Nonintrusive Test Complements ATE to Meet PCB Test Needs
Test engineers have a variety of options for testing printed-circuit boards—ranging from in-circuit testers to the latest in embedded-instrument technology. Despite the decline of PCB physical access as component sizes shrink and test points disappear, in-circuit test retains a key role in high-volume circuit-assembly test, and flying probers continue to serve low-volume and new-product-introduction (NPI) test, as described in the “Test Application Special Report” in our February print issue, which you can read online here.
As that article notes, an effective test strategy will likely employ nails and probes as well as functional test and the latest in embedded instrumentation and nonintrusive test techniques. In this article, industry experts from ASSET InterTech and Aeroflex elaborate on the PCB-test options available. This article concludes with comments on the future of automated test beyond the PCB space.
Kevin Tomkins, ATE product manager at Aeroflex, listed key PCB test-technology options:
In-Circuit Test (ICT),” he said, “…is a powerful tool for checking the integrity of an assembled printed circuit board.” By accessing many, if not all, of the PCB circuit nodes via a bed of nails fixture, he said, each component can be measured for value, orientation, functionality, and other features, regardless of the other components connected to them. “Functionality of digital circuits can also be measured through a stimulus source and monitor circuitry,” he said, “although their increasing complexity usually makes a full check uneconomic.” By combining analog and digital techniques, he said, “…it is possible to undertake a very comprehensive form of printed circuit board test, ensuring that the circuit has been manufactured correctly and has a very high chance of performing to its specification.”
Functional Test (FCT), Tomkins said, “…is always the last stage in the manufacturing plan. It provides a go or no-go decision on finished PCB, module, or final product. The key difference between in-circuit and functional test is that you are testing the functionality of the unit under test (UUT) as a whole and not whether the soldering is good or the components within it are correctly oriented or in tolerance. In general, you are checking that the UUT works as it should, by applying signals that mirror its final electrical environment. Therefore, rather than accessing each circuit node or circuit cluster, a functional test would interface via a board or module connector.”
Vectorless Test, Tomkins explained, “…means that no 'test vectors' are required, therefore negating the need for application engineers to write costly device tests that apply a combination of test patterns in order to establish correct operation.” Tomkins listed several vectorless test techniques: parasitic diode, capacitive, or inductive coupled lead-frame. “All have been developed to identify whether or not integrated circuit devices or connectors are correctly soldered to the PCB by establishing device pin connectivity without detailed information of the device under test.” The first, parasitic diode, technique, he said, “…measures current flow through the parasitic diode junctions in a device (clamping diodes, substrate junctions). If a device leg is not soldered, current will not flow.” In contrast, “The second and third techniques rely on a fixture-mounted plate that forms a capacitor or inductor with the IC’s lead-frame metal during the test,” he said. “The target pin is stimulated with an AC voltage source, while the device’s remaining pins are connected to ground.” The signal then capacitively or inductively couples to the sensor plate mounted in the fixture, and the tester’s detector then does or doesn't measure this signal, thereby establishing good or bad connectivity to the PCB.
Boundary scan, Tomkins said, “…is defined by the IEEE 1149.x (JTAG) standard; it provides a means to test interconnects between integrated circuits on a board without using physical test probes.” Boundary scan overcomes test access issues imposed on boards densely populated with devices such as FPGAs, CPLDs, and BGAs. “This capability is implemented as the IC design stage and provides a boundary-scan cell that includes a multiplexer and latches to each pin on the device,” he said. A dedicated port (TAP) captures data from each pin or forces data onto pins, and the captured data is serially shifted out and compared with expected results, thus determining a failing pin, he said, adding, “More recently this approach has been developed to allow memory test, EEPROM, and flash programming.”
In-System Programming (ISP), Tomkins, concluded, covers the ability of some programmable logic devices to be programmed while installed on a PCB or in a system, rather than requiring the device to be programmed prior to placement. “The primary advantage of this feature,” Tomkins said, “is that it allows manufacturers of electronic products to integrate programming and testing into a single production phase, rather than requiring a separate programming stage prior to assembly. This allows manufacturers to program the chips in their own system's production line instead of buying preprogrammed chips from a manufacturer or distributor, making it feasible to apply code or design changes during a production run.”
Elaborating on Embedded Instruments
Glenn Woppman, president and CEO of ASSET InterTech, elaborated on the roles of various test techniques, such as in-circuit test, flying-probe test, functional test, and boundary-scan test. “Historically,” he said, “intrusive, probe-based test technologies like oscilloscopes and logic analyzers have been employed during design to validate and test PCB prototypes for the purposes of board bring-up. Once the PCB design was released to manufacturing, a completely new set of tests would be developed to test the PCBs on another set of intrusive, probe-based test technologies, which included in-circuit test, flying probe, and manufacturing defect analyzers.” In addition, he said, “Functional tests would be applied late in the PCB prototype board bring-up process because the traditional functional test suite could not be applied until the PCB’s firmware and possibly its operating software were integrated with the hardware.” As for functional test, it would often be incorporated late in the manufacturing process. “Functional test often suffered from poor diagnostic capabilities—that is, functional tests could determine that the PCB did not operate to spec, but they had difficulty isolating the root causes of failures,” Woppman said.
