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PCBD-Dec2014

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40 The PCB Design Magazine • December 2014 Role of PCBs in Medical Devices Almost every medical device PCB is heav- ily influenced and constrained by the work of other disciplines. It is rare that a board can be designed without heavy consultation through- out the design cycle with mechanical engineers or industrial designers. There is usually no clear owner to the mechanical constraints of a PCB. A board outline may be proposed, then shoe- horned as the housing design gets hashed out. Rearranged as the main, large components, con- nectors and things get roughly placed; it will be tweaked again by the mechanical engineers or industrial designers. Final- ly the PCB designer will add their final touches and throw it back. This process is highly iterative and relies more and more on accurate 3D modeling of the board, connectors and wiring. PCBs are structural ele- ments with all sorts of great me- chanical properties: they are fair- ly rigid, they're pretty strong and their tolerances are often excellent. The collaboration doesn't stop there either. Whoev- er is doing the firmware, software and logic de- sign will also have some strong opinions about the board. It's best to get everyone together early and often to minimize the chance of later rework or worse. Medical devices have some of strictest in- dustry EMC requirements and regulations. Ad- dressing these constraints is primarily achieved at the PCB level. This is yet another factor driv- ing PCB density and manufacturing tolerances. Design Considerations and Tips As PCBs in medical devices become more complex and dense, it is more important than ever to ensure that the designs are correct and will function as required. Although a typical medical product will see three or more signifi- cant PCB design cycles it is important that the value of each cycle is maximized. The more functionality is designed into a board, the more possibilities for errors and faults there is. General Rules Electronic design automation (EDA) soft- ware now has sophisticated automatic design- rule checking, providing the rules are set up properly. These cannot prevent a bad design from getting into production, but they can pre- vent very common and frequent mistakes and oversights from getting out. As PCBs get more complicated, it is helpful to partition designs hierarchically. The top level of a hierarchical design is much like a block dia- gram of the board. Putting all connectors on the top level schematic sheet if possible, makes that sheet very useful during troubleshooting and design reviews. Child sheets encapsulate lower level func- tionality and sub-circuits. These can be referenced mul- tiple times if a circuit is re- peated, rather than cut and pasted as separate sheets. This allows for easier duplication of layout and is less prone to mis- takes creeping in. Once the schematic design is done, sub-circuits are often routed first, without con- straining them to the board outline. This leads to a better overall layout compared to the tradi- tional approach of component placement fol- lowed by routing. Once the sub-circuit layouts are routed, these are then pushed and shoved together like puzzle pieces. It is helpful to mini- mize the number of layers of sub-circuits even 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 PCBS FOR MEDICAL APPLICATIONS—A DESIGNER'S PERSPECTIVE continues article Once the sub-circuit layouts are routed, these are then pushed and shoved together like puzzle pieces. It is helpful to minimize the number of layers of sub-circuits even if the board will be manufactured with many. " "

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