Get the Most From AOI
A growing number of electronics assemblers are integrating automated optical inspection (AOI) into their lines, particularly as board density increases and component size decreases. But, it’s far from a ubiquitous technology.
Even among those who have adopted AOI, there’s debate over how best to use the technology. Some experts suggest that AOI should be used early in the assembly process—that is, before soldering—because that’s when most defects occur. It’s easier and less expensive to fix defects when they’re found early. And, AOI can provide real-time feedback on machine performance so that operators can make adjustments and prevent faulty boards from being produced.
Others argue that this expensive technology is best used after soldering, when board value is highest. When placed at the end of the line, AOI has more opportunity to catch every possible defect.
Boris Mathiszik, director of marketing and business development at Machine Vision Products Inc. (Carlsbad, CA), says the best place to use AOI depends on the manufacturer and its goals. Manufacturers with high-mix lines tend to use AOI after reflow to ensure that no defect goes undetected. In contrast, manufacturers with dedicated, high-volume lines tend to use AOI after printing or part placement, as a way of monitoring their finely tuned processes.
Paste inspection is a good application for AOI. Paste printing accounts for approximately 10 percent of a PCB’s value but 60 percent of its defects. AOI after paste printing can measure the height, shape and volume of paste deposits. It can detect missing, excessive, inadequate or offset fillets.
For the same reasons, AOI can also be used to inspect deposits of surface-mount adhesive. At least one company—Smart Sonic Corp. (Van Nuys, CA)—supplies AOI equipment dedicated to that application.
A more common spot for AOI is after component placement but before soldering. At this stage, AOI ensures that the right component has been placed in the right spot, in the correct polarity and with the correct side face up. AOI can find components that are missing, rotated, translated, misplaced or "billboarded." It also detects shorts, opens and lifted leads.
If the line has more than one placement machine, many manufacturers prefer to use AOI after the high-speed machine, but before the high-precision machine. In this way, AOI covers most of the components on the board, as well as the paste deposits for fine-pitch components and ball grid arrays (BGAs). Inspection after fine-pitch placement or odd-form assembly still detects missing or misplaced components, but it also finds broken, bent or lifted leads.
"I like to have the AOI machine right after placement, because this will tell you if the pick-and-place machine has misplaced a component or put down the wrong component," says Cal Driscoll, applications product manager for Manncorp (Huntingdon Valley, PA). "In some cases, you can also tell if there’s a minimal amount of solder on the pads. Once the board goes through reflow, all bets are off, because the components center themselves and you can’t get a good idea of solder volume."
Besides any defects caused by upstream processes, inspection after reflow can find dry joints and shorts. It also finds shifted or "tombstoned" components.
"We promote the use of AOI after the soldering process," says James F. Fishburn, sales manager for inspection systems at Omron Electronics Inc. (Schaumburg, IL). "Even if you use AOI after paste application or chip mounting, you can still have additional problems after soldering. By putting AOI at the end of the line, you find any soldering defects, and you still capture faults that were created by those earlier processes."
Though post-reflow AOI will catch the most defects, the trade-off is the inability to get timely information on line efficiency. "A key justification of AOI is to find defects early and respond in real time," says Mathiszik. "If you’ve installed the wrong reel of components, the AOI will detect that and notify the operator so you don’t produce hundreds of bad boards."
Another trade-off is higher rework costs. "It’s easier to fix a defect if you find it before reflow. For example, you might simply be able to bump a misplaced component with a pair of tweezers," says Driscoll. "Once it’s reflowed, the board has to be reworked with a soldering iron. That’s labor-intensive, and it introduces more heat to the board."
In the end, assemblers might get the most value from AOI by using it as close as possible to their biggest source of defects, advises David Clark, product manager for AOI with GSI Lumonics Inc. (Northville, MI). "If most of your defects are related to solder paste, you should use AOI after the screen printer," he says. "If most of your defects are component-related—and a lot of people are seeing that now with small components like 0402s—you should use AOI after the pick-and-place machine."
