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Medical Device & Diagnostic Industry Magazine
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Originally Published August 2000

A Strategy for Quality: Testing in Contract Electronics Manufacturing

The growing complexity of outsourced medical electronics assemblies poses challenges for increasing yields and reducing defects.

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.

"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.

IMPLEMENTING AN INSPECTION AND TESTING SYSTEM

As the CEM industry grows, electronics design and packaging are undergoing a rapid series of transformations in terms of both assembly size and complexity. Throughout this process, the need to generate usable information remains a key element in maintaining the quality of outsourced products. Product quality that cannot be adequately monitored or measured is difficult, if not impossible, to maintain. Although quality systems are designed to generate an abundance of quality data, converting this raw data into useful information to guide corrective action can provide a challenge. CEMs striving to shift from being data poor to data rich are relying increasingly on software-based systems that can satisfy information needs—from measuring yield to monitoring production and quality. In addition, systems used by CEMs to measure quality levels ideally give OEM customers real-time access to information that can be considered in implementing timely corrective action. Otherwise, the first indication of a manufacturing problem is likely to be an increase in the quantity of returned product.

Tom Lyons of Manufacturers' Services Ltd. (MSL; Athlone, Ireland) has described the use of a software-based system for ensuring the quality of CEM-manufactured PCB products. The goal of the project was to create a single system for storing and analyzing data, and ultimately to achieve a paperless test area.

The basis of this software system is a set of tools that enables connections to be made with in-circuit and functional test equipment using network locations. The system also allows output from a com port, or the interception of messages sent to a printer. Transmitted data usually consist of failure information or successful test data. The system also includes a test, analysis, and repair subsystem for in-circuit and functional test areas. When error data are automatically entered from the test equipment, test technicians perform the analysis of the PCB and enter the defect causing the error. If the board then passes the test, the analysis becomes a verified defect, which is stored within the system and is subsequently available to technicians and quality analysts.

Education and training were important aspects of implementing the software system, according to Lyons. Operators were required to learn a new list of defect codes that were needed for rapid defect entry. The operators also had to adjust to using a mouse and keyboard to enter data. Training was provided to ensure that correct data were being sent and received.

With tracking as a primary goal, Lyons explains that "the system records serial numbers. Once each product has a unique serial number, it can be traced through the process." Proper label design and placement was also a key element in implementing the software-based system. Each PCB produced at the MSL plant now has a bar code that is scanned at each step of the manufacturing process. Typical labeling also includes the product name, product revision, and serial numbers, and the bar code serial number. All process documents were altered to include information regarding the content and placement of the PCB labeling. Because some OEM customers specify bar code content and others do not, the company created a standard label format to reduce setup time during the label generation process. Modifications were needed only in cases where a customer required a new label format.

Increasingly, OEMs are turning to CEMs for design ideas regarding PCB layout.

According to Lyons, the system "generated vast quantities of data on the system database." The software system was capable of transforming compiled data into useful information by generating customized reports in real time, yielding feedback that can be used to identify and correct process problems more quickly. "The test is a solid method that has withstood every test thrown at it to date. To be 100% sure that no assemblies are missed, one can develop an application to verify that the serial number has been entered into the correct table," Lyons explains. He adds that Microsoft Access offers a good basis for developing such an application.

Subsequent implementation of plans to make the collected data available via an extranet system will allow OEM customers to access real-time information about their products.

TECHNOLOGY DRIVES THE NEED FOR BETTER TESTING METHODS

The advances in fundamental device technology, such as increased complexity and density of electronic assemblies, are also posing new and more difficult challenges for quality inspection and testing strategies. Among the recent challenges are microvia technology that reduced the number of vias—holes or feed-throughs in the laminate between layers of circuitry that are used for electrical connections or thermal dissipation—that can be used as testpads, radio-frequency boards with ground or power planes that preclude the use of test pads, increased test complexity, component miniaturization, and the introduction of flip-chip and similar technologies that are being combined with microvia.

