Timely inspection—accompanied by real-time results analysis and prompt corrective action—avoids scrap, improves quality and saves costs. But printed circuit board (PCB) assembly entails many sequential operations, leading you to ask: At which production stage is inspection most beneficial, and what inspection technique is best at each step?
The typical PCB assembly starts with a bare board, followed by solder-paste application and component placement, IR reflow, flow solder, and possibly hand soldering of additional components. The order of these operations may vary depending on product characteristics.
Inspection may be performed to address these concerns:
Bare Board: assurance that there are no shorts and opens, tracks have adequate current-carrying capability, and the integrity of plated-through-holes is maintained.
Solder Paste: application should provide adequate but not excessive solder volume, and be coplanar, uniform and correctly located.
Component Placement: correct components should be at every location with proper orientation and accurate alignment.
Solder Quality: solder joints should be electrically and mechanically congruous, without opens or solder bridges.
Since these concerns are varied, an inspection-system implementation perfect for one application may not be ideal for other inspection tasks. Some systems, however, do address almost the entire inspection spectrum. These may have a higher acquisition cost, but they can be very cost-effective in specific production environments.
Most often, there is a particular technology associated with a specific inspection task, although the same equipment may carry out two or three additional types of inspection.
X-Ray and Eddy Current
During the manufacture of multilayer boards, the copper plating thickness must conform to specifications, and the registration between layers must be proper. Directional X-ray equipment, such as the FeinFocus FXS-100.82 and the Innervision system from Optek, can assure that there is no misregistration by providing inner layer-specific alignment data and information on distortion and shrinkage.
“For example, the Hewlett-Packard facility at Loveland, Colorado, employs a real-time CAD-programmable, computer-controlled automatic X-ray system to check layer-to-layer registration of their multilayer boards,” said Roger Bryan, Vice President Sales and Marketing at Operations Technology (Optek). “The process is fast and feature-specific, and allows for immediate analysis of all parameters. It not only facilitates post-lamination drilling optimization, but also directly improves the production control process prior to lamination.”
The proper copper weight of internal tracks can be critical to assure adequately low impedance and avoid excessive heat dissipation. A measurement of plated copper thickness before and after etch may be performed by the MRX system from CMI International. It uses a combination of eddy current and microresistance techniques to provide a direct readout of material thickness.
While companies that perform PCB assembly may wish to perform bare-board registration and copper-adequacy inspection only at the source, through-hole plating integrity is often verified at incoming inspection. The PTX probe from CMI, which operates on the same eddy current and microresistance principle as the MRX system, can be used for this purpose. It provides an accurate readout of through-hole plating thickness on a 3-digit LCD, regardless of board thickness.
Light Beams and Video
Correct component placement and solder-paste deposit locations are readily verifiable by 2-D image examinations. Evaluations may be performed by direct or optically aided observation via video cameras or scanners and frame grabbers, followed by automatic image analysis.
However, assessing solder-paste adequacy in the third dimension, as well as measuring its volume, requires height information. Methods commonly employed to obtain the 3-D information include illuminating the object from varying directions, measuring shadows, assessing reflections, and triangulation. Light sources may consist of structured light or laser beams. Receivers may be photo-electric cells or video cameras.
Component and solder-paste inspection may be done with a combination of 2-D video image analysis and 3-D laser thickness measurement, or by 3-D laser scanning alone. In either case, data gathering and processing should be fast since it is usually desirable, if not essential, to inspect 100% of all pads, leads and joints.
“Video cameras capture many features in a single frame, and frames can be captured at rates of 50, 100 or 200 per second,” said Tom Trozzi, Chief Engineer at Machine Vision Products. “Features within a frame can be processed by high-speed, reasonably priced, general-purpose CPUs operating in parallel.
“Competitively priced equipment can inspect 400 joints (or leads) per second while scanning at 20 to 30 square inches per second,” Mr. Trozzi continued. “Video-system repeatability is in the range of 0.5 mil. Defect detection is in excess of 99% and false calls are in the range of 50 to 500 ppm, depending on the stability of the process and whether the inspection is on solder joints or more regular features such as pre-solder component location.”
High-speed 3-D laser scanners can cover a board at rates of one in.2/s to several square inches per second, according to David Clark, Director of Marketing at the SVS Products Group of View Engineering. Normal speeds for laser scanning technology are about 30 pads/s for fine-pitch quad flat pack (QFP) devices and up to 200 pads/s on ball grid array (BGA) sites.
“With all vision systems, there is always a trade-off between inspection speed, resolution and accuracy,” Mr. Clark explained. “Consequently, systems can change resolution during inspection to provide high-accuracy measurements for critical areas and then allow high-speed coverage for the rest of the board at lower resolution. In general, 3-D laser systems can inspect an entire PCB within the cycle time of the production line.”
About the advantages and shortcomings of laser and video techniques, Mr. Trozzi commented: “Perpendicular axis viewing is tolerant of warped boards and can discern low-profile components between tall components. Laser techniques can provide high-precision assessments but capture only a very small area with each sample, and measurement accuracy can be affected by board warpage.”
While automated inspection may be essential for high-volume or high-speed production operations, do not ignore human visual inspection. Some military and NASA contracts demand it and the acquisition cost of simple video systems or optical microscopes is substantially lower than that of automated systems.
Optical systems also provide very high resolution. The Metron 3-D Scanner, for instance, resolves 57 line pairs/mm at 4X magnification and 141 line pairs/mm at 10X magnification. The high intellect of the sensor, in this case a human eye and brain, also provides a definite advantage.