And if more than one process is a major source of defects? Experts say it’s rarely, if ever, necessary to install more than one AOI machine in the line. "It depends on the complexity of the board," says Thorsten Niermeyer, AOI product manager with Agilent Technologies Inc. (Loveland, CO). "If you’re building boards for high-reliability products, such as air bags, antilock brake systems and aviation applications, you might want to inspect them at multiple steps."
Inspection OptionsThe good news for electronics assemblers is that most AOI equipment is flexible enough to be used anywhere on the line. The programming may change subtly depending on where the machine is used, but the hardware usually stays the same.
This is not to say, however, that all AOI equipment is the same, or that there aren’t special features available to solve particular problems.
One such feature is optical character recognition (OCR). With OCR, an AOI machine can read the markings on a component and convert them into a string. The machine then compares this string with one or several user-defined strings assigned to that part. If the string matches, the system knows that it has the correct part.
Color imaging is another option for AOI. Color systems produce higher contrast images than gray-scale systems. "A gray-scale system may have difficulty identifying a green component on a green substrate," says Fishburn.
Color is often necessary to check the polarity of components or to identify specific parts, such as connectors. Some components, such as tantalum capacitors, come in different colors depending on the supplier. "Without color sensitivity, some AOI systems would treat a yellow capacitor as absent and a black one as present," says Driscoll.
Assemblers that are particularly concerned with inspecting solder fillets should consider an AOI system with 3D imaging. Such images can be captured two ways. Color AOI systems get 3D information from a 2D image by illuminating the board from three different angles with red, green and blue light.
The other way to get information about the board in 3D is with a scanning laser. When the laser scans the board, the light is reflected onto two position-sensitive detectors. Changes in height are measured as changes in the location of the reflected beam on the detectors. Using triangulation, the system combines position data from the two detectors to produce a single height data point. The system then maps millions of height measurements into arrays, creating 3D images.
"The amount of solder paste on a pad determines the quality of the joint, and amounts can vary with the performance of the squeegee or the viscosity of the paste," says Clark. "However, you can’t always tell how much paste is on a pad just by looking down at the deposit with a camera."
With 3D information, an AOI machine can ensure that through-hole components are fully seated. It can check the height of pins and even the tightness of screws. In addition, height information can often obviate the need for X-ray inspection of BGAs, flip chips and other components with hidden solder joints.
"A BGA should sit planar to the board," explains Driscoll. "If there’s no solder paste on one side of the BGA or there are balls missing, it’s going to sit differently on the board. If you take a laser measurement of the four corners of the BGA, they should be equal in height."
Choosing the Right MachineWith more than 25 companies offering AOI equipment, choosing the best machine for a particular line necessitates some homework. Will the machine be used in-line or off? Will it be used early in the process or late? Some machines excel at inspecting solder paste; others are better at examining components.
First, says Mathiszik, manufacturers should identify their most common defects, then look for the best machine for detecting those defects. "If you have a lot of problems with insufficient solder, lifted leads or hairline bridging on [quad flat packs], a simple machine that compares captured images to a ‘golden board’ is not going to solve your problem," he says. "If you’re concerned about placing the wrong components, you may need a machine with OCR ability.
"A lot of companies get hung up on detecting obscure defects that rarely happen."
The next step is to consider the machine’s performance. Does the machine let too many defects slip through? Does it make too many false calls? Will it keep pace with the rest of the line? Does it provide statistical process control? Is it easy to program? This last question is particularly important in high-mix environments.
Solectron Malaysia (Penang) has developed a unique method for evaluating AOI equipment. First, it sends questionnaires to several AOI suppliers. The questionnaires cover such issues as cycle time, loading time, delivery, warranty, machine size and cost. Based on the survey results, the contract manufacturer selects four finalists to test for false reject rates, false accept rates, repeatability, inspection time and programming time.
To test a machine, Solectron uses three different batches of assemblies. Each batch includes PCBs with defects, to determine the machine’s false accept rate, and good PCBs, to measure the machine’s false reject rate. The machine with the best performance gets the nod.