In-circuit test helps ensure PCB quality

Despite the difficulty of probing hidden nodes on dense circuit boards, in-circuit ATE still provides valuable test data.

Rick Nelson, Senior Technical Editor -- Test & Measurement World, 4/1/2001

font face="Arial" color="black">The limited access afforded to circuit nodes by PCBs densely populated with tiny components seriously challenges traditional PCB test strategies. But while optical and x-ray inspection can help fill the test-coverage gaps and final functional test can provide an ultimate pass/fail decision before a product ships, the venerable in-circuit tester remains an integral part of the test process.

The key to effective in-circuit test is to plan ahead, beginning at the design stage. Test engineers and design engineers must work together. Designers must recognize the benefits of direct electrical access to as many test points as possible. Test engineers must recognize that, to be economical, a product cannot have significant portions of PCB real estate available for test-probe access.

Compromise is critical and can range from the straightforward to the complex. As a straightforward approach, designers might choose to bury relatively reliable parts, such as passive devices, while preserving direct electrical access to more fragile parts, such as dense mixed-signal systems-on-chips (SOCs). Complex schemes include using boundary-scan compatible components (Ref. 1) that give testers virtual access when I/O pins are not accessible to test probes.

Test evolution

Figure 1. Able to support 5200 nodes, the Agilent Technologies 3070 Series systems perform analog and digital tests with logic levels programmable on each pin.

Figure 2. PCBs measuring as large as 24x24 in. can be tested with GenRad's Pilot LX four-probe flying prober.

In-circuit test evolved when PCBs were one-sided affairs with every circuit node accessible to a bed of nails contacting the board on the side opposite the components. Test-probe access has become exceedingly more difficult as boards have evolved through double-sided boards populated with ever-smaller surface-mount components to today’s multilayer boards containing I/O-hiding BGAs. Yet, in-circuit test is still possible.

The “in-circuit testing” concept brings to mind a floor-standing system integrating instrumentation and a bed-of-nails fixture (Figure 1). Or, you might visualize the flying prober, such as GenRad’s Pilot LX (Figure 2), which also integrates the necessary instrumentation but replaces the bed of nails with robotically controlled probes that successively probe PCB test points.

As the name implies, such testers evaluate individual components “in circuit”—after they’ve been soldered to the PCB. While inspection systems can identify missing, misaligned, or misplaced components and can make judgements about the integrity of solder joints, and while functional testers can exercise and grade the performance of a board by simulating the electrical environment it will face in the target product, only an in-circuit tester coupled with a bed-of-nails fixture (for parallel access to many test points) or a high-speed flying prober (for fast sequential access to many test points) can pump test signals into and out of boards with sufficient speed to quickly isolate opens, shorts, or bad or faulty components.

Manual approaches

But in-circuit test isn’t limited to high-speed bed-of-nails or flying-probe systems. You can also choose bench models or semiautomated or manual systems targeting rework applications. These systems come with sets of probes, often dedicated to specific package types. ABI Electronics, for example, offers probes for PLCCs, QFPs, and DIPs. To maximize measurement flexibility, some companies, such as DiagnoSYS Systems, offer in-circuit products that work with external instruments, which you can connect through an IEEE 488 bus. You’ll find various combinations of instrumentation, automation, and probing options as you peruse the product survey chart in Table 1.

Just as the ways of probing vary from system to system, so too do the basic capabilities of in-circuit test systems. At one end of the in-circuit-test spectrum, manufacturing defects analyzers (MDAs) concentrate on verifying the integrity of PCB solder joints. They generally operate with an unpowered DUT, and they look for opens (indicating missing components or inadequate solder) or shorts (indicating pin-to-pin solder bridges), but they don’t evaluate the electrical performance of individual components.

Moving up the spectrum, some MDAs, such as Checksum’s Analyst ft, add power-on functional test capability. Full-blown in-circuit testers check connection integrity as well as the functionality of individual components; they include software libraries of permissible device behavior against which they compare test results. Such systems apply digital patterns (vectors) derived from design data to individual logic devices and then monitor device response; Agilent’s 3070 systems can generate up to 20 megapatterns per second using 32-channel pin cards. Full-blown testers also support various logic levels to accommodate DUTs populated with multiple logic families. GenRad’s TestStation systems can handle up to 26 logic levels. To make accurate analog measurements, high-end in-circuit testers employ six-wire guarding schemes (Ref. 2), which isolate node voltages and currents of a component under test from the loading effects of electrically neighboring components.

Vectorless test

Figure 3. Proprietary test techniques extend fault coverage on Teradyne's Javelin 1004 flying-probe test system. (a) The DeltaScan approach makes use of a stationary probe, two flying probes, and on-chip protection diodes, while (b) the FrameScan technique makes use of capacitive coupling to check for opens.

Functional in-circuit test depends on the tester’s being programmed to recognize a device’s behavior—the tester must generally know, for example, what digital output pattern a logic component should send in response to an input vector. But even if a device’s response isn’t included in the tester library, a tester can still make estimations about whether it functions using so-called vectorless test approaches. Teradyne’s DeltaScan vectorless test (Figure 3a), for example, enables the Teradyne Javelin flying prober to evaluate a chip based on parasitic diodes built into that chip. And the company’s FrameScan technology, as embodied in the Javelin ( Figure 3b), uses retractable capacitance probes to look for opens.

Such approaches work fine when nodes are available to probing systems. For other nodes, boundary scan is becoming increasingly important. One study (Ref. 3) found as much as 25% of board surface area dedicated to test pads. The company that can pack more functionality into that 25% of surface area without adversely affecting test coverage will gain a significant edge in the marketplace.

An original promise of boundary scan was to provide test access through a PCB’s edge connector or through a test-access-port (TAP) connector on the board surface, and you can choose from a variety of dedicated boundary-scan-controller products to test boundary-scan-compliant circuitry in circuit (Ref. 4). On complex boards having multiple TAPs and boundary-scan-compliant components intermixed with noncompliant parts, a bed of nails or flying prober can be effective for delivering and measuring boundary-scan test data as well as other test signals. Consequently, most in-circuit tester manufacturers have incorporated boundary-scan capability into their equipment.

The result is a synergistic combination of traditional direct-electrical-contact test and the virtual-test capabilities enabled by boundary-scan; this synergy will continue to develop as digital boundary scan gets a boost from emerging analog boundary-scan components (Ref. 5). But boundary scan won’t supercede in-circuit test—it will simply give each tester pin, with physical access to a single node, virtual access to many nodes. T&MW

References

1. Rolince, David, “Extend the frontiers of boundary-scan test,” Test & Measurement World, February 2001. p. 37. 

2. Nelson, Rick, “High Speeds and Fine Precision Knock PCB Traces Off Pedestal,” Test & Measurement World, January 2000. p. 17. 

3. Neal, Bob, “Design for Testability—Test for Designability,” Agilent Technologies Manufacturing Test Division, Loveland, CO. www.ate.agilent.com/EMT/LIBRARY/intelligent_test/Bneal_dft_dfd.pdf.

4. Nelson, Rick, “Systems Expand IEEE 1149.1 Test,” Test & Measurement World, February 2000. p. 30. 

5. “Boundary Scan Gains ITC Attention,” Test & Measurement World, November 2000. p. 6.

Rick Nelson received a BSEE degree from Penn State University. He has six years experience designing electronic industrial-control systems. A member of the IEEE, he has served as the managing editor of EDN, and he became a senior technical editor at T&MW in 1998. E-mail: rnelson@tmworld.com.