As electronics technology forces manufacturers to rethink board-test strategies, a new class of hybrid testers is evolving. Although called manufacturing defects analyzers (MDAs), they are more than traditional MDAs–they offer a variety of unconventional features, including vectorless test, a CAD interface, in-circuit test and functional test.
The Old Way
Traditional manufacturing test relies on power-up in-circuit test, functional board test and/or final functional test of the product after assembly. The advantages of this approach are good test coverage and the use of a minimum number of test systems. The disadvantages, however, can outweigh the pluses. For example, all assemblies must be routed as a separate process step to the factory testers, leading to more manufacturing time. Failure feedback occurs after an entire batch of UUTs is completed, rather than on a UUT-by-UUT basis. In the case of a systematic process problem, more rework is required. Then there are scheduling complications; and, if a tester breaks down, an interruption of the entire factory production may occur.
This traditional approach originated from batch-oriented production rather than JIT, high purchase and programming costs of ATE, and relatively high failure rates for individual components on the boards being tested.
Things Have Changed
In the last few years, many technological trends have prompted the reorganization of factory test (Table 1):
The defect rate of ICs has been reduced to the ppm level, virtually eliminating the need to test and isolate to the faulty IC level. This has greatly reduced the requirement for traditional power-on, vector-based, in-circuit-test (ICT) methods.
Manufacturing has evolved from a production-line approach to a cell-oriented approach. In manufacturing cells, a work group is responsible for a number of operations on each assembly.
To support this approach, localized board test has become necessary. Requirements include low tester cost because more testers are required, quick UUT changeover, the capability to test at multiple assembly levels, good diagnostics since cells may not have technicians, and simplicity of use.
The widespread use of surface-mount technology (SMT) has created new test problems, such as a higher probability of open circuits than with through-hole technology. To diagnose these faults quickly and at low cost, new vectorless methods test ICs and connectors for open circuits. Other new challenges with SMT assemblies are fixturing of close component leads, open holes in assemblies, and board-flex considerations during test.
Lower-cost, easier-to-use systems make test cost-effective in many more applications, such as in cell-based manufacturing.
Test systems with integrated manufacturing defects analysis and functional test provide a broader test spectrum.
New high-density circuitry, such as SMT and micromodule technology, and environmentally safe manufacturing methods, such as no-clean fluxes, have forced revisions of fixturing techniques.
To support this current set of testing needs, a new class of hybrid testers has evolved. While these new testers are called process testers or manufacturing defects analyzers (MDAs), they fall into the middle ground among traditional MDAs, power-on in-circuit testers and functional testers (Figure 1).
Some testers also can be configured with options to cover the capabilities of one of the other tester types shown in Figure 1. Here’s what you can expect:
Bed-of-Nails Fixture Systems
Traditional fixturing uses the industry-standard GR-2270-style vacuum interface. More recently, the use of mechanical and pneumatic fixturing has increased. There are several reasons for this trend.
First, the new no-clean flux has a tendency to prematurely degrade vacuum gasketing. There is no gasketing required with pneumatic and mechanical fixturing.
Second, the use of dual-sided probing requires some sort of cover or overclamp. Since a top is inherently present on pneumatic and vacuum fixturing, the incremental cost is much lower. Many times, top-side probing is required for normal test points or for vectorless test for open connections, such as HP TestJet technology probes.1
Third, high-density SMT UUTs often cannot be vacuumed down because of excessive probe loading or open vias. The theoretical limit of excessive loading is about 25 probes/in.2.
Finally, board flex can harm SMT connections. UUT board flex is inherently lower in properly constructed mechanical or pneumatic fixturing.
Vectorless Test for Open Connections
This new technology finds open connections to components on the UUT. The most popular methods use a noninvasive top probe on each IC. The test system then stimulates each pin from the already present probes on the bottom of the UUT to each network, measuring the capacitance to the top of the component. Good readings are typically in the 30 to 100 fF region, with open connections less than 10 fF.
This technology can be used for ICs, connectors, traces and other parts on the UUT. Several variations of this technology are on the market, such as HP TestJet technology, Teradyne WaveScan and GenRad Opens Xpress. Some of these patented technologies are also available under license from other vendors.
Traditional MDA Features
These include testing for opens and shorts, and verifying the correct installation of resistors, capacitors, inductors, transistors, diodes, ICs, relays, FETs, zeners, transformers, jumpers, potentiometers and other parts. Modern MDAs offer sophisticated measurement and guarding systems to provide high test coverage.
About 85% of all faults are found with these features. Typical test times are a few seconds per UUT.
Networking and Statistical Analysis
Modern MDAs can be connected to standard PC networks for data archival, analysis and program storage. Standard software can include statistical process control to obtain reports such
as Pareto, X, Cpk and Yield. These can be used to analyze production and UUT trends.
MDAs now offer standard packages to convert board-design CAD files into test programs and fixture wiring specifications. This can help generate test programs and fixtures more quickly, accurately and economically.
In-Circuit Test Functions
In addition to traditional functions, modern MDAs can offer features such as testing for transistor beta, transformer polarity, relay actuation, FET on/off actuation, zener measurements, capacitor polarity and voltage measurements. When coupled with integrated functional test, other traditional ICT testing can be performed.
Integrated Functional Test
For applications where performance testing is necessary, the same tester can provide integrated MDA and functional test. First, the MDA test detects shorts and other defects that could cause power-up problems. If there are none, the same tester, often using the same test-point electronics, can power up the UUT and provide stimulus signals and measurement capability to check all, or critical, circuit performance. This power-up can also help the operator make UUT adjustments prior to final assembly. In some cases, it can be the entire final test before product shipment.
Modern MDAs are inexpensive; complete installations range from $8k to $75k, with typical installations about $10k to $15k. They are quick and easy to program. Programs are generally written and debugged in about a day, and provide very good diagnostics information. A functional test capability adds about $3k or more to the price of a test system.
Electronics manufacturing has evolved through the years to meet new challenges of the production environment such as cell-oriented manufacturing, new generation manufacturing techniques and new environmental demands. To meet these needs, testers have changed. Today’s MDAs provide far more than their predecessors. They offer new fixturing methodologies, lower costs, easier programming, higher performance, and techniques such as vectorless and functional test capabilities.
1. HP TestJet technology is protected under U.S. Patent Number 5,254,953.
About the Author
Brian Laine is the Sales and Technical Support Manager at CheckSum. One of the founders of CheckSum, he previously held engineering and engineering management positions at John Fluke Manufacturing and Summation. Mr. Laine has a B.S. degree in computer science from Oregon State University and an M.B.A. degree from Seattle University. CheckSum, Inc., 19009 61st Ave. N.E., Building 4, P.O. Box 3279, Arlington, WA 98223, (360) 435-5510.
Traditional Test Strategy
New Test Strategy
Assembly-line batch manufacturing
Autonomous work cells
Expensive shared factory-wide ICT
and functional test systems
Low-cost localized MDA
or integrated MDA/
Networked stations with
Assume components are bad
Assume components are good.
Look for wrong, missing or
incorrect assembly of parts
New methods for
accommodating SMT needs
Copyright 1995 Nelson Publishing Inc.