Boards keep shrinking, but the decreasing size of test targets doesn't have to be a problem.

As technology advances, an ever-larger array of electronic products that do bigger and better things is hitting the market. At the same time, the size of the printed circuit board is shrinking faster than ever to keep up with the demand for smaller product packaging.

To fit more components in a smaller area, designers have no choice but to decrease the size and spacing of test targets commonly used on printed circuit boards. These test targets - namely, component leads, test pads and via holes - allow electrical access to the unit under test through spring probes in a bed-of-nails test fixture. The traditional challenge for the test department has been to reliably hit these smaller, densely spaced targets while achieving accurate test results.

This is now possible with socketless spring probes. Now PCB designers can shrink boards, test engineers can continue to test with larger test probes and contract manufacturers can maintain the fixture reliability they so desperately need to ship good boards.

Compared to flying probes, X-ray, built-in self-test (BIST), boundary scan software, and optical inspection - all of which combine to enhance the testing of high-density PCBs - the bed-of-nails fixture continues to offer the best combination of speed and test coverage in a high-volume manufacturing environment. Of course, as with any technology, there's always room for improvement. A reliable test fixture is critical to reducing false component failures, which are costly to retest and slow the process of producing known good boards.

Since the beginning of automated PCB testing, designers have pushed fixture and probe manufacturing companies to build a better bed-of-nails fixture. However, any new product innovations must be balanced with design for testability (DFT) guidelines that have evolved over time to keep pace with the latest advances in PCB and fixture manufacturing. The challenge is getting everyone in the design, manufacturing, and test departments to coordinate their efforts and agree upon the test guidelines.

Spring probes conventionally used in bed-of-nails test fixtures are held in place by a socket, which in turn is permanently mounted in the fixture's probe plate and wired to the test system. For decades, this type of system has produced reliable results when used on .100" centers. However, as the size of circuit boards continues to shrink and the demand to use smaller probes on 0.075," 0.050" and even 0.039" centers grow, test fixtures are becoming more expensive and increasingly difficult to build. Because of their smaller diameter, such probes are not only costly to manufacture, they are less robust, easier to damage, harder to maintain, and tend to need replacing more often than larger probes, all of which drive up test costs. Smaller probes also offer fewer tip style and spring-force options.

By functioning without the need for conventional sockets, the socketless tester allows the use of larger, more robust probes on smaller centers, resulting in a more reliable production fixture.

Going Socketless

While the concept of socketless probing isn't new, the technology wasn't widely used throughout the industry until smaller test targets started calling for the use of smaller, more delicate test probes. Today, socketless probe technology is widely recognized for its ability to use larger, longer-lasting probes on high-density PCBs. This is made possible by the joining of two parts: a probe and a termination pin, mb .

In the example shown in Figure 2, the interconnect pin at the top of the termination fits securely into the modified interconnect receptacle at the bottom of the probe tube. Because the termination pin is mounted directly beneath the socketless probe, it stays within the diameter of the probe tube while still providing a reliable electrical connection to the test fixture and performing all the functions of a standard socket.

Compared with conventional probes, the benefits of using socketless probe technology in bed-of-nails test fixtures are many:

• Higher probe-pointing accuracy due to the elimination of the single-press ring socket.
• A lower per-point cost, currently less than that of a conventional probe, for 0.075", 0.050", and 0.039" centers.
• The use of larger, more robust probes less prone to damage in a production environment, resulting in fewer maintenance issues.
• More tip styles and spring-force options with the use of larger probes on closer centers.
• Long-stroke socketless probes make dual-level testing on .050" centers if possible.

All these advantages of the socketless design combine to increase productivity and lower the overall cost of testing HDI boards with closely spaced test points.

Wiring methods for socketless fixtures offer just as many options as conventional ones, including wire-wrap posts, wire jacks, and pre-crimped and wireless terminations. To successfully probe a 0.050" connector lead, for example, termination pins must be set 0.050" lower than probes designed to hit test pads or vias. The capability to adjust heights for various test targets helps guarantee proper probe travel for each point in the fixture.

Due to their larger size, socketless probes accommodate a wider range of tip styles in copper-beryllium and hardened steel, as well as higher spring forces. When the choice is made to use socketless technology, test fixtures are generally designed to house only socketless probes. However, if there is a high ratio of larger to smaller center probes, it may be more cost-effective to mix socketless with conventional ones. In some cases, small areas of larger socketless probes may be added to a conventional fixture to replace areas of smaller probes like those typically found under ball grid arrays, high-density connectors, and other closely centered test points (Figure 3).

Regardless of their position on the fixture plate, whenever conventional probes and sockets and socketless probes are mix-mounted, vacuum leaks may result between the two systems. To prevent such leaks, simple gaskets and similar sealing methods can be incorporated into the design.

Easy maintenance and repair are important factors when considering the use of any technology. However, these aren't considerations when comparing conventional and socketless technologies since replacing dull, worn out probes is easy in both types of fixtures.

Probing Ahead

Today, conventional probes and sockets are still the most commonly used products in bed-of-nails test fixtures. But as more components crowd onto smaller PCBs, test targets continue to shrink, and center-to-center spacing decreases, the need to use smaller-diameter probes will keep driving up the cost of testing.

Hence, socketless probing like that offered by QA Technology, for example, will become an increasingly popular alternative. Acceptance of socketless technology continues to grow, and such advances in electronic testing can't help but universally benefit PCB designers, fixture houses, and end-users alike.   PCD&M

Matthew Parker is a product design engineer with QA Technology. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it.. Jeff Smith is North American sales manager at QA Technology. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

• Prev
• Next