Interest in Automated Inspection Booms But Which Technique Is Best?

What is behind the sudden interest in PCB inspection equipment? And why are the major ATE suppliers supplementing their functional and in-circuit test systems with new automatic inspection system product lines?

The answers lie in everyone’s aim to solve the triple quandary of how to ensure that densely populated PCBs with minimal test access are correctly assembled, fault free, and less costly to produce and test. As always, there is not a single best solution. Instead, we must determine what is to be inspected, which of the many inspection techniques is suitable for your application, and what feedback data your inspection system should provide to minimize cost and maximize quality.

Inspection Targets

After specification compliance and functionality of bare boards and components have been verified, either by incoming or source inspection, solder deposition and PCB assembly begin. These inspection tasks usually are performed during or subsequent to manual, machine-aided, or fully automated assembly operations:

Verification that the correct components have been inserted at their designated locations.

Confirmation that components are correctly positioned, not inverted (wrong polarity or incorrect placement of pin 1 of ICs) and not excessively skewed.

Determination that all solder-related processes are under control and that quality solder joints are being produced.

The latter task is the most critical of the three, not only because of the quantity of the joints involved, but also because of the number of process variables. Solder quality is affected by the quantity of solder paste deposited, the positional exactness of the paste-deposition process, the solder composition, the solder reflow time and temperature, and the thermal characteristics of the board and components.

In the old days when an inspector found that most solder joints were acceptable, he would use his soldering iron to touch up the remaining ones so that the PCB could pass. But this procedure is impractical now.

With some of today’s fine-pitch components, hand touch-up does more harm than good due to their small contact areas and tight lead spacing. And for other packages, such as ICs in ball grid arrays (BGAs), the connections are on the underside of the device and inaccessible. As a result, the only viable alternative is to obtain enough data to ensure that each phase of the soldering process is kept under control.

Controlling Operations and Processes

Ideally, production operations always should be defect free. However, differences in material composition, unforeseen build-up of fixture and placement-machine tolerances, or human error make this difficult.

Achieving a near-defect-free manufacturing environment requires that inspection equipment is not merely used for finding and classifying defects, but also for preventing them from ever occurring. “To achieve this goal, inspection results need to be available in real time and easily comprehensible to help operators recognize what and when adjustments need to be made,” said David Clark, director of the SMT Product Group at General Scanning.

“To achieve process stability, the inspection system also should present data in a format suitable for in-depth statistical analysis,” Mr. Clark added. “This allows engineers to perform process experiments and optimize each step of the operation. For example, our customers have used solder-paste inspection to determine optimal settings for the screen printers. In addition, the inspection systems are used to qualify new equipment and stencils.”

But even optimized processes gradually drift, and defects are likely to occur. When that happens, prompt defect recognition is essential. However, modern computer-aided PCB inspection systems do more than that. They monitor process trends and generate early warning alerts so defects can be minimized.

For instance, Teradyne’s Graphit™ SPC Software which runs on a PC accumulates the data and measures production-quality variables and their underlying patterns and trends. “When a process deviates from acceptable tolerances, the operator is alerted to take immediate action,” said Dominic Haigh, Business Unit Manager of the Automated Inspection Systems at Teradyne. “Early prompting allows corrective measures to be taken before production slowdowns and costly repairs are required. The system also provides an assortment of graphical displays for quick and easy interpretation of production defects.”

Similarly, the GenRad GR Vision System generates early warnings and key statistical tools to effectively manage manufacturing processes. It also prepares Pareto charts and management reports on yield and equipment uses.

Technologies Targeted to Tasks

Not all PCBs are populated with ICs contained in fine-pitch, flip-chip, or BGA packages. For many of these less complex boards, simple inspection aids enable human inspectors to easily identify placement-related as well as solder-related defects or deficiencies.

One method for direct visual inspection compares newly produced boards against a known-good sample, a technique aided by equipment such as Metron’s Circuit Board Comparators. “Images of the known-good master board, or a photomaster, and the new board appear on a horizontally split screen as moving mirror images coming out of a center dividing line,” explained Paul Kempf, president of Metron Optics. “Mismatches are errors and are quickly identified by the operator.”

Another comparison implementation that lends itself to automation—and is applicable for conventional as well as fine-pitch PCBs—uses thermal-imaging techniques. “An infrared camera equipped with accessory lenses can be used for this task,” said Lisa West, manager of application engineering at FLIR Systems. “Image subtraction from a known-good board easily identifies misplaced or missing components. A standard lens or wide-angle lens displays the entire PCB and, with a simple change to a 5-mil close-up lens, the individual components can be investigated. Microscopic lenses allow image resolution to 5 microns.”

