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PCBD-June2016

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70 The PCB Design Magazine • June 2016 (GCPW) structure. Both of these structures can have lower insertion loss and they get some loss benefit due to their fields using air. Air is the lowest-loss medium for electromagnetic waves, and these waves use electric and magnetic fields. When a microstrip or GCPW is covered with soldermask, some of the fields which were using air as the dielectric medium are now us- ing soldermask instead. Soldermask will never be as low-loss as air, so the soldermask will always increase the inser- tion loss. A thin layer of soldermask causes less insertion loss than a thick layer of soldermask. In the case of GCPW, there are fields between the ground-signal-ground conductors on the outer layer. When soldermask fills the gaps be- tween these conductors, the insertion loss is in- creased significantly more than for a microstrip circuit covered in soldermask. The previous is an issue that can cause confusion where one type of circuit can be impacted by loss much more than another type of circuit. When applying soldermask over RF conduc- tors, proper design considerations must be ap- plied. For example, if a multilayer PCB has two outer copper layers made of 20 mil RO4350B laminate, which has a Dk of about 3.73 at 3.5 GHz, the propagating wave on a microstrip transmission line circuit will have a wavelength that is about 1.85" (47 mm). The design can have patches of soldermask covering portions of the microstrip circuitry and are less than 1/10 wavelength or 0.185" (4.7 mm). When the sol- dermask is less than 1/10 wavelength, there is no significant impact on the RF circuit perfor- mance. This is often done for RF components that are soldered onto the PCB and the patches of soldermask act like "solder dams," which constrain the solder flow in a specific area dur- ing the reflow operation. If the RF circuitry cannot be covered by soldermask, the designer often uses electroless nickel immersion gold (ENIG) as a final finish so the copper will not oxidize and cause prob- lems later. The addition of ENIG will also in- crease insertion loss, but for a different reason than the soldermask. Nickel has about a quarter of copper's conductivity, so it will cause more conductor losses, which will cause an increase in insertion loss. However, the insertion loss impact due to ENIG is frequency-dependent due to skin depth and at certain frequencies the nickel may not cause as much loss as other frequencies. Engi- neering is a game of tradeoffs and sometimes RF circuit designers will consider the difference of losses due to ENIG and soldermask. They may choose one over the other for their particular application due to the operating frequency or the working environment. PCBDESIGN John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns or to reach Coonrod, click here. THE DILEMMA: SOLDERMASK FOR HIGH-FREQUENCY PCBS A new disposable battery that folds like an origami ninja star could power biosensors and other small devices for use in challenging field conditions. Seokheun "Sean" Choi, assis- tant professor of computer and electrical engineering at Bing- hamton University, along with two of his students, developed the device, a mi- crobial fuel cell that runs on the bacteria available in a few drops of dirty water. Choi previously developed a paper-based origami bat- tery. The first design, shaped like a matchbook, stacked four modules together. The ninja star version, which measures about 2.5 inches wide, boasts increased power and voltage, with eight small batteries con- nected in series. Paper-based biosensors include pregnancy tests and HIV tests. Origami Ninja Star Inspires New Battery Design

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