A Strategy for Quality: Testing in Contract Electronics ManufacturingA Strategy for Quality: Testing in Contract Electronics Manufacturing
Medical Device & Diagnostic Industry MagazineMDDI Article IndexOriginally Published August 2000The growing complexity of outsourced medical electronics assemblies poses challenges for increasing yields and reducing defects.Gregg Nighswonger
August 1, 2000
Originally Published August 2000
The use of contract electronics manufacturing services has become a widely accepted business strategy in the highly competitive medical device marketplace. Outsourced manufacturing remains among the fastest growing segments of the electronics industry overall. Current forecasts for overall growth of the industry range from 15 to 25% through 2003, with significantly greater increases predicted for some segments. Board building, for example, is expected to rise by as much as 70%, according to some estimates. Many contract manufacturers are continuing to strengthen their business positions by upgrading existing facilities and acquiring new plants (nearly 65% of all contract manufacturing growth has reportedly been related to acquisition of OEM manufacturing facilities), engaging in mergers, and expanding the types of services offered. Some are providing management of supply-chain functions, and others are handling certain aspects of packaging and order-fulfillment operations.
According to Technology Forecasters Inc. (Alameda, CA), medical electronics outsourcing now represents an estimated 4% of the total market for electronics outsourcing. Says Pamela Gordon, president of Technology Forecasters, "Manufacturing of small, high-volume products such as hearing aids and heart monitors has been outsourced for many years. Contrast this with the large, low-volume products such as medical laser equipment." She adds that, "At the start, the electronics contract manufacturer provided mainly the assembly of the electronics portion; more and more, these contractors are building the entire product or nearly so. Both ends of the spectrum have received more attention from electronics contract manufacturers as they aim to also provide premanufacturing (design, and supply-chain management) and post-manufacturing (test, distribution, repairs, and upgrades) services."
Gordon explains that contract manufacturers should carefully assess areas of focus. "We urge contract manufacturing executives to choose a manageable number of industry specialties, and to get all the certifications and specialist employees necessary to be expert in that field; the medical electronics field is especially demanding of focus and expertise owing to good manufacturing practices and other FDA requirements."
The reasons for this continued growth of medical electronics outsourcing are simple enough. Outsourcing offers significant advantages to medical device OEMs, including reductions in capital risks, increased asset productivity, increased access to current technology, the ability to focus on core capabilities (including design, R&D, product development, and marketing), and reductions in time to market. The role of contract electronics manufacturers (CEMs) has also expanded significantly as OEMs have restructured in efforts to control costs and optimize productivity. CEMs appear to be emerging from their more traditional production role to offer increasingly comprehensive services, including design (see below). By working more closely with contract manufacturers, OEMs can address production issues more effectively and at an earlier stage.
Growth in contract manufacturing is also increasing in response to the emergence of "virtual manufacturers"—firms that excel in product design and development. Increased global and domestic competition are additional factors. As competition escalates and the need for appropriate production capabilities also increases, outsourcing offers one way of maintaining the necessary access to manufacturing technology.
FOUNDATION FOR A STRATEGIC RELATIONSHIP
The relationship between medical device OEMs and CEMs has been described as being more like working with a partner than with a vendor. In addition to meeting fundamental production needs, a contract manufacturer must also provide the foundation the OEM will rely on to expand its business—something seldom required of most vendors.
An essential element in the outsourcing strategy is the need to maintain adequate quality levels in outsourced products. Effective test strategies need to be developed to ensure that manufacturing quality and reliability can be maintained by the CEM while meeting time-to-market requirements and increasing product yields. Additionally, a CEM's claims of quality should be backed by adherence to appropriate certifiable standards and approvals, such as FDA's quality system regulation and ISO 9000 standards. Compliance with recognized standards serves to ensure that the OEM can use these standards for quality assurance. Furthermore, a CEM's adherence to recognized standards suggests that it has quality measures in place to ensure overall operational quality.
Schematic capture, flex circuitry, flip-chip (shown above), and surface-mount technologies are among the services offered by contract electronics manufacturers.
Because there is no single inspection or testing system that will meet the needs of every manufacturing environment, a number of factors must be considered in adopting any given strategy. Among these are product design and testability, availability of equipment for testing, and the manufacturing process being used. Test systems might include built-in self-test firmware, automated optical inspection systems, in-circuit testing, x-ray testing, and functional test and environmental stress screening.
PREDICTING QUALITY PERFORMANCE
The ability of OEMs to effectively compare the quality levels of different contract manufacturers also poses greater challenges as electronic assemblies become more complex. Although the fundamental goals continue to be higher yields with lower defect rates and reduced costs, greater emphasis is being placed on standardizing quality measurement. The difficulty lies in standardizing quality assessment across assemblies and components of differing levels of complexity. Obviously, as the complexity levels of various electronic assemblies increase, defect rates are also likely to increase while yield rates decrease. New measurement protocols are being developed, however, that will resolve these issues and provide a basis for predicting quality performance.
