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JULY 2019 I PCB007 MAGAZINE 45 Remember, the test is looking for milli-ohm changes in resistivity from a good barrel to trigger a fault. The detection guard percentage is adjustable and typically set to around 25%. If you have excess copper in the circuit, the total resistance end-to-end is 1 ohm, and the fault trigger is 25%, you would have to see a 250+ milli-ohm change in that circuit to trig- ger a fault. That is far too high to detect the type of fault in question. The main solution is pre-planning with these types of product. Small hole size, high aspect ratio product requires in-process screening. Trying to perform 4-wire Kelvin test on fully masked and finished product will not identify the potential latent defect unless the barrels are accessible from both sides. This test should be performed before solder mask and after all plating processes are complete. This allows the direct probing of the high aspect ratio barrels, which will deliver the most accurate results. This can also be a sampling from each flight bar from plating to identify if there was a po- tential systemic issue across the entire load or just perhaps an issue with just one flight bar alone. Performing the test at this stage increas- es your confidence in reliability as resistance fluctuations will be detected before costly fi- nal processes are performed. If the test is fatal and caught early enough, a restart can be per- formed with as minimal an impact as possible on delivery. What we have seen today is that "passed" is not always passed. We must be diligent to scrutinize the failures found during routine electrical test as a high yield in ET may not indicate high reliability. Improperly reviewing the failures, and especially overlooking the po- tential impact of a detected void, can turn a 96% ET yield into a 0% yield in the field. This may result in a devastating monetary hit to the manufacturer not to mention the reputation hit in this extremely competitive market. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns or contact Kolmodin, click here. Establishing the Ultimate Limits of Quantum Communication Networks Right now, sensitive data is typically encrypted and then sent across fiber-optic cables and other chan- nels together with the digital "keys" needed to decode the information. However, the data can be vulnerable to hackers. Quantum communication takes advantage of the laws of quantum physics to protect data. These laws allow particles—typically photons of light—to transmit the data using quantum bits, or qubits. Multinational corporations are now building inter- mediate-size quantum computers with an increasing number of quantum units or qubits. Once they scaled up to larger sizes, these devices will have far-superior capabilities than current classical computers. One challenge will be to connect quantum comput- ers together to create a quantum-version of the Inter- net or "quantum internet." However, an important but unanswered question remains: What is the ultimate rate at which one can transmit secret messages or quantum systems from one remote quantum computer to another? "Studying quantum networks is notoriously difficult, but recent mathematical tools developed in quantum information theory have allowed us to completely sim- plify the analysis," he said. "An outstanding question was to compute the maximum number of elementary quantum systems [known as qubits] that could be re- liably transmitted from one user of the network to an- other, or similarly, the maximum number of completely secret bits that these remote users could share. This number has now a precise analytical formula." (Source: University of York)

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