As the size of circuit boards continues to shrink, the demand grows for smaller probes to use on 0.075", 0.050", and even 0.039" centers. Smaller-diameter probes, however, are not only costly to manufacture, they also 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.
Test Electronics
Base Socketed Probes
All White PapersAs the size of circuit boards continues to shrink, the demand grows for smaller probes to use on 0.075", 0.050", and even 0.039" centers. Smaller-diameter probes, however, are not only costly to manufacture, they also 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.
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. Now, socketless technology is widely recognized for its capability to use larger, longer-lasting probes on high-density PCBs. This is made possible by joining two parts: a probe and a termination pin.
In the example shown in the drawing on the right, 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 stand-ard socket.
While other types of ATE, including flying probers, X-ray, built-in self-test (BIST), boundary scan software, and optical inspection, 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. But, as with any technology, there's always room for improvement.
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 for dual-level testing on 50-mil centers.
Compared to smaller conventional probes used in high-density PCB testing, socketless probes offer more choices for probing test targets. 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 generally are 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 conventional and socketless probes. 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.
Easy maintenance and repair are important factors when considering the use of any technology. When comparing conventional and socketless technologies since replacing dull, worn out probes is just as easy in both types of fixtures.
In the socketless design, probes sit atop termination pins that are permanently mounted and wired into a fixture, making the probes especially easy to replace. These pins are designed to withstand a high number of probe replacements while still maintaining a reliable electrical path. However, when damaged, they can be removed and replaced, using special termination insertion and extraction tools, just as easily as conventional sockets.
Socketless probes also score high in compatibility since they now can be used in every standard, in-circuit, and functional test fixture. This includes fixtures for GenRad, Agilent Technologies, and Teradyne test systems, whether pneumatic, mechanical, or vacuum type. Wiring methods for socketless fixtures offer just as many options as conventional ones, including wire-wrap posts, wire jacks, and precrimped and wireless terminations. The probes, which now are readily available, also work well with the wide range of test targets found on PCBs because they can be set at various heights