A number of new methods and strategies are being developed to accommodate inspection and testing of denser assemblies, including automated optical inspection (AOI), flying-probe testing, and others. Although these new methods are not intended to replace conventional visual and functional testing strategies entirely, they are proving to be useful alternatives and adjuncts to strategies intended to ensure device quality.

Automated Visual Inspection. The basic function of an AOI system is to view a circuit board in a manner similar to the human eye. AOI systems suffer no fatigue during long hours of operation, however, and can be designed to work effectively with very small, dense assemblies. This allows them to function more rapidly and more efficiently than human inspectors, ideally yielding measurable improvements in terms of both cost and quality. Use of AOI systems has also been found to result in higher yields (see below).

Typical AOI systems consist of a video camera, lens, and lighting, configured to provide output to an image-processing computer. The camera is moved over the surface of the electronic assembly in a preprogrammed pattern, inspecting the entire surface with sufficient resolution to locate and identify defects. Images taken by the system are processed by the software and a comparison is made against component data and assessment algorithms. Among the defects that can be detected using AOI are missing, skewed, or misplaced components; bent or lifted leads; short or open circuits; insufficient or excessive solder; solder voids; contamination; and polarity errors.

X-Ray Systems. In instances when no test pad is available and there is no via access for an electrical test, x-ray imaging is generally considered the only effective method for testing ball-grid arrays for solder quality and hidden solder balls. The method has also proven effective for inspecting embedded components. Two-dimensional x-ray inspection can be used to examine an entire board at once, whereas the three-dimensional method examines the PCB from various angles by generating multiple images. Three-dimensional methods have been found to be especially effective on double-sided circuit boards, and are capable of simultaneously testing both sides of the unit under test with no additional fixture requirements. The principal limitations to x-ray inspection are a high false-failure rate, high per-board cost, and speed limitations.

Flying-Probe Technology. Testing methods for low-volume production of PCBs and for prototypes are also being challenged by the increasing complexity of electronic assemblies. Manufacturers of low-volume production runs or prototypes often cannot justify the cost of in-circuit testing. In such cases, use of flying- probe technology can sometimes provide a cost-effective alternative to more-conventional methods.

Flying-probe testing is performed using a fixtureless manufacturing defect analyzer capable of detecting defects such as shorts or opens. Typical systems use four independent probes controlled by software that allows either manual or on-line testing. The probes can be used to simultaneously generate stimuli and record measurement results. The technique usually requires the same skill level as does in-circuit testing. Pass/fail data are generated at the end of the test, providing a basis for the debug and repair stages.

Although it cannot achieve coverage comparable to that of in-circuit tests, the flying-probe technique is often a good first-stage test prior to conducting more-comprehensive functional testing.

This test strategy allows debug and repair times to be reduced—particularly in very complex PCBs. This can make the technology well suited to applications that require rapid turnaround, such as inspection and testing of PCBs with limited test access. The method is continuing to gain popularity as a result of advances in the mechanical accuracy, speed and general reliability of such systems.

With positionable test probes, flying-probe systems can be configured to assess active components. The addition of a vision system enables the flying-probe system to perform testing of component presence or absence, perform multiple-object testing and image recognition, and inspect component orientation. Assembly test points can be "learned" by flying-probe equipment that incorporates vision systems. A known "good" board can be used to train the system to locate assembly markers and to identify specific areas to be tested.

CONCLUSION

Use of contract manufacturing has clearly become an important strategic tool for medical device OEMs. The increased complexity of electronic assemblies, however, poses new challenges in terms of maintaining levels of both quality and product yields.

Developing adequate strategies for testing and monitoring quality remains an essential part of establishing and maintaining successful partnerships between OEMs and CEMs. An effective testing strategy provides the tools needed to ensure that CEMs meet the criteria established by an OEM in forming such a partnership.

The focus of such a strategy must be to generate data in a volume and format that allow analysis using common tools. In addition to ensuring that yield and quality expectations are consistently met, the generated test information should also allow the OEM to make realistic comparisons of performance by different contract manufacturers of medical electronics components.

Gregg Nighswonger is executive editor of MD&DI.