For high-quantity production of dense boards, fully automated inspection equipment becomes imperative. “For instance, an automated optical inspection (AOI) system consistently achieves >95% fault coverage, while human inspectors, on a good day, might average 50% to 60% fault coverage,” said Teradyne’s Mr. Haigh. “That difference shows up in several areas: High fault coverage results in a more reliable product, higher yields at final test, and better feedback for controlling the manufacturing processes.”

While all automated PCB inspection systems can identify a range of defect types, some are targeted toward finding or assessing the causes of a particular class of defects. Other systems use techniques that may provide a particular capability not offered by others.

A PCB inspection system produced by CR Technology, for instance, uses a proprietary white-lighting technique, video cameras, and a vision processor to inspect for component-related defects as well as the presence or absence of solder and bridging. “But checking for the correct component and its orientation—by matching the label on the part to templates stored in the system’s library when necessary—is a key inspection task that the system offers,” said Don Miller, vice president of sales at CR Technology. This particular strength is attributed to the fact that component and label matching is facilitated by the capabilities inherent in vision processing.

GenRad’s GR Vision Automatic Optical Inspection system also is designed to inspect component placement. It checks 100% of every PCB produced at the beat of the line. The system speed is achieved by using line-scan camera technology.

General Scanning’s inspection equipment finds solder-paste and component- placement defects. “The systems use a laser scanning technique that supplies 3-D process data,” said Mr. Clark of General Scanning. “For solder paste, the equipment provides solder-paste thickness and volume measurements. For component inspection, 3-D imaging means that the systems are not sensitive to changes in board color or material in the same way that camera-based inspection systems are.”

Teradyne’s AOI systems have angled cameras and a programmable lighting dome, which give higher defect coverage for solder-joint inspection, according to Mr. Haigh. “The angled cameras allow the system to view an area from several perspectives, which is particularly useful in inspecting J-leads and detecting lifted leads that elude a single overhead-camera configuration,” Mr. Haigh said.

The programmable lighting dome contains multiple light sources in the form of programmable LEDs that are densely packed within a hemispherical dome. By selecting and turning on groups of LEDs, the system effectively moves the light source to any position in the hemisphere. The result is a brightly reflective image of a fillet, lead, or component that can be analyzed for a number of defect categories.

A two-stage inspection system, with each stage targeting a series of inspection tasks, has been designed by Panasonic. A color camera in the first station inspects component presence, placement accuracy, polarity, and skew of components.

“The second station is equipped with a gray-scale CCD camera and a laser unit,” explained Eric Covelli of Panasonic. “These facilities detect insufficient solder paste or no solder paste by determining whether or not fillets are found at the terminals and leaded areas. The station also detects lifted chips or lifted leads.”

A unique color-processing algorithm enhances image interpretation. Color cameras capture images as red, green, and blue signals, each containing color as well as light-intensity information. A nonlinear conversion transforms the primary color signals into an HSI format in real time. In the HSI model, H denotes the hue, S is saturation (indicating the intensity of the color), and I is the brightness.

Use of separate hue and saturation data significantly enhances the correctness-rate of image-quantification judgments. “For instance, the difference in color from component to component or the shadowing of one object on another does not affect the inspection process,” commented Mr. Covelli.

The Hewlett-Packard automated cross-sectional X-ray inspection systems address solder-joint quality-verification issues. “New packaging techniques such as BGAs, flip chips, and chip-on-board, combined with multichip modules, often result in many hidden solder joints,” said Tim Rees, product marketing manager at the Manufacturing Test Division of Hewlett-Packard. “With X-ray technology, you can look through barriers or obstructions to the actual solder joint

“However, traditional X-ray systems provide images that are frequently cluttered with devices and solder joints from multiple levels on each side of the board,” Mr. Rees continued. “X-ray laminography, in contrast, allows you to control the depth of the image that is viewed. As a result, you can chose the level or side of the board that you wish to inspect without turning the board over or removing obstructions. In addition, the system creates a 3-D view of objects to verify the complete characteristic of each solder joint.”

Conclusions

Many techniques are available to perform inspection to ensure that production processes are well controlled and PCBs are defect free. While most inspection systems address the entire fault spectrum, some use techniques that make them particularly effective for finding—and determining the causes of—specific classes of defects.

Which type of system will serve you best not only will depend on the types and quantities of PCBs produced, but even more so on the production processes used at your facility. Since cost and quality are the prime success-determining factors of any manufacturing and test operation, it is essential to work with potential inspection- system suppliers to assess the suitability of their equipment for your application, predict the defect reductions achievable, and perform the expected return-on-investment calculations for your various options.

