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July 2016 • The PCB Magazine 77 when doing on-board testing, it is impossible to know what the pitch of the test points is going to be. In order to do that, suppliers have devel- oped groundless variable pitch probes that can be used on circuit without damaging the pads. This is a huge advance in the industry that has gradually become common. 5. Coupon and board design In order to have similar impedance measures on board and on coupon, the design of the cou- pon should be similar to the design on board. If there is a large area of copper surrounding the impedance lines on board, there must be an area similar to that on coupon for better copper distribution. We know this is never easy when we deal with better usage of the panel, but this is the challenge for the CAM engineers. Some- times, coupon and circuit must have different etch compensations to achieve the same results. Impedance has been changing the whole process of PCB fabrication, especially inspec- tion and data analyses, and it has challenged companies to pursue their best. Surely nobody will have the perfect solution, but if you pursue it you will improve a lot. Even without achiev- ing perfection, you will find a better solution than the one you had. PCB Renato Peres is an industrial en- gineer and production coordinator with Circuibras Circuitos Impressos Profissionais. THE EVOLUTION AND REVOLUTION OF IMPEDANCE CONTROL IN PCB PRODUCTION Flexible pho- tovoltaics made by researchers in South Korea could power wearable electronics. The ultra-thin photovoltaics are flexible enough to wrap around the average pencil and could power wear- able electronics like fitness trackers and smart glasses. The researchers re- port the results in the journal Applied Physics Let- ters, from AIP Publishing. Thin materials flex more easily than thick ones— think a piece of paper versus a cardboard shipping box. The reason for the difference: The stress in a material while it's being bent increases farther out from the central plane. "Our photovoltaic is about 1 micrometer thick," said Jongho Lee, an engineer at the Gwangju In- stitute of Science and Technology in South Korea. The researchers made the ultra- thin solar cells from the semiconductor gallium arsenide. They stamped the cells directly onto a flexible sub- strate without us- ing an adhesive that would add to the material's thickness. The cells were then "cold welded" to the electrode on the substrate by applying pressure at 170° and melt- ing a top layer of material called photoresist that acted as a temporary adhesive. By transfer printing instead of etching, the new method developed by Lee and his colleagues may be used to make very flexible photovoltaics with a smaller amount of materials. The thin cells can be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, Lee said. Ultra-thin Solar Cells Easily Bend around a Pencil