Woppman said ASSET's ScanWorks platform for embedded instruments can be deployed across a spectrum of test strategies covering the entire PCB life-cycle. “Beginning with new product board bring-up, ScanWorks is able to test and validate prototype boards faster than legacy intrusive hardware-oriented tools like oscilloscopes and logic analyzers. For example, a ScanWorks platform might be configured with boundary-scan structural-test tools, HSIO tools for high-speed I/O and memory bus test and validation, and processor-controlled test (PCT) tools to functionally debug and test PCBs before any of the board’s firmware is in place.” Then, the same tests and others can migrate and be reused in low- or high-volume manufacturing where the nonintrusive tools would overcome the access limitations of ICT and MDA tester. “Unfortunately,” Woppman said, “today’s best practices in PCB design frequently call for the elimination of test pads. Fortunately, ScanWorks does not need the external test access provided by test pads since it uses embedded instruments to test from the inside out.” In a low- or high-volume manufacturing setting, he continued, “ScanWorks might employ boundary-scan tools to detect structural shorts and opens; processor-controlled test tools to perform board debug, device initialization, at-speed functional test with structural diagnostics, and other test functions; and HSIO tools to thoroughly stress and test multiple lanes on high-speed buses simultaneously…. Also during manufacturing, the PCB test and diagnostic tools of the ScanWorks platform can be deployed in repair applications to recover the cost of PCBs that fail as they come off the manufacturing line and end up in the proverbial ‘bone pile’ of non-functional PCBs.”
Woppman elaborated on the evolution of PCB test strategies. “Over the last decade, the role of non-intrusive embedded instrumentation with regards to PCB test has become increasingly more important because legacy test technologies have seen access for their test probes disappear from PCBs. Scopes and logic analyzers could once use device pins for test access, but chip-scale packages, like ball grid arrays, place pins beneath the silicon die, essentially eliminating probe access.” Board designers have not helped matters, because they have eliminated test pads on most high-speed PCBs to minimize the signal reflections, ringing, and other anomalies that test pads can induce. “As a result of these and other factors,” Woppman said, “the role that nonintrusive embedded instrumentation is now playing in PCB test has consistently grown from year to year as speeds and PCB densities have increased. Most notably, the nonintrusive software-based test technologies like boundary-scan test, processor-controlled test, HSIO test, and FPGA-controlled test are playing an essential role in PCB test strategies throughout every phase of a PCB’s life-cycle, including design, manufacturing, and field service.”
Woppman elaborated on the future of embedded instrumentation. “With JTAG-based soft access, an embedded instrument tool platform can operate embedded instrument-intellectual property (IP) based on industry standards”—including IEEE 1149.1 boundary scan and its descendants, such as IEEE 1149.6 and 1149.7. Also playing a role, Woppman said, are the new IEEE P1687 Internal JTAG (IJTAG) standard as well as proprietary embedded-instrumentation IP, such as Intel’s Internal Built-In Self Test (IBIST), which is being embedded in all of Intel's advanced processors. “We expect that the imminent passage of the IJTAG standard will accelerate the growth of an embedded instrumentation ecosystem that will include instrument IP from a wide range of sources, including open-source standards-based instruments and proprietary instruments,” Woppman said. “The ability to integrate instruments from any source into a cohesive test platform will be necessary to capitalize on the great value of embedded test resources.”
The Future of Automated Test
As our February print-issue “Test Application Special Report” on PCB test was going to press, National Instruments was preparing to release its “Automated Test Outlook 2013,” highlighting the company’s research into the latest test-and-measurement technologies and methodologies. The report does not specifically comment on PCB test but does examine trends affecting a variety of industries, including aerospace and defense, automotive, consumer electronics, semiconductor, telecommunications, and transportation—for all of which PCB quality is a significant issue. In addition, the PXI platform that NI pioneered is often used in functional-test applications.
For example, as quoted in the February special report, NK Chari, director of worldwide marketing and support for the Agilent Technologies Measurement Systems Division, said, “We are improving throughput with a new functional test system for automotive systems and other industrial electronics. This new PXI-based test system will be faster than current-generation solutions by at least 20% to 30%.” Chari said the company will share more details on its new board-test initiatives at Apex Expo 2013 Feb. 19-21 in San Diego.
The 2013 version of NI's annual “Automated Test Outlook” covers the following five trends: test economics, big analog data, software-centric ecosystems, test software quality, and Moore’s Law meets RF. Click here for more information.