data with part number information, together with storage location. This comprehensively defined each component used in PCB production and integrated with the BX library database.

Operating experience
As the database has been growing, set-up times for entire boards have got faster. For example, a board was programmed within 25min of receiving the CAD data even though it contained 123 new components not yet in the library. After set-up, inspection has proved to be fast too. An inspection of a double sided board after solder pasting took 23s to determine that the board was ready for soldering. Component presence and position were checked.

The machine also uses OCV to check polarity for each component and also that it is the right part by reading its label. Another useful feature was the AOI’s ability to check component height above the board, as an over-height component could indicate a problem such as a smaller part trapped beneath a BGA being inspected. A further 23s inspection after reflow was sufficient to check for shorts, dry joints, lifted leads and other solder faults.

Soundcraft Studer mounted its AOI benchtop machine on a trolley so it can be rapidly moved to any point in the production line where inspection is required. This is a significant improvement on their earlier situation in which inspection was performed by an x-ray machine permanently in position after reflow. And with just x-ray inspection, it was not possible to detect incorrect components or orientation, and solder is often masked by components in the same position on the top and bottom.
Batch sampling has proven to be well worthwhile with the new system as it can catch problems within the manufacturing process that go beyond a single board failure.
With smaller batches 100% inspection is best as, for example, it is possible for a component reel loaded into a pick&place machine to be populated with mixed components when kitting small lots. This fault would not be visible, and the pick&place machine would be unable to detect it as it only reads the barcode label on the reel which would probably be as expected. However a series of boards rejected by the AOI system for the same ‘wrong component’ failure would soon alert an inspector to the existence, nature and source of the fault.

Two months down the line following installation and Soundcraft Studer has found the AOI to provide the inspection resource essential for its low volume, high mix production environment. Dave Birtwistle, SMT Team Leader at Soundcraft Studer says, “We appreciate the benchtop system’s agility as well as its speed, as this allows its rapid reconfiguration for different product types and process stages, and it is easy to program as well as fast in operation. Above all we can report that to date it has found every fault it has encountered.”