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98 PCB007 MAGAZINE I MAY 2019 standard opens and shorts test but also buried passive, four-wire Kelvin, and HiPot testing. They can still provide the strict, resistive-only test still required by some OEMs and high-re- liability applications, but probers can also pro- vide indirect testing by signature comparison. This is accomplished by developing a capaci- tive master from a test specimen that has gone through and passed the strict resistive test. The subsequent boards have their capacitive signa- ture compared to the master, and any nets that are atypical compared to the master are retest- ed in resistive mode to guarantee they are ac- ceptable. Historic requirements of PCBs did not allow probers in some military requirements or per- formance classes. This was due, in part, to the lack of ability to detect defects in the 3D envi- ronment. Probers of today have the ability to scan the horizon as well as the third dimen- sion or the Z axis. This is known as "line of sight" horizontal or "Z-axis" vertical adjacen- cy. Statistical probabilities have been accepted that if a defect is going to occur, it will occur in a given window between two adjacent nets. This is referred to as the "adjacency window." Historically, this was limited to the horizontal axis or the same layer. This window can now be expanded to layers adjacent in the vertical axis. This enhancement in prober technology is one of the main reasons that some military and higher-performance class products can now be tested utilizing flying probes. This has allowed quicker turn time and reduced cost for electrical test. However, you will not find this technology available in older probers. Software and meter- ing systems in older equipment just didn't have the capacity to perform some of the required tests of today. One needs to inquire about the capabilities of the given prober versus the re- quirements you may have. Remember your de- liverables. Some equipment may provide only some of your solutions while others may de- liver the full package and more! Fixture Testers There is still no question that fixture testers provide the ultimate opens and shorts test. The fixture tester still provides the full parametric test. The simultaneous compression and test of all nets will still detect possible defects that no flying probe will ever find. This is because the fixture tester applies stress to the PCB. The simultaneous compression may detect micro- fractures to traces and/or barrels during the test that a flying probe may not. They also do not have the limitation of the adjacency win- dows as the flying probes have. The isolation test covers all nets to all nets regardless of proximity. A gross short defect covering a long distance will be detected where on a flying probe, it may be out of the adjacency window and not tested. Although rare and statistically low in occurrence, this type of defect would always be detected on a fixture tester. Fixture testers are still the answer to high- volume production. Automation and dedicated fixtures process thousands of boards per time segment compared to a much smaller amount with flying probes. Although automation does exist for flying probes, they will never be able to handle the volume per capita compared to the fixture tester. Conclusion As you can see, the evolution of the PCB and electrical test have come a long way in the last few decades. The advances in PCB manufac- turing and design have forced the evolution of ET and the methods used. ET has become as much of a new science as the new manufactur- ing designs coming from the OEMs. Just check- ing for opens and shorts is no longer enough. Requirements now not only include the opens and shorts test but also the guard zones of bur- ied passives that are independent of the IPC Historic requirements of PCBs did not allow probers in some military requirements or performance classes.

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