"With OEMs outsourcing more and more of their manufacturing, they want to be able to objectively measure the quality levels they are receiving from their printed circuit board (PCB) assembly suppliers," said Brian Coll during the Apex 2000 meeting in Long Beach, CA. "At in-circuit or functional test, the first-pass yield is a traditional metric but is not a good indicator of process quality." He indicated that defects per unit (DPU) and defects per million opportunities (DPMO) are among the metrics now being used to provide a more standardized basis of quality measurement.
DPU is a measure of the average number of defects on each PCB. To allow valid comparisons between assemblies with differing complexity levels, DPMO uses a normalizing factor, called the opportunities for defects (OFDs). Coll suggested that using a standard OFD denominator for all products being manufactured allows a true comparison to be made among contract manufacturers.
COMPUTER-ASSISTED DESIGN OF PCBs IS ESSENTIAL TO SUPPLY-CHAIN SOLUTIONS
To remain competitive, CEMs must be able to offer medical manufacturers a total supply-chain solution that includes design, product and system assembly, manufacture, and support services. In view of this, many CEM providers have added PCB layout and design to their range of services. Development of such capabilities as schematic capture, surface-mount technology (SMT), through-hole, flip chip, flex circuitry, multiple layers with blind vias, and double-sided multiple-layer surface mount is, of course, fueled by the never-ending drive toward smaller, smarter medical device components. For example, flip-chip technology provides advantages in meeting the requirements of high-performance chips. The technology involves attaching silicon chips directly to PCBs without requiring a wire bonding process. Flip-chip soldering uses a solder bump at each die pad location to complete the electrical and mechanical connection between the die and the substrate. The advantage of this technology lies in the fact that the flip-chip connections are arranged over the entire area of the die. This allows the I/Os to be arranged over the entire area of the substrate—enabling significantly higher densities.
Taking PCB design in-house allows CEM providers to provide turnkey services and deliver products much faster, as well as to pass along savings in redesign to medical manufacturers. It also enables OEMs to take advantage of their CEM's engineering and production expertise, and to control their product's cost drivers, speed up their product's overall development cycle, and shorten its time-to-market.
INDUSTRY'S SHIFT TOWARD OUTSOURCING
For many OEMs, the shift to outsourcing has become an essential part of their strategic business plan. Although many CEM providers are taking the lead in developing turnkey manufacturing processes to support advances in component and process technology, industry-wide standardization and cooperation (like acceptance of the 2.4-GHz universal telecom frequency) will become essential in developing platforms for emerging technologies, packages, substrates, components, attachments, and test processes.
This strong PCB design and assembly outsourcing trend has spurred an increase in the capacity and infrastructure capabilities of many CEM providers. In order to cost-effectively integrate and maintain ball-grid array (BGA), microBGA, chip-scale package (CSP), and other high-density, small-form-factor technologies into their assembly lines, CEM providers are using aggressive PCB design to their advantage. Among the benefits are rapid turnaround, enhanced thermal management, and the incorporation of power and ground planes within the package substrate, allowing a reduction of layers within the semiconductor device.
EVOLUTION OF THE PCB DESIGN PROCESS
How does integrated PCB development work? OEMs come to CEM providers for design ideas regarding PCB layout in addition to product assembly, test, and manufacturing services. PCB layout is, in one sense, a necessary evil in going from raw engineering design to the product's final production.
PCB design has evolved from a non-CAD, hand-laid-out, and photo-negative process to become a fully computerized, e-mail– and modem-based operation. The schematic is now sent out by e-mail, the optimized PCB design is often returned within five to seven days, and production begins. So-called breadboarding in the development cycle is seldom a consideration.
Although CAD systems can now automatically analyze signals and lay out PCBs accordingly, CAD is not necessarily the final word in layout design. The human element is still important— especially considering that PCBs used in medical devices are now very dense and expected to become denser still.
Some factors that may require human knowledge and intervention include significantly smaller boards, more complex hookups, ultrafine-pitch circuitry, capacitance relationships, crosstalk between lines, the desirability of multiple layers, the resolution of mechanical and electronic compatibility problems, and EMC/EMI—which must be considered in the initial design. The desirability of flex circuitry should be assessed, as well as any advantages that might be implicit in the design of a PCB with components on a flexible base.
In an environment of shrinking footprints and increased I/O per square inch, and ever smaller array package sizes, PCB design can be problematic—even with highly computerized design equipment. In light of all the interdependent relationships, processes, and possible pitfalls, the value to OEMs of a turnkey service that includes optimized PCB design is evident.
