Best-in-class testers combine multiple AOI, text verification and impedance testing in one machine.

Assembling a single PCB and thoroughly inspecting it before turning on a production line – also known as first-article inspection (FAI) – is critical to a successful PCB assembly process. It ensures the right components are in the right places, and that the assembly meets its manufacturing specification. FAI is important at all stages of a product’s life. In the prototype stage, it delivers correct assemblies so designers can debug the circuits instead of the manufacturing process; in production, it verifies the correctness of the first product before building hundreds, or even thousands, of them.

Failure to capture mistakes in circuit assembly methods, equipment setups or programs can result in delayed development progress, lost production, risky rework or even product recalls – all of which cost unbudgeted time and money.

Typical defect modes found in PCB first article inspections include wrong component values or sizes, misplaced components, reversed polarity or missing components.
Counterfeit components that slipped past incoming inspection may also be identified at FAI.

Different PCB assemblers employ different FAI techniques. Checks for component presence, polarity, placement or size are performed by operations that range from manually comparing the PCB with a drawing to semiautomated or fully automated optical inspection (Table 1). Current methods of first article inspection on a PCB assembly are mainly optical. (Ironic, isn’t it, considering that the product being tested is electronic?) Optical inspection is only a partial answer at best. Electrical verification is also required, especially on high-reliability products. Many companies now specify electrical verification as part of their FAI requirement, especially those employing the AS-9000 Quality System (sidebar).

[Ed.: To enlarge the figure, right-click on it, then click View Image, then left-click on the figure.]

Electrical verification of the values of unmarked components may be done manually with a meter and a BoM checklist, or by in-circuit testing, if available. On prototype runs, ICT is often not available, so verification is performed manually. This can take hours, keeps the assembly line down, and is prone to human error and fatigue. Some assemblers do not test the electrical integrity of the PCB assembly in the prototype stages because of the time requirement. Electrical verification should not be optional when manufacturing electronics assemblies, as component values are critical to the product’s functionality.

A new genre of FAI testing equipment is emerging to support the need for more efficient, effective inspection. These machines typically employ semiautomatic optical inspection methods, which improve inspection speed and accuracy and ease operator fatigue. They are easy to program with BoM and Gerber files, and are intuitive for operators to use. Their cost and portability between assembly lines enhances their ROI, and their ability to prevent costly mistakes makes them highly desirable for prototype or batch-style assembly operations.

FAI inspection systems vary greatly in feature content and user benefits. The most basic machines scan images of the PCB assemblies and display programmed areas of the scanned images for operator inspection. More advanced systems offer higher resolution cameras on X-Y gantries to allow operators to pan, zoom or change lighting in real time to better see the objects under inspection. The most sophisticated systems integrate automated and semi-automated optical inspection with electrical testing to provide the most comprehensive first article verification.

The fully integrated machines utilize key aspects of AOI, electrical test and semiautomatic inspection as they apply specifically to FAI:

• The AOI function inspects for component X/Y location, rotation, and size.
• An Optical Character Verification (OCV) function inspects the text or other markings on the component.
• An LCR meter measures the impedance of chip capacitors and resistors (Figure 1) to verify the values listed on the BoM. Value checking on parts smaller than 0402s is critical because the components are too small to bear identifying text and cannot be verified optically.

If automated checks identify any discrepancies throughout the optical or electrical test sequences, the machine uses a semiautomatic mode to step nonconforming components under the camera for positive verification. Figure 2 shows the high-magnification camera view of the component test probes.

[Ed.: To enlarge the figure, right-click on it, then click View Image, then left-click on the figure.]

After the first article has been verified, the data can be stored and uploaded to various data management programs and quality assurance reports. These systems can replace nearly all paper-based FAI documentation like diagrams, checklists or other recording media. The operation speed and automatic report generation can reduce the time needed to perform an FAI and improve the accuracy of the entire process.

Summary

FAI testers validate PCB assemblies by various methods. Best-in-class FAI testers combine multiple methods into one machine: AOI, text verification and electrical impedance testing. They also eliminate the paperwork of inspection documentation and report generation.

Simplifying, automating and documenting the PCB FAI process according to international standards, while incorporating both optical and electrical tests, will contribute to improved product reliability and lower manufacturing costs.

References

1. Goodrich ISR Systems, First Article Training Ref AS9102, Rev A.
2. Robert Morris, “Enhance First Article Inspection,” Quality Magazine, July 16, 2007.

Matthew Holzmann is president of CGI Americas (cgi-americas.com); matt@cgi-americas.com. Chrys Shea is founder of Shea Engineering (sheaengineering.com); chrys@sheaengineering.com.