Long gone are the times when board ATE was synonymous with high acquisition costs. Today even the largest high-throughput board ATE sells for a fraction of the cost of its ancestors. It also is easier to program and better able to pinpoint defect symptoms and locations.
To complement- or in some cases replace- multifunctional high-throughput test systems, a variety of small, very low-cost ATE is evolving. These systems range from simple fixtureless in-circuit testers with clip-on leads to boundary scan testers, PC plus plug-in board test systems, and VXIbus-based ATE. Custom-configurable test systems that employ standard modules and require minimal configuration engineering effort, such as the Racal Freedom Series and the Wayne Kerr 2930 and 3930 Series, also are widely used.
Two factors have fostered the emergence of such a variety of ATE. First, IC variances, in function as well as in appearance, have led to greater divergences in UUT characteristics. Secondly, today’s test-environment needs are more disparate than ever before. ATE is not only used in the factory for process-integrity verifications and functional tests, but also employed to a greater extent in field service repair centers and even during equipment development.
Despite greater device complexity, more densely populated UUTs and diverse test environments, the common goals of minimizing program and fixture development are still paramount. At the same time, achieving high fault coverage and accurate diagnostics is equally essential. To help achieve these goals, more vectorless test as well as program and fixture-design automation software are now available.
Vectorless Test
The TestJet™ technique developed by Hewlett-Packard was the first vectorless test implementation to win wide acceptance. It identifies open connections and is based on capacitive coupling between transducers and device lead frames.1
Besides HP, several test-fixture and ATE suppliers now provide TestJet under licensing agreements. Implementations similar to TestJet have been developed by GenRad and Teradyne.
“The primary benefit of vectorless test is a significant reduction in programming time compared to vector-based test methods,” said Craig Pynn, Marketing Manager, Assembly Test Division at Teradyne. “Several different vectorless techniques, such as analog junction, capacitive and inductive, have been developed. However, each technique has certain fault-coverage limitations.
“For example, capacitive techniques are ineffective on most ball grid arrays, and analog junction techniques provide limited coverage for arrays, such as memory circuits. To provide the highest fault coverage for vectorless test, Teradyne offers MultiScan, a comprehensive tool set consisting of capacitive, inductive and analog junction vectorless techniques.2 MultiScan provides higher overall fault coverage in less total programming time than does a single technique,” Mr. Pynn explained.
Several companies have developed other vectorless technique derivatives. In addition to providing TestJet, Testronics offers a measurement algorithm to detect opens on bused IC pins.
“This technique is really an extension of the diode junction measurement methods,” explained Cliff Call, Applications Engineer at Testronics. “It treats the junctions within the IC not as diodes, but as transistors. We included this utility because many users do not want the additional fixture costs associated with the HP TestJet measurement method.”
Program and Fixture-Design Automation
Developing fixtures and generating even the simplest vectorless, MDA or in-circuit test (ICT) programs are key activities that must be completed before testing can begin. In many modern high-yield circuit-board manufacturing environments, timely availability of fixtures and programs for ICT is particularly important.
“In fact, the development of ICT programs and fixtures is often on the critical time path for production ramp-up,” said Mr. Pynn. “But unfortunately in the realities of the production environment, often only partial board data is available when test programming and fixture design must begin. To complicate matters, frequent design revisions which commonly occur during ramp-up slow down program and fixture development.”
To address these problems, many companies have developed automatic test program generation and fixture design aids. Hewlett-Packard’s software tools, for instance, include automatic layout of a variety of fixtures, including the short-wire type.
Teradyne developed Momentum™, a comprehensive tool set that accepts inputs from a variety of sources, such as CAD data, a bill of materials or a netlist, to quickly create the program and generate a complete fixture design. “By providing one easy-to-use, comprehensive environment to manage the entire process from CAD data to final program and fixture design, Momentum cuts data collection time, minimizes fixture build time and ensures timely availability of stable, reliable test programs. This removes ICT program and fixture development from the critical path,” concluded Mr. Pynn.
Making Your Selection
While only vectorless test and program and fixture development have been addressed, many other factors must be considered when making your selection. The accompanying chart lists some key parameters of typical ATE or test-system building blocks.
All of these systems are expandable in functionality, pin count or memory/pin. Most of the PC plug-in cards and modules can be ganged or intermixed to accommodate higher pin counts or to provide more test capabilities.
Since the hardware as well as the software features differ so widely, you may ease the selection process by following these steps:
Define the test environment; for instance, high-volume low-mix manufacturing test, low-volume high-mix manufacturing test, or field service or depot maintenance test.
Estimate annual volume and required throughput rates.
Define presently known UUT circuit characteristics, such as digital, memory, analog, mixed-signal and RF, as well as the physical configuration, such as double-sided, surface-mount or density.
Determine appropriate test vector rates, frequencies and accuracies.
Define future requirements.
Define and quantify the expected defect spectrum.
Submit this data to several ATE suppliers who have appropriate systems. Once they respond, clarify any misunderstandings and generate an evaluation matrix. List all selection criteris on a matrix and assign a numerical value based on the importance of each characteristic.
Make your selection based on the evaluation score.
References
1. “New Technique Reduces Test Development Time for SMT Opens,” Evaluation Engineering, February 1993, pp. 62-71.
2. Pynn, C. T., “Vectorless Test Boosts Fault Coverage and Cuts Cycle Time,” Evaluation Engineering, August 1995, pp. 37-41.
Copyright 1996 Nelson Publishing Inc.
February 1996