Connector grief? Today's inspection equipment must handle multiple pin heights within a connector and quickly identify defective pins.

Ever-tightening cost control environments continually challenge us to find new, cost-effective manufacturing solutions. One solution explored by original equipment manufacturers (OEMs) has been the outsourcing of connector assembly to electronics manufacturing services (EMS) providers. Many of these OEMs, though, continue to complain about connector quality issues from their EMS partners. Despite the automated press fit assembly process, defects escape to customers due to process and/or post manufacturing handling errors.

Connectors can have pin lengths of up to 12 mm, and their defect pareto charts can include missing pins, bent pins or alignment errors, bowed pins, connector-to-connector misalignment, broken connector shrouds or even debris among the pins. Repair of soldered connectors is often difficult, if not impossible, as a result of thermal stress on the printed circuit board (PCB) and the laborious nature of the repair process.

Automated connector inspection is becoming a necessary requirement, especially for backplane applications. Here, reliable inspection of thousands of pins on a board in just a few seconds is key. The inspection tools must provide flexibility to handle multiple pin heights within a connector and to quickly identify to the repair specialist the location of defective pins.

In today's surface-mount environment, mixed surface-mount device (SMD) and press fit defect detection can be combined, reducing a multistage test and inspection methodology to one inspection system. This combination drives efficiency and quality in the right direction. The connector pin world is a three-dimensional (3-D) matrix defined by pin tip location and height on which successful connector mating depends. Providing direct pin height data, relative to the connector base-plane, provides information to aid process setup, control and continuous improvement.

To fully define the right machine vision solution, we need to better understand the process tolerances and variations for pin location and height using automated optical inspection (AOI). Utilization of 3-D AOI systems with multi-angle illumination and angled cameras, along with zaxis control, provides the necessary focal range and imaging dexterity to separate the good, the bad and the ugly of connector defects.

Here's How

Combined SMD and press fit quality assurance requires inspection at multiple focal planes (Figure 1). One plane is at the board level; others are at z-axis positions that coincide with the plane of the pin tips, which is especially important when different pin lengths are within one connector package.

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In this application, the key to height and position measurement is focusing on the board level and imaging quality of fiducials used for registration and alignment to computer-aided design (CAD) information.

Another factor associated with the measured positional accuracy is the parallax effect on the pin tip. Parallax effects are lateral position errors associated with the angular relationship of the pin position to camera and the center of the field of view. They are proportional to the measured height of the pin.

Analyzing the defect detection factors, the x and y location of the tip and the pin height information are isolated as the primary mechanisms for future system assembly failures, especially for backplane boards.

To fully evaluate both of these aspects on a production board, a number of elements are required in the AOI system optical head design. For example, z-axis movement of the board is required. A stationary lighting and camera arrangement also provides maximum optical stability and the highest image quality possible for high-speed inspection needs. To measure height and detect anomalies, the optical system should have a combination of top-down and low angle illumination with CCD imaging (Figure 2). Due to the number of pins, the pin spacing and their heights, many inspection angles provide the necessary 3-D capabilities.

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An Optimized Imaging System

A large depth of field with z-axis movement is needed to incorporate the full range of component heights in mixed SMD and connector applications, and situations may arise where high-resolution inspection may be required for small parts like 0402s or 0201s. Some of these components are shadowed by the relatively tall connector bodies, which means that full flexibility, high-resolution imaging adds to the reliability of inspection for the low-profile component types.

What makes this approach most effective is the combination of stability and sophistication built into the optical head. An ideal AOI system may incorporate up to 13 CCD cameras and 18 different illumination sources with direct and diffuse xenon flash lighting. These cameras and illumination sources should be used in different combinations to provide maximum variety of inspection conditions, much like the human inspection process. The optical head design, without any moving parts, should provide high quality and stable imaging during high-speed inspection.

Key to the press fit application is the fine resolution that may be achieved with a z-axis board handling stage (5 [micro]m steps). Control of the board movement can be fully automated using board handling control macros, with acceleration and deceleration control.

Detection Algorithms

Using a component library, CAD data should be translated into an AOI inspection program that automatically assigns appropriate algortihms and associated threshold settings. For standard SMD components, presence, absence, offset, polarity and solder joints should all be inspected.

For press fit pins, programming can be implemented using a connector software macro that takes the location of a single pin and expands the connector's pin matrix according to the component definition (Figure 3). Different pin types can be assigned to certain locations. Ground pins have a lower height and require different inspection parameter settings than the majority of connector pins. The macro can provide pin type variants to be combined under one component body type.

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Additional inspection and feature extraction techniques may be added to the standard inspection algorithms to address the pin inspection needs. Presence or absence of the pin can be determined by analyzing the region of interest in the vicinity of the defined pin location (Figure 4). The pin's location can be determined by optimizing the goodness of fit correlation in the pin tip feature search. Typically, the pin diameter is 1.0 mm with a lateral offset tolerance of 0.2 mm, although half pin width may be a general rule applied with traditional non-measurement inspection methods using 3X or 6X microscope techniques.

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The value of inspection is not only in defect detection but also in the prevention of future defects. The way inspection information is interpreted and used to improve processes and yields is where the added value lies.

Yield Improvement

For repair process efficiency, identifying to the repair specialist where the defect is among the complex connector landscape is necessary. Graphical tools at the review station should represent the location of lateral tolerances to help the operator evaluate defects, process indicators or any false calls that may occur. In Figure 5, the outer blue frame marks the location of the maximum offsets allowed, as seen from this particular direction. This image is what the operator sees at the verification station.

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When the defect information has been verified, the pin height information, measured using angled views and by moving the board in the z-axis until the pin tip is located, can be displayed in yield management software tools for statistical process control (SPC) purposes (Figure 6).

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Defect trend analysis can be performed using pareto checks that can highlight repeat offenders at the pin or part level. This analysis provides the clues to understanding what the root cause of the defects is--for example, a process-related effect or a handling procedure prone to error. Either way, the process advisor tools can lead the operator in the right direction.

Conclusion

Press fit and connector inspection is a demanding inspection application. The 3-D nature of the features is the key hurdle. Combining both SMD and pin inspection presents even greater challenges but also offers the maximum return on capital investment.

Optimization of the optical and board handling mechanism of AOI equipment, to extend the z-axis range and improve the overall quality of 3-D imaging capabilities, is the key advancement enabling accurate defect coverage and robust inspection performance for this application. Technology exists today to address these requirements.

David Doyle is the marketing manager for the electronics assembly group at Orbotech, Inc., Billerica, MA; [phone]; e-mail: [e-mail].