With Flying Probe, Electrical Test Depends on Mechanical Precision

Estimated reading time: 3 minutes

Flying probe testers use motorized test probes to roam around the surface of an assembled PCB, poking test access points (TAP) to complete circuits. This form of electrical test is more flexible than the typical bed of nails method, which has prompted many low-volume/high-mix (LVHM) assemblers to consider a flying probe machine investment. Electrical test experts can hit roadblocks when comparing mechanical specs on flying probers, says Greg Leblonc, P.Eng., product manager at Accculogic. He described the impact of motor mechanisms, probe layout, and probe angle on test accuracy and repeatability at a recent Acculogic seminar.

Several mechanical drive systems can be used to power the probe movements. With a lead screw mechanical design, probes are mounted on two levels. The top level requires longer probes at a more obtuse angle (up to 16°) than bottom probes. Dual-gantry forcers use an I beam layout, and run into the same differences with two probe lengths and two mounted angles. Bearings and ball screws are subject to wear over time, which will cause a prober to lose accuracy. The farther away from 0° a probe is mounted, the more likely it will have trouble testing around taller components, and the angled impact may tear a test pad. On a mono-planar drive system, all of the probes are mounted on the same level. Probe angle is programmable, and doesn’t reach the same obtuse angles as the first two options.

Other factors that can influence accuracy and repeatability are the probe’s wiggle room inside its shaft, probe tip aging, board warpage, and the fiducial-recognition vision system’s accuracy.

How can you determine true accuracy and repeatability? First, Leblonc says, clarify the difference between these metrics. Accuracy is how close to a given target the probe hits. Repeatability is hitting at or near the same point over and over again. Accuracy can be influenced by routine programming; repeatability is a function of machine performance. To quantify the real accuracy and repeatability of a flying prober, simulate production test. Program all of the probes to hit one test point on a board. This measures accuracy. Next, send all of the probes to their own test points multiple times, re-using the same board. Inspecting the probe hits on or around these test points will reveal repeatability. Most importantly, perform these tests on a typical PCB assembly for your business.

How long will tests take? Again, this depends on machine design, as well as PCB complexity and design. Better vision systems can increase accuracy, but they can also slow probing. Probe movement around the board is wasted time, and should be reduced as much as possible. Leblonc offers the equation n(n -1)/2 = measurements per movement, when n is the number of probes. This test speed measurement more closely mimics production conditions than recording the speed of the motors. In addition to increasing test coverage, probes on the top and bottom side of the board can reduce test times. If a flying prober can perform via verification, powered-up test, and basic AOI functions, it shortens test & inspection time by eliminating additional steps at other stations.

Leblonc states that systems with higher repeatability will naturally lend higher accuracy in real production test. Testers with more flying probes will finish test programs faster than faster moving systems. And flying probe stations with built-in boundary scan, AOI, and dual-sided probing capabilities will save even more time.

Meredith Courtemanche, executive editor, SMT

For additional information about flying probe design and operation, and what to look for in a flying probe system, contact Leblonc at greg.leblonc@acculogic.com.

Interested in the ROI equation for flying probe? Read Flying Probe Testers Lower PCBA Costs by Bob Steel, North American sales manager for Acculogic.

Subscribe

Join the PennWell SMT Group on LinkedIn

Become a Fan on SMT's Facebook Page

Post your electronics manufacturing, SMT-related material to the #SMT community on Twitter. Use the #SMT hashtag.