Issue link: https://iconnect007.uberflip.com/i/410144
26 The PCB Magazine • November 2014 if the board will be manufactured with many. This makes it easier to re-use portions of layouts or adapt to last minute changes in layer counts. More and more boards have high frequency differential pairs or impedance critical single- ended signals. Use a finite element analysis (FEA) calculator to determine the correct trace width and spacing. FEA gives more flexibility than the analytical solutions although they are often not far off each other. Do not neglect the effects of adjacent ground pours or traces. If you are un- able to guarantee the impedance to the closest plane layer then adjacent ground pours are a nice way to control the impedance. Make sure to stitch the pour to the ground plane all along the controlled impedance traces. These calcula- tions can then be compared with the controlled impedance results from the PCB manufacturer. Unfortunately, the industry standard for PCB manufacturing files is still the old Gerber format. These files never contain quite enough information to be sure the boards will turn out right. They don't contain units, the decimal place can be ambiguous and even the origin is uncertain. It is good practice to disable zero suppression, and use an absolute origin for both Gerbers and drill files. It is also good practice to inspect all files with Gerber viewing software other than the EDA package that generated them. More and more PCB manufacturers now have automated workflows with little human intervention or interaction. Layer misalign- figure 4: treatment control device for age-related macular degeneration (amd). Custom PCb design enabled device prototyping for evaluation of the technology in clinical settings. PCBS FOR MEDICAL APPLICATIONS—A DESIGNER'S PERSPECTIvE continues