Board Inspection Products

System Provides

Real-Time Imaging on PC

The Tracer™ Thermal Imaging System records and analyzes thermal-event sequences at real-time frame rates up to 60 Hz on a Windows-based PC and offers a 12-bit dynamic range. The system includes the company’s Prism DS infrared camera, a Pentium^®-based PC, a digital recording system, and software. Recording rates, areas, and acquisition depth are user definable. The Tracer senses temperature differences of <0.1°C @ 30°C and measures temperatures from -10°C to +2,000°C with an accuracy of ±2%. Call company for price. FLIR Systems, (800) 322-3731.

Optical Inspection Systems

Offer Improved Fault Coverage

The Interscan 5539 and 5529 Series Automated Optical Inspection Systems visually inspect components and solder paste on loaded PCBs. They are used during pre- or post-solderwave and post-reflow or post-placement of components. The 5539 Series features a five-camera head design comprised of one vertical and four angled cameras. The 5529 is an entry-level system featuring one camera. The company’s Component DESigner toolset facilitates off-line program development and library customization. $130,000 to $200,000. Teradyne, (800) 227-1620.

Inspection System Offered in

On-Line and Off-line Versions

The RTI-6500 PCB Inspection System checks populated boards for correct assembly, determines proper screen-printed solder-paste application prior to parts placement, and identifies solder defects. Programming is accomplished by automatically extracting part information from CAD files, loading a known-good board, or self-learning component information. Part orientation may be verified by matching acquired label data with stored templates. Typical inspection scan times are 800 parts/min. The RTI-6500 also assists rework with results files that highlight incorrect components. Off-line: $70,000; on-line $90,000. CR Technology, (714) 448-0443.

New Software Provides

Expanded Real-Time Data

The SVS 8100 Measurement Software v4.2 provides better access to and presentation of data gathered by the SVS 8100, a solder-paste and component- placement inspection system. The software offers an enhanced operator environment, simplified reporting, configurable user access, and on-line help. Displays include a board map, defect lists, Pareto charts, and yield and X-Y-theta target charts. The Model 8100 performs high-speed, in-line data acquisition and image-processing and provides data for prototyping, process characterization, and real-time process control. Call company for price. General Scanning, View Engineering Division, SMT Products Group, (805) 578-5000.

Optical System Performs

High-Speed In-line Inspection

The GR Vision Optical Inspection System provides automated visual inspection of PCBs in an in-line, integrated, real-time manufacturing environment. Defect detection and reporting are programmable and include inspections for presence or absence of components, orientation or polarity, offset in the X and Y axes, and rotation. The system typically loads, scans, and unloads a 9″ × 14″ board with 700 components in less than 45 s. From $120,000. GenRad, (978) 589-7000.

System Features

Three Inspection Modules

The VC40C Visual Inspection System consists of three inspection modules and interfaces with peripheral devices. Using image recognition and laser-supporting technology, the system inspects the mounting and soldering status of components. The system accommodates boards ranging from 50 mm × 50 mm to 330 mm × 250 mm. The inspection area is selectable from 5.1 mm × 4.8 mm to 20.4 mm × 19.2 mm, and the inspection time is 0.25 s per area. Call company for price. Panasonic Factory Automation, (847) 288-4400.

Visual Software Builds

Machine Vision Applications

VisionBlox™ Software provides a new way for system integrators, OEMs, and volume end users to develop custom machine-vision applications without spending time to develop core vision algorithms. VisionBlox custom controls are linked with standard controls from the development environment by dragging them onto an application window. The Windows-based architecture is open and supports frame grabbers as well as high-performance vision processors. When using a simple frame grabber, all VisionBlox algorithms are run on the host CPU. Call company for price. Integral Vision, (810) 471-2660.

Monoscope Offers 3-D Effect

With 8″ Working Distance

The Super 3-D Monoscope provides a resolution of 1.2 line pairs/mm at 5× magnification and a depth of field of 15 mm at an 8″ working distance. An optional 7.5× magnification lens offers 144-line pairs/mm resolution. The working distance allows room for soldering and assembly. Monoscope with 5× lens $4,000. Metron Optics, (619) 755-4477.

X-Ray System Inspects

PCBs in 60 to 90 s

The HP 5DX Series II Cross-Sectional X-Ray Inspection System checks densely populated, double-sided PCB assemblies for solder-joint defects at a rate of one board every 60 to 90 s. It offers real-time statistical process control and has the imaging capability to inspect boards with ball grid arrays, flip chips, and tape automated bonded components at in-line speeds. The Model 5100A is suited for inspecting standard pitch PCBs, the Model 5300A for high-volume inspection of very fine-pitch PCBs, and the Model 5400A for ultra-fine-pitch PCBs. From $349,000. Hewlett-Packard, (800) 452-4844, ext. 5364.

March 1998