WHAT THE FUTURE HOLDS
What can be surmised about the future direction or growth of outsourcing? What are the trends? Employing CAD systems in the design of PCBs is certain to continue to reduce turnaround time in the manufacturing cycle. With this fast turnaround, microscale linewidths, and the high frequencies, engineers are bypassing breadboarding. The trend toward multilayer board design has been advanced by the move to smaller, more compact boards, which provide better ground planes for reducing emissions and enhancing EMC/EMI control. Multilayer boards of up to 18 layers are now regularly produced, with each layer being only 1 mil thick. In fact, for such PCBs, the epoxy substrate is really unnecessary and is added only to make the board rigid.
Of course, every development is influenced by cost. Nearly all PCB designs are now produced using CAD, which tends to speed up everything in the development cycle. Using this technique, however, requires additional cost considerations. In spite of cost, growth in storage media and memory systems is bound to continue. Miniaturization and PCB density increases are also expected to continue.
As soon as limits appear to have been reached, new technologies are certain to spring up that will continue these trends. Perhaps in a few short years, a new development will make the use of PCBs in medical devices obsolete altogether.
Luke C. Kensen is director of business development at Express Manufacturing Inc. (Santa Ana, CA).
"In-process defect levels can be measured using DPU for an individual process, such as surface-mount technology or wave solder, whereas DPMO is fast being recognized as the industry standard," Coll stated. "It allows one to compare quality levels among products of varying complexity." He adds that it can also be used as a basis for rating CEMs and for benchmarking against international standards. Coll emphasized that, beyond reactive use as a measurement tool, data generated using DPU and DPMO methods can be applied as a predictive tool to develop statistical models to forecast yields on new products. "This has enabled manufacturers to better understand processes and subsequently drive improvement both at the design and manufacturing levels," Coll noted.
Given an understanding of the DPMO of a given manufacturing process, according to Coll, OFDs can be calculated during the design stage, allowing product yields and defect levels to be predicted prior to beginning the manufacturing process. Essentially, the process provides a basis for improving production quality and a quality performance benchmark.
AOI SYSTEMS IMPROVE MEDICAL DEVICE COMPONENT YIELDS
In recent years, yield improvement and rework systems have emerged as areas of significant interest to manufacturers of PCBs used in medical devices. This interest is largely motivated by the fact that an immense volume of process and test data is generated by a typical PCB manufacturer during test operations. The volume of data generated continues to grow as automated inspection instrumentation improves. In addition, with the advent of improved defect or failure analysis tools, data accuracy is also improving. Rapid defect identification and analysis can be advantageous because it can reduce the response time, rework, and costs caused by defects in manufacturing. As a result, manufacturing engineers and quality control personnel are constantly challenged by the need to rapidly collect and analyze any new data and use it to improve yield rates of medical device components.
One of the primary tools used by contract PCB and electronics manufacturers is automated optical inspection (AOI). When the efficiency and repeatability of AOI systems are compared with those of human inspectors, the appeal of the automatic systems can be better understood. For example, a typical PCB assembly line at an electronics contract manufacturer employs between two and four inspectors for an inspection and rework operation. In contrast, an AOI system requires only one operator to select the programs, detect defects, and perform rework on failed PCBs—reducing a firm's per-shift requirement for labor by a significant degree. Before a manufacturer makes an investment in automation, however, a realistic evaluation of all the factors that influence yield improvement and return-on-investment (ROI) should be conducted.
AOI systems were first introduced to the PCB assembly industry in the early 1980s. Designed to replace human inspectors in the task of inspecting PCBs for visible defects such as missing parts and placement errors, these first systems were expensive, slow, and difficult to program.
Recent improvements in PC processing power, software, and imaging technology have enabled new generations of AOI systems to overcome these limitations. These systems can be utilized in various modes at several points along an assembly line. In a typical surface-mount technology (SMT) scenario, inspection is performed after parts placement, reflow, or final assembly.
The types of defects found by an AOI system include missing parts, incorrect components, polarization errors, placement errors, and solder defects. All this defect information is identified and reported for rework. There are definite system limitations, however. Although AOI systems do an excellent job on optical inspection of the board surface, they can only analyze visible features. With ball-grid arrays (BGAs), micro-BGAs, chip-scale packages (CSPs), flip-chips, and other hidden-connection devices, manufacturers have to opt for x-ray systems to analyze the critical solder joints of these new-generation packages. Also, double-sided PCBs must be "flipped" for a proper inspection of both sides.
Today, AOI systems are typically used to complement in-circuit test (ICT) methods, thus providing an inspection spectrum wider than either process alone. In addition, increasing numbers of engineers in mid- to high-volume manufacturing environments employ AOI postreflow as an SMT process-inspection tool. The AOI system provides engineering analysis data that can be used to perform a variety of analytical tasks such as sourcing of components, facilitating product traceability, increasing test yields, boosting mean-time-before-failure, decreasing the number of "escapes" to the field, and improving process control. In addition, trend analysis is a time-tested method of monitoring for changes (both good and bad) in a manufacturing line. If a desirable trend is found, the AOI system can help the user pinpoint the changes that resulted in the improvement. Alternatively, if an undesirable trend is found, the AOI system can assist the user in determining what went wrong.
