Answers to Common Questions About MDAs

Many systems on the market today—manufacturing defects analyzers (MDAs), in-circuit testers (ICTs), functional testers, combinational testers and hot mockups—test PCBs. Usually, the first system that tests a PCB in the manufacturing process is an MDA. True, MDAs look for manufacturing defects, but the name really doesn’t tell the whole story. To learn more about the capabilities and attributes of this tester, here are answers to some of the more commonly asked questions about MDAs.

How are MDAs used?

Primarily, MDAs test populated circuit assemblies, both surface-mount technology (SMT) and though-hole. Other common applications include backplanes, flex circuits, subassemblies, cables and harnesses.

What kind of faults do MDAs find?

MDAs focus on problems that occur during the manufacturing process. Typically, the failures include opens, shorts, wrong components, missing components and components installed in the wrong orientation.

What measurement capabilities do MDAs have?

Typical measurement capabilities include resistance, capacitance, inductance, semiconductor junctions, standard and zener diodes, LEDs, transistor beta, IC orientation, transformer polarity and voltage.

Can most components be measured in-circuit? It seems as if parallel circuits would interfere with making good measurements.

Some measurements are hard to make in-circuit, but MDAs provide methods that allow very high test coverage. Some of these methods are:

Low-amplitude ranges that prevent diode turn-on.

Complex-impedance measurements that can individually discriminate among capacitive, inductive and resistive components of parallel networks.

Choice of measurement frequencies to optimize impedance of measured components compared to parallel components.

Use of multipoint guarding to eliminate or reduce the effect of parallel impedances.

What kind of faults cannot be found by MDAs?

Faults that are difficult or impossible to check generally are caused by unguardable parallel circuit paths in the UUT. These same testing problems are present with in-circuit testers. For example, if a low-impedance transformer winding has a small parallel capacitor, often times the capacitor cannot be checked since its impedance is very small compared to the transformer winding.

Another testing problem is a small capacitor (in the low pF region) at the input of an IC. In many of these cases, the input of the IC has a very large inherent capacitance compared to the external capacitor, making the external test impractical.

Still another common testing problem is all of the parallel decoupling capacitors on power supplies. Typically, these have a large capacitor (for example, 10 m F +20%) in parallel with many smaller capacitors (for example, 0.1 m F). Because the variability of the higher-value capacitors is large compared to the smaller capacitor values, it is not practical to detect missing smaller capacitors on an individual basis. If one is missing, it usually does not cause a performance problem on the UUT.

How much do MDAs cost?

Many MDA systems—for example, ones with measurement electronics, 400 test points, a computer, a fixture receiver and software—are available in the $10k to $15k range. These systems are complete except for fixture power, such as air pressure or a vacuum source, and the fixture/test program that is customized for the particular assembly being tested.

A ready-to-use test fixture with a test program typically costs about $1,500 to $3,000. System sizes range from 200 test points to 8,000 points.

What is the difference between MDAs and the much-more-expensive ICTs?

Both are very similar for most testing operations. ICTs add the capability to power-up the UUT and individually test the ICs for proper operation. In most cases, the incremental test coverage is minimal since the failure rates of ICs are in the parts-per-million range. MDAs assume the parts are good and look for problems that occur during assembly. MDAs are also called analog in-circuit testers or process testers.

What kind of fixturing is used?

Bed-of-nails fixturing is used for the majority of MDA testing. Smaller assemblies can be fixtured with low-cost mechanical fixtures. Higher point-count UUTs typically are fixtured on pneumatic (air-powered) or vacuum-powered fixtures. We have seen a significant movement from vacuum fixturing to pneumatic and mechanical fixturing in the last few years. There are many reasons for this, but the use of SMT on today’s boards is the most notable.

How is programming done?

There are a couple of methods. MDAs can accept CAD data such as net lists and component files to generate the test programs and fixture wiring lists. They can also be manually programmed by entering the information about the UUT. Some of the tests, such as opens, shorts and IC orientation, can be auto-programmed from a known-good UUT.

Typical MDA test programs can be completely written and debugged in a day or two. Small programs can be written in a few hours. Technician-level personnel with little specialized training can program the testers.

Are MDAs suitable for SMT?

Yes. The main issue on SMT is fixturing. For best results, the PCBs should be laid out with bottom access to each network. Compared to through-hole technology, SMT manufacturing has a higher occurrence of open connections. MDAs offer vectorless testing tools such as HP TestJet technology to help detect these faults.

What kind of data tracking do MDAs offer?

Typical fault tracking consists of a strip-printer output of failures for the UUT. This printer output then goes with the failed UUT for repair. However, systems provide standard tracking and reporting of statistical process control parameters such as pareto, Cp, Cpk, X-bar/Sigma and yield reports. Systems can be linked on standard PC networks for data sharing, storing and analysis.

Do MDAs do anything else other than MDA testing?

Yes, some MDAs offer optional integrated functional test that can be used for power-up testing. These systems can power up the UUT on the same fixture as an MDA, then test for UUT operation. These tests can range from a few limited tests, such as oscillator frequencies, power supply voltages or pot adjustments, to full functional test of the UUT.

When integrated testing is performed, the system first does a manufacturing defects test that finds the most faults. If there are no failures, the system continues to functional test. The functional test checks from UUT input to UUT output and confirms operation of the UUT as a whole rather than on an IC-by-IC basis as with an ICT.

About the Author

 


 

Brian Laine is the Sales and Support Manager at and a founder 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, 19009 61st Ave. N.E., Building 4, P.O. Box 3279, Arlington, WA 98223, (360) 435-5510.


Copyright 1996 Nelson Publishing Inc.

August 1996

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