Adapt Your Board-Test ATE for High-Speed Measurements

High-frequency probes and instruments plus multistep testing can extend bed-of-nails capabilities to gigahertz frequencies - at least for a component or two.

Rick Nelson, Senior Technical Editor -- Test & Measurement World, 4/1/2000

As PCB speeds increase, you might find your in-circuit, board-test ATE running out of steam. As speeds rise beyond the 7- to 10-MHz limits of standard bed-of-nails fixtures, you can either find an alternative to in-circuit test or else modify your test setup to handle high-speed measurements. 

Figure 1. You can adapt your in-circuit fixture for microwave measurements by employing subminiature A (threaded, shown here) and B (snap-on) connectors, which can handle frequencies to multiple gigahertz. (Courtesy of Everett-Charles Technologies.)

Figure 2. A wireless test fixture employs double-ended nails to connect a PCB under test to in-circuit ATE.

If you decide to forgo in-circuit testing in favor of another option, you’ll find that each of the alternatives has drawbacks. For example, you could eliminate in-circuit test and rely on functional test only. This approach tests your product at the rated speed, but it burdens your production line with having to complete the manufacture of parts into which faults were introduced early in the manufacturing process.

Whereas in-circuit test would promptly detect a bad component on a loaded PCB, reliance on final functional test means the bad board would be assembled along with good components into your product. And once you uncover the problem during functional test, it may be impossible to economically rework the board, so you’ll end up with high levels of scrapped products that contain mostly good components.

As another option, you could incorporate built-in test circuitry such as boundary-scan cells. Boundary scan can effectively look for manufacturing defects, but you must include boundary-scan components in your product, and your tester must be able to generate boundary-scan test patterns and gauge the circuit response. Boundary scan can operate at higher speeds than a bed of nails, yet it tops out at only 25 MHz.

Naturally, you could obtain a test platform dedicated to testing high-speed boards and modules. This approach is the ideal one for high-speed test, but it’s expensive. Before you embark on that road, consider these ways in which you can extend the capabilities of standard in-circuit ATE—ranging from fine-tuning your test fixture to incorporating microwave test probes (Fig. 1):

1) Test Slowly
Consider using low-speed in-circuit testing on your high-speed PCB. A low-speed test won’t determine if an RF amplifier operates within spec, but it can tell you if the device is missing or if leads are shorted together. If you’re currently uncovering high levels of manufacturing defects at final functional test or in a hot-mockup test, an early in-circuit test stage can be beneficial. In general, you want to find defects as soon after wave solder as possible.

Figure 3. A two-stage test can enhance tests of sensitive components. (a) In the first stage shown here, all UUT nodes—A through F—are probed. (b) In the second, only A and E are contacted (by nails with longer travel), minimizing interference from signals on nodes B, C, D, and F.

The wireless fixture is no magic bullet—if you’re careful about keeping wire lengths as short as possible and using twisted pairs, you can get a wired fixture to operate as fast as a wireless one. The wireless fixture, though, can make it quicker and easier to get there: It provides better repeatability; it better withstands the wear and tear of the production floor; and it is easier to duplicate for multiple test systems.

Figure 4. A double-nest fixture can help you test a few RF components on a UUT. You first position your UUT over the low-frequency nest and then move it over the RF nest to make microwave measurements.

Figure 5. An effective probe length of one-half wavelength (l/2) terminated in an open circuit minimizes current flow at the node under test.

 4) Build an RF Nest
By following the first three suggestions, you can improve the speed of your bed-of-nails testing, but only by a small percentage. At high frequencies, the two-stage test runs out of steam, because even the disconnected pins in the second stage act as an antenna field that disturbs the measurements of the connected pins. If you have just a few RF components on an otherwise low-speed board, you probably won’t want to buy an RF ATE system. Instead, you can adapt a standard ATE fixture to handle the tests. (Of course, you will have to install appropriate RF instrumentation in your ATE system. Some in-circuit testers offer RF connectors that facilitate the integration of such instruments.)

According to Chuck Clark, an applications engineer at Agilent Technologies Manufacturing Test Division (Loveland, CO), the key to testing a few RF parts in-circuit is to completely isolate the RF tests from other tests. Using a double-nest approach (Fig. 4), you essentially divide your test fixture into two subfixtures, or nests. One acts as a standard bed of nails, making low-frequency tests. When those tests finish, you move the UUT to the RF nest, which is populated by RF probes, to make microwave measurements.

When employing the dual-nest approach, be sure to use appropriate circuit terminations on the RF probe circuits in your tester. A common technique is to employ a half-wavelength path from your node under test to the tester, where you terminate the path in an open circuit. That in turn results in minimal current flow into the test probe and consequently minimal circuit loading (Fig. 5). T&MW

You can contact Rick Nelson at rnelson@cahners.com.