As with any good business decision, the investment in automation should reduce the cost per function and improve the current yield. A major improvement, obviously, is a reduction in labor. According to research conducted by Teradyne (Boston, MA), trained inspectors will identify only an estimated 50% of the detectable visible defects on circuit boards. This low effectiveness percentage is due to several factors, primarily the repetitive and demanding nature of the work, which makes concentration difficult to maintain. The monotony also results in a high staff turnover, with consequent costs in hiring and training personnel. Inspection may also be influenced by such factors as the attention span of the particular inspector, the specific day (inspection may be less effective on Friday afternoon than on Monday morning), or the time of day (inspectors may be less effective in the afternoon than in the morning). In contrast, AOI, as a machine automation process, is able to deliver high defect coverage consistently and repeatably (often as high as 99%), and allows virtually no escapes to the next production stage.
REWORK DATA SERVER
A new rework data server (RDS) being developed is expected to dramatically increase the effectiveness of AOI systems. With the RDS, defect data gathered by the inspection systems will be channeled via open database connectivity to a central server for storage. The Web-based software tool will then generate a series of Web pages that will guide repair technicians through the rework processes on the defective PCBs. Computers anywhere on the LAN will be able to view the defect data using a Web browser.
The RDS will provide defect and yield information in the form of board statistics, master tables, defect maps, and defect classifications. Defect information will be listed in table format while defect locations are indicated graphically on a user-friendly map. After performing rework, technicians will be able to log the date, time, and reworker's name. In addition, the system will provide defect statistics for multiple assembly lines. Because it is Web-based, the RDS will present data across the company's intranet, or users may choose to have the data available on the Internet for an audience anywhere in the world.
FEWER PCBs TO DIAGNOSE, REPAIR, AND RETEST
The uneven performance of human inspectors was an ongoing concern for SMS Manufacturing Technologies Inc., a contract electronics manufacturer in San Diego. "We introduced AOI systems on our SMT assembly lines because there were simply too many defects getting through," says Allan Stein, director of manufacturing. "We even find AOI effective on double-sided boards. We look at one side, then flip the board and check the other side," he adds. "Now, by using both AOI and human inspectors, we've improved both our quality and our yield."
The higher yield at ICT as a result of improved AOI coverage means fewer PCBs to diagnose, repair, and retest. In some manufacturing operations, this improvement in ICT yield has been so dramatic that the manufacturer has eliminated ICT altogether—with a consequent savings in labor, capital, and floor space.
In the case of functional board tests, the savings can be even more dramatic if one considers the benefits of AOI. These savings can be brought about by shortening test times, reducing the number of failed PCBs requiring diagnosis, decreasing the use (and cost) of skilled technicians, and virtually eliminating "fatal" defects that require boards to be scrapped.
QUALITY AND YIELD
Many visible defects are not detectable by either ICT or functional board tests. Although many of these defects can be detected by human inspectors and AOI, operators typically miss 50% of the defects. The result is that these undetectable defects pass through all the inspection and test stages, but can cause various problems such as the following:
AOI can reduce the incidence of these failures, which should help manufacturers meet their customers' requirements for increasing product quality. For example, a dimensional aberration of a single part could conceivably shut down an assembly line until the problem was discovered and rectified. AOI should eliminate this type of problem.
Improving product quality usually increases product yield, ensuring that key elements for sustaining long-run costs remain low. The way to meet cost pressures is to consistently improve process yields. This falls into two categories:
Visual/electrical correlation is an important yield-improvement and failure-analysis tool. The ability to correlate visual defects observed during processing with electrical test failures can be a powerful aid in reducing failure-analysis time and improving failure analysis efficiency, thus increasing the yield.
THE NEED FOR REAL-TIME MONITORING
Real-time monitoring is another aspect of improving product yield. It can alert the line manager as soon as a process upset occurs, and can help rectify catastrophic occurrences, such as selection of the incorrect placement program. Data can be collected, product quality analyzed, and repair procedures initiated at critical steps in the manufacturing process. Finding problems quickly can prevent the manufacture of large numbers of defective boards.
Long-term process improvement is a concept that requires input from as many sources as possible. And a system where defect data are automatically collected, maintained, and monitored is an ideal starting point for improving both product quality and yield. A well-managed yield improvement system can be a cost-effective means for achieving and maintaining high production yields of PCBs for medical device applications.
Don S. Miller is vice president of sales at CR Technology (Aliso Viejo, CA).
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