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Integrating AOI systems at key locations in the assembly line has proven to be a cost-effective solution for improving yields.
By Mark Norris
The days of inspection by the naked eye are over. The demand for higher throughput, combined with higher board densities, complex mixed-technology boards and smaller devices, have all but eliminated individual inspectors on the assembly line. Despite advances in the performance capabilities of process equipment, the need to ensure board quality is as essential as ever. With systems stressed to maintain speed and precision while handling finer pitched components, defects are a fact of life during paste deposition, component placement and reflow.
Unfortunately, waiting for in-circuit test (ICT), functional test or end-of-line (EOL) inspection is often too time consuming for programming and analysis, too difficult to ensure traceability of problems and too late for cost-effective rework. ICT during assembly is also becoming less practical because of the need for fixturing, difficulty in accessing contact points and the inefficiencies of boards being taken off-line. The answer is 100 percent inspection of boards at critical locations in the assembly process.
Over the past few years, in the effort to overcome the limitations of manual inspection, various types of automated and semi-automated equipment X-ray inspection, laser scanning, automated optical inspection (AOI) and X-ray/AOI hybrids have been developed for printed circuit board (PCB) assembly. Transmission (2-D) and cross-sectional (3-D) X-ray systems offer the advantage of being able to "see" the solder bump interconnects on the underside of area-array components. They are also useful for checking components that are enclosed for protection by radio frequency (RF) shielding, such as components found in mobile communications products. While X-ray machines are available for in-line applications, such use is limited to applications where throughput is of no concern, because the major drawback of the technology is the time required to perform 100 percent inspection of solder joints. As a result, X-ray systems are most often installed off-line for random or batch inspection of boards and for examination where defects have been noted by optical inspection or electrical testing.
Laser scanning employs a beam of highly energized photons that sweeps across the board being viewed, creating a series of images. Using imaging technology, computer-aided design (CAD) data for the board and measurement routines, the height and volume of solder paste deposits can be determined. Laser scanning has some applications for component placement, such as detection of "tombstoned" chips and errors of coplanarity for quad flat pack (QFP) components. In these cases, the laser can be operated at line speed. However, laser inspection is reserved almost exclusively for solder paste print inspection due to the complexity of programming and the high number of false errors.
AOIOnly one type of inspection technology AOI permits an assembly line to operate at its maximum throughput speed, while inspecting paste deposits, component placement/nomenclature and the condition of reflowed solder joints. Insufficient, excessive or inaccurate deposits; missing or misaligned components; improper components or component polarity; bent leads; incomplete solder joints; and solder shorts can all be determined at inspection speeds of more than 100,000 components per hour. What this means is that in-line AOI systems can perform 100 percent inspection of boards while keeping up with the fastest chipshooters on the market.
Figure 1. AOI inspection systems permit PCB assembly to operate at maximum throughput while maintaining board quality.
The particular AOI system shown in Figure 1 employs image analysis software and leading-edge vision technology for measuring components, confirming their value and polarity, and ensuring placement accuracy. Featuring Windows NT 4.0, it can be programmed using standard CAD data or Gerber files. The system incorporates statistical process control (SPC) software tools and a synthetic component library, against which components are inspected, measured and analyzed. The system can be network-linked to repair stations for data transmission and to peripheral equipment for remote access programming and control.For optimum contrast and image definition, different types of programmable lighting can be selected. The operator is able to designate either axial or radial light sources in combinations of orange and green that result in almost infinite variations of color and intensity to suit the needs of the specific application. The smallest markings can be read, even the component values of both leaded and area-array devices, using special optical character verification software.
Figure 2. A fully integrated, post-inspection repair station allows operators the flexibility to fix errors in process or to buffer defective boards off-line for later repair.
Where and When To InspectAs shown in Figure 2, AOI inspection systems can be inserted in PCB assembly lines immediately after key processes. One critical point is after the stencil printer has deposited the paste on the board studies have shown that up to 70 percent of the defects in assembly can be attributed to this process. Not only can the stencils be misaligned, there can be too much or too little paste deposited, leading to bridging, shorts and tombstoning during component mounting. Occasionally, components cannot be mounted at all, simply because there is no paste on the pads; this is caused by clogged apertures or because the stencil pencil is low on paste. Although the source of the problems is the printer, defects such as wetting problems and fillet forming become noticeable after reflow.Another key location where most AOI inspection systems are located is after the chipshooter. Here the system can be used to inspect not only the correct placement, polarity and value of the chips placed, but also critical paste deposits for ball grid array (BGA), QFP and even flip chip components. Sometimes devices can fall off the placement nozzle and bounce around the board, coming to rest on other pads where there is paste, or not adhere to the board after placement because of inadequate paste deposits. Missing components can also be the result of feeders in the placement machine being jammed or out of components.
