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October 2014 • The PCB Magazine 81 todd Kolmodin is the vice president of quality for gardien services usa, and an expert in electrical test and reliability issues. to read past columns, or to contact Kolmodin, click here. WHAT IS 4-WIRE KELVIN? continues role due to the contact area of the two probes onto the pad. If only one probe hits the pad the test will be jeopardized. Other areas of consideration when using 4-Wire Kelvin: • Kelvin test should be done prior to solder- mask as an in-process QA step. It should not be performed as an FA process as the final board will not provide optimal or accurate results due mask on via holes • Due to excess copper travel, the use of Kel- vin for buried microvias is at the venders discre- tion. The copper may add too much resistance to the master values and therefore allow pos- sible defects to go undetected • Kelvin testing for possible microvia copper issues should be done at the sub-part level • For optimum test results, the barrels must be probed directly from side to side. It is best to test each hole twice using an overlapping pattern I hope this helps explain 4-Wire Kelvin and its uses in today's manufacturing marketplace. OEMs are requiring more and more from their suppliers and 4-Wire Kelvin test is just one of the expanded requirements. PCB new research published in the journal advanced functional materials suggests that graphene-treated nanowires could soon replace current touch-screen technology, significantly reducing production costs and allowing for more affordable, flexible displays. the majority of today's touch-screen devices, such as tablets and smartphones, are made using in- dium tin oxide (ito), which is both expensive and inflexible. researchers from the university of surrey and amber, the materials science centre based at trinity College dublin have now demonstrated how graphene-treated nanowires can be used to produce flexible touch screens at a fraction of the current cost. using a simple, scalable, and inexpensive method the researchers produced hy- brid electrodes, the building blocks of touch-screen tech- nology, from silver nanowires and graphene. dr. alan dalton said, "the growing market in devices such as wearable technology and bendable smart displays poses a challenge to manufacturers. they want to offer consumers flexible, touch-screen technology but at an affordable and realistic price. at the mo - ment, this market is severely limited in the materials to hand, which are both very expensive to make and designed for rigid, flat devices." lead author dr. izabela Jurewicz commented, "our work has cut the amount of expensive nanow- ires required to build such touch screens by more than fifty times, as well as simplifying the production process using graphene, a material that can conduct electricity and interpret touch commands whilst still being transparent." Co-author Professor Jonathan Coleman added, "this is a real alternative to ito displays and could replace existing touch-screen technologies in electronic devices. even though this material is cheaper and easier to produce, it does not com- promise on performance. we are working with industrial partners on future devices and it is clear that the benefits will soon be felt by manufacturers and con- sumers alike." the research benefitted from funding and collabora- tion with m-solV, a touch- screen manufacturer. Nanotech Gets Big Push from Graphene

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