“Human visual inspection is particularly effective for high-density boards because human vision (from infancy) sees whole objects (Gestalts) and not just lines (as machine vision),” said Paul Kempf, President of Metron Optics. “Edge detection is also not a problem for human visual inspection.
“For high-density PCBs,” he continued, “Metron offers Circuit Board Comparators, which give a mirror-image comparison between a master (known correct) board and a new production board. Moving images are projected on a high-resolution screen and viewed at 4X, 7X or 11X magnification.
“Instead of using an actual master PCB, a photomaster can be used as a known-good standard,” Mr. Kempf continued. “The large number of different boards produced, especially by contract assemblers, can make it impractical to save a master board of each design. Photomasters showing both sides of the board are easily made and conveniently stored in minimum space.”
X-Ray for Solder Inspection
Solder bridges can usually be seen by all types of vision systems, but that is not necessarily so for solder voids or hidden solder connections, such as those under BGAs. In these cases, you must rely on the technique that can penetrate objects and make hidden views visible—the X-ray technique.
Three forms of X-ray implementations are being used for PCB inspection: a point X-ray source and detector plane consisting of an image intensifier or film; a large X-ray scanning source and point detector, an arrangement referred to as the reverse geometry X-ray technique;1 and a rotating arrangement such as that used in X-ray laminography.
The point-source X-ray implementation is used for most medical and engineering analysis applications today. There is a downside, though. It may be difficult to interpret images of objects that overlap each other in the X-Y plane but are located at different positions in Z, such as components on double-sided PCBs. Also, volumetric information is not readily available, since it is usually derived by inference from gray-scale interpretations.
However, point-source X-ray implementations are most useful for many ordinary PCB inspection applications. Some systems, such as the RTX Mini from Glenbrook Technologies, are small and light enough to be moved easily from one production line to another, according to Gil Zweig, President of the company.
To enhance the usefulness and application potential of X-ray systems, some have concurrent vision-inspection facilities. The Glenbrook Dual-VU System presents a 15X magnified view of the surface of a PCB on one monitor and an X-ray view of the same area on a second monitor.
X-Ray Laminography
While many of these techniques and systems are suited for specific inspection tasks, they do not necessarily encompass all the capabilities needed for evaluating the quality of the overall assembly process.
“For example, automated laser triangulation systems can effectively inspect solder-paste deposits prior to component placement, but they cannot inspect the quality of reflowed solder joints,” stated Steve Rooks, a consultant for Hewlett-Packard. “Likewise, automated optical inspection systems are ideal for identifying missing and skewed components, but they cannot properly assess the quality of fine-pitch QFP joints because the heel joint, the key to reliability, is hidden from their line of sight.
X-ray laminography can isolate and measure critical solder-joint features in cross-sectional X-ray images selectable at any height in the Z-plane. “By analyzing the solder-joint measurements taken by an X-ray laminography system, such as HP’s Four Pi 5DX system, you can characterize and improve every step of the entire assembly process,” said Mr. Rooks.
“For example, variations in average solder thickness or volume for solder joints across a single PCB or among several PCBs provide insight into the quality level of and the sources of defects in the paste-printing process. Similarly, variations in pin-to-pad offsets provide information about the component-placement process, and variations in heel-fillet heights and fillet lengths provide data on the solder-reflow process,” Mr. Rooks concluded.
Defect Detection and Avoidance
Before selecting the production stage(s) where inspection is most beneficial, answer these questions:
Which process is most likely to go out of control if it is not properly monitored?
Which inspection operation(s) provides the highest return on inspection equipment and labor expenditures, taking into account saved rework costs, scrappage avoidance, customer satisfaction, and limitation of potential liabilities?
The first question is usually easily answered from observation and experience. But a detailed analysis is required to answer the second one.
It is necessary to assess the diagnostic capabilities needed after each process step to detect defects and reduce rework. Then, the extent to which inspection equipment can identify these defects and provide timely feedback must be evaluated. A concurrent cost/benefit analysis will reveal at which stage(s) targeted inspection equipment will provide the best returns.
Regardless of the equipment and production steps selected, accurate deficiency reporting and pinpointing of defect locations to the exact solder joint or component are essential to reduce residual rework cost. In addition, a graphical representation of the board with defect locations highlighted can shorten repair times by 50%, commented Mr. Rooks. “And a paperless graphical reporting system eliminates the need to attach repair tickets to the board and eases the collection of defect and repair data for process control improvements.”
Since defect prevention is always preferred over defect correction, the capability of the inspection process to achieve a zero-defect goal is paramount. Monitoring process results in near real time, analyzing deviations and generating prompt alerts when established guardbands are reached all help to achieve control.
References
1. Albert, T. “Application of Reverse-Geometry X-Ray Technology to PCB Inspection,” Proceedings of the Technical Program, NEPCON West ’94, pp. 1,371-1,372.
These companies provided information for this feature:
Boeckeler Instruments (602) 573-7100
CMI International (800) 678-1117
Control Automation (609) 520-0333
FeinFocus USA (770) 717-0179
Glenbrook Technologies (201) 361-8866
Hewlett-Packard (800) 452-4844
Machine Vision Products (800) 260-4MVP
Metron Optics (619) 755-4477
Operations Technology (908) 362-6200
View Engineering (313) 665-1140
Copyright 1996 Nelson Publishing Inc.
March 1996