Consistent drifts in the placement of components, noted by the inspection system and provided as data in a feedback loop, can signify the need to calibrate and adjust the chipshooter, allowing timely and preventive maintenance.
Another key location for in-process inspection equipment is after the fine-pitch placement machine. Here, boards are checked for missing or misaligned components and bent leads, to ensure that components are placed on pasted pads, and for a myriad of other problems that can occur with the mounting of complicated devices. AOI can also identify improper polarity and incorrect component values and type. This type of error may result in expensive rework because of the high cost of fine-pitch components.
Post-reflow (EOL) is also a point where inspection of solder joints should occur to ensure the quality of the reflow process itself. Inspection for short circuits and dry solder joints can be carried out very effectively at relatively low cost and with high throughput. Waiting until this point, however, to inspect for errors that occurred upstream, i.e., during stencil printing or placement of components, is too late for such defects to be dealt with in a cost-effective manner. Another limitation of EOL is that tracing the process problem is more difficult. A tombstone found after reflow may be the result of insufficient solder paste during stencil printing, incorrect placement or a nonsuitable reflow profile.
Where should inspection stations be integrated? Ideally, at each of the locations mentioned. Multiple installations are the only way to achieve "full in-process" inspection. In some cases, cost-effectiveness and return on investment may not allow such wide implementation of AOI. In these cases, many companies are choosing post-printing and pre-reflow positions. In situations where only one AOI machine is to be implemented in the line, the ideal position is after the chipshooter to inspect both components and fine-pitch paste deposits.
Finding and reworking process errors prior to reflow is significantly more cost-effective than repairing components and pads after reflow. A fully integrated, post-inspection repair station, either in-line or used in batch mode off-line, is a necessity for today's high-speed production lines. This rework system should have the added advantages of showing the repair operator the exact error with a video image as well as the exact location of this error on the PCB. Graphical representations of errors or line drawings are not sufficient.
The Dollars and Sense of Multiple Inspection StationsPutting inspection equipment at key locations in an assembly line will certainly improve the assembly process, but is it practical? Does the investment make sense? Will the savings in scrapped boards and the reduction in rework costs offset the amortized capital expense of the added machines? Will production schedules be easily met and will improvement in assembly line quality be measurable? Such questions can only be answered by a detailed analysis of existing line conditions, including defect rates, scheduling requirements, types of boards being processed, etc. But in most instances, full in-process inspection is proving to be cost-effective and a significant benefit to both OEMs and contract manufacturers.
Important performance factors for AOI systems are:
- The system must not become a bottleneck and slow down the line, which means the equipment must be able to keep up with the fastest placement machine in the line
- It must have a small footprint, so it can be installed after the stencil printer and fit in front of the carriage feeder of the chipshooter
- Equipment must be easily programmed using an existing component library and CAD download
- It should have rework capability either in-line or off-line with real-time defect data transmission
- The system must have real-time SPC and networking capabilities.
With the aforementioned performance factors, inspection equipment becomes an integral, unobtrusive element of the assembly line, with the final result being PCBs with consistently higher quality and reduced production costs without sacrificing throughput.
ConclusionWith the challenges facing PCB manufacturers in terms of higher board densities and reduced component size, coupled with the need to maintain throughput, board quality remains an important issue. To minimize rejects and ensure the quality of PCBs, 100 percent or full in-process inspection of boards is necessary. With the ability to keep up with the fastest placement machines in the line and to provide precise inspection at each process point, AOI inspection systems are a cost-effective solution for reducing rework. At the same time, they optimize quality, help meet delivery schedules and monitor assembly line performance.
In-process vs. End-of-Line
Defects can occur anywhere in the line during stencil printing, placement of components, pre- or post-reflow. EOL is too late in the process, forcing manufacturers to rework defects without giving them the advantage of tracing the defect back to its origin.
After the Stencil PrinterSystems installed after the stencil printer primarily check pads for proper deposition and for missing, excessive or offset paste.
After the ChipshooterSystems installed after the chipshooter inspect not only the correct placement, polarity and value of chips placed, but also critical paste deposits for BGA, QFP and even flip chip components. AOI systems installed at this point must keep pace with the fastest chipshooters on the market, upward of 100,000 components per hour.
After the Fine-pitch PlacerSystems installed at this point check for proper component placement and orientation, as well as for bent, twisted or missing leads.
Post-reflowEven though EOL inspection alone is not good enough, inspecting after reflow is an important step during in-process inspection. Installed at this point, systems check for solder joint integrity, i.e. dry joints and shorts, and ensure against excessive component shifting or tombstoning.
MARK NORRIS is the CEO of Vision Inspection Technology LLC, 179 Ward Hill Ave., Haverhill, MA 01835; (978) 372-1230; Fax: